4.chemistry by Design
4.chemistry by Design
4.chemistry by Design
Chemistry by Design
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AA CD O MD
Chemistry by Design
Aspects of Agriculture
AA2.1 AA3.1 AA3.2 AA3.3 AA4.1 AA4.2 AA4.3 AA5 How does temperature affect the rate of a reaction? What is the nitrogen content of soils? (Optional extension) The nitrogen balance in UK agriculture Revising for end of course exams Dilemma over malaria Partition equilibrium What makes an active pyrethroid? Check your notes on Aspects of Agriculture 236 248 251 253 254 256 258 260
Colour by Design
CD1 CD3 CD4.1 CD4.2 CD4.3 CD4.4 CD4.5 CD5 CD6 CD7.1 CD7.2 CD9 Changing colours chemically Seeing colours Using reflectance spectra to identify pigments What factors affect the drying potential of an oil? Investigating paint media Identifying a pigment Finding a perfect match Comparing hydrocarbons Making azo dyes Dyeing with a direct dye and a reactive dye Different dyes for different fibres Check your notes on Colour by Design 264 266 267 270 272 275 278 280 283 285 287 2
Chemistry by Design
The Oceans
O1.1 O1.2 O1.3 O3.1 O3.2 O4.1 O4.2 O5 What is the relationship between a solvent and the substances that dissolve in it? What changes occur when an ionic solid dissolves? What factors affect the enthalpy change of formation of an ionic compound? The enthalpy change of vaporsation of water What crystals form when a solution is cooled? Finding out more about weak acids Investigating some buffer solutions Check your notes on The Oceans 293 295 296 297 299 300 301 3
Medicines by Design
MD1.1 MD1.2 MD3.1 MD3.2 MD3.3 MD3.4 MD5.1 MD5.2 MD6 Aldehydes and ketones BAC determination using gas-liquid chromatography Making a toolkit of organic reactions Classifying reactions Using the toolkit to synthesise medicines Manufacturing salbutamol (Optional extension) Making and testing a penicillin A closer look at the structure of penecillins (Optional extension) Check your notes on Medicines by Design 307 309 311 316 317 324 330 334 336
AA2.1
How does temperature affect the rate of a reaction?
In this activity you will develop a method for studying how an increase in temperature affects the rate of a reaction.
Requirements
G G G G G G
0110 C thermometer boiling tubes test-tubes burettes (or 1 cm , 2 cm and 5 cm graduated pipettes) potassium iodide solution, 0.2 mol dm3 (25 cm3) potassium peroxodisulphate(VI) (K2S2O8) solution, 0.01 mol dm3 (35 cm3) sodium thiosulphate (Na2S2O3) solution, 0.01 mol dm3 (20 cm3) freshly made starch solution (10 cm3) stopwatch 250 cm3 beaker Bunsen burner, tripod and gauze CARE Eye protection must be worn.
WEAR EYE PROTECTION
potassium peroxodisulphate(VI)
3 3 3
HARMFUL
OXIDISING
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What you do
Use the iodine clock technique from Activity EP6.4, and devise a procedure to investigate how the rate of the reaction varies in the temperature range 20 C to 90 C. It is important that the concentrations used throughout this experiment are kept constant: only the temperature is varied. Suitable volumes to use are:
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3 cm3 of 0.2 mol dm3 KI(aq) 2 cm3 of 0.01 mol dm3 Na2S2O3(aq) 1 cm3 of starch solution 4 cm3 of 0.01 mol dm3 K2S2O8(aq)
Collect your results in a table. Plot a graph of reaction rate against temperature, with temperature on the horizontal axis. Describe the effect on the rate of the reaction of increasing the temperature. There is a rough rule that a rise of 10 C in temperature causes the rate of many reactions to be approximately doubled. Is this true here? You can read about the effect of temperature on the rate of a chemical reaction in Chemical Ideas 10.2.
QUESTION
How does increasing the temperature affect the rate constant (k) for the reaction?
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AA3.1
M v ? Requirements
G G
Activities AA2.5 and AA3.1 form a pair of activities involving soils. You should aim to do at least one of them. This activity allows you to measure the amount in moles of nitrogen present in soil as ammonium ions and as nitrate(V) ions. You will be able to practise the techniques of steam distillation and titration.
oven-dry soil (30 g) 250 cm3 stoppered bottle potassium chloride solution, 2.0 mol dm3 (200 cm3) lter funnel lter paper 250 cm3 conical asks (2) and bung apparatus for steam distillation (Figure 1) 50 cm measuring cylinder magnesium oxide (0.5 g) boric acid, 1% solution (5 cm3) Devardas alloy (0.5 g) burette and titration apparatus sulphuric acid, 0.00500 mol dm3 (50 cm3) 100 cm3 volumetric ask 10 cm3 pipette 25 cm3 pipette safety ller indicator solution (pH range 56), 1:1 mixture of Methyl Red and Bromocresol Green (100 mg in 100 cm3 of ethanol)
3
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indicator solution
HIGHLY FLAMMABLE
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Introduction
Nitrogen available to plants exists in soil in two forms: as ammonium ions, and as nitrate(V) ions. The method you will use to determine the nitrogen content of your soil sample is a method still used for accurate determinations. The quicker, more convenient methods are less sensitive. Work in groups for this activity. Half the students in each group can determine the nitrogen present as ammonium ions, in Part 2. The other half can determine the total nitrogen present as nitrate(V) and ammonium ions, in Part 3. You can then combine your results to work out the nitrogen present as nitrate(V) ions. Part 1 can be done in advance and the ltrate stored in a stoppered ask in the fridge until required. There will be enough solution for both halves of the group. Ammonium ions are displaced into solution by ion exchange when the soil is shaken with a solution containing excess potassium ions. If the solution is then made alkaline, ammonia is formed, and can be distilled off. A steam distillation technique is used (see below) to make sure that ammonia remains in solution and does not escape as a gas. The ammonia produced is absorbed in a 1% solution of boric acid: NH3(aq) + H3BO3(aq) D NH4+(aq) + H2BO3 (aq) The borate formed can then be titrated with a strong acid, like sulphuric acid: H2BO3(aq) + H+(aq) H3BO3(aq) A mixed indicator solution containing Methyl Red and Bromocresol Green gives a sharp end-point from blue-green to pink. The amount in moles of ammonia is equal to the amount of H+(aq) used in the titration. This titration procedure is used to reduce the risk of loss of ammonia.
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AA3.1
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Nitrate(V) ions are also present in the soil solution at the end of Part 1. These are converted to ammonium ions by a reducing agent. Distillation as before, followed by titration, gives the total available nitrogen in the soil. The amount present as nitrate(V) can then be found by subtraction.
What you do
Part 1: Extraction of ammonium ions
1 Put 200 cm3 of 2.0 mol dm3 potassium chloride solution into a bottle. Add 30 g of dry soil. Stopper the bottle and shake it for 10 minutes. 2 Filter the mixture into a conical ask. Stopper the ask and store it in a fridge until required.
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QUESTIONS
a Write an equation to represent what is happening as the soil is shaken with the potassium chloride solution. b Why is such a large volume of 2.0 mol dm3 potassium chloride solution used?
HEAT INTERMITTENTLY
3 Set up the apparatus for steam distillation, as shown in Figure 1. 4 Place 50 cm3 of the ltrate from Part 1 into the round-bottomed ask and add 0.5 g of magnesium oxide. (The MgO reacts with the NH4+ ions, liberating ammonia.) Put 5 cm3 of boric acid in the measuring cylinder you are using to collect the ammonia solution. 5 Heat the water in the steam generator. When it boils, steam passes through the mixture in the round-bottomed ask. This ask should be heated gently to prevent steam condensing. Steam distil until about 40 cm3 of distillate have been collected. The distillate should contain all the ammonia released from your soil sample. The distillation will take some time. While it is being carried out, one of you should set up a burette lled with 0.00500 mol dm3 sulphuric acid ready for the titration. 6 At the end of the heating be careful to avoid any sucking back. Before turning off the heat, lower the receiver and rinse it with a little distilled water into the measuring cylinder, and then disconnect the steam generator.
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7 Transfer the distillate to a 100 cm3 volumetric ask and make up to the mark with distilled water. Stopper the ask and shake it well. Using a pipette tted with a safety ller, transfer 10 cm3 of the solution to a 250 cm3 conical ask. Add 2 or 3 drops of indicator solution and do a rough titration to get used to the end-point. This will also allow you to decide on the most suitable volume of solution to remove for the accurate titration. Make a note of this volume. 8 Now carry out an accurate titration and record the volume of sulphuric acid you used.
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9 Set up the apparatus for steam distillation as shown in Figure 1. Put 5 cm3 of boric acid in the measuring cylinder you are using to collect the ammonia solution. 10 Devardas alloy contains copper and aluminium and acts as a reducing agent. It reduces nitrate(V) ions to ammonia in alkaline solution. Place 50 cm3 of the ltrate from Part 1 into the round-bottomed ask, and add 0.5 g of Devardas alloy and 0.5 g of magnesium oxide. (The MgO reacts with the NH4+ ions, liberating ammonia.) Steam distil the mixture as described in steps 5 and 6 until you have collected 40 cm3 of distillate. During the distillation, one of you should set up a burette lled with 0.00500 mol dm3 sulphuric acid ready for the titration. 11 Transfer the distillate to a 100 cm3 volumetric ask and make up to the mark with distilled water. Stopper the ask and shake it well. Using a pipette tted with a safety ller, transfer 10 cm3 of the solution to a 250 cm3 conical ask. Add 2 or 3 drops of indicator solution and do a rough titration to get used to the end-point. This will also allow you to decide on the most suitable volume of solution to remove for the accurate titration. Make a note of this volume. 12 Now carry out an accurate titration and record the volume of sulphuric acid you used.
QUESTIONS
c Write an equation for the reaction between ammonium ions and magnesium oxide in steps 4 and 10. d Write an equation for the reaction of the distillate solution with sulphuric acid.
AA3.2
M v ? What you do
The nitrogen balance in UK agriculture
The purpose of this activity is to allow you to become more familiar with the nitrogen cycle and to help you appreciate the quantities of nitrogen involved.
Figure 1 shows a modied version of the diagram of the nitrogen cycle from Storyline AA3. The reserves of nitrogen in the soil and in the atmosphere are shown, but the ovals showing the nitrogen uxes have been left blank. 1 Using data from Table 1, enter values for the total annual uxes of nitrogen in UK agricultural land into the appropriate spaces in the diagram. You may like to use different coloured pens for the input and output gures.
Input Thousands of tonnes per year 275 14 1150 26 1020 9 24 150 Output Thousands of tonnes per year 1367 326 536 50 9 380
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Rain Seeds Fertilisers Sewage Livestock excreta Silage efuent Crop waste Biological N2 xation
Crops and grasses (mostly for animal feed) Leaching Loss of ammonia as gas from livestock excreta from crop wastes from sewage Denitrication and immobilisation
Table 1 Nitrogen balance in UK agriculture. The gures (produced by the Royal Society in 1983), are estimated totals for the UK in thousands of tonnes of nitrogen per year, showing the inputs and outputs of nitrogen in agricultural land. These estimates, although originating in the 1970s, still remain the best available data. It is accepted that the proportions between the various inputs and between the outputs are similar to those obtained now.
2 Refer to the section on the nitrogen cycle in Storyline AA3. Mark on Figure 1 the formulae for the different forms of inorganic nitrogen in the cycle to show the conversion from one form to another.
QUESTIONS
a Work out the total input of nitrogen into agricultural land in the UK in thousands of tonnes per year. Now work out the total output of nitrogen. Comment on the relative sizes of the two gures. Compare the size of these uxes with the size of the total reserves of nitrogen for the UK. b Make a list of the inorganic ions and molecules involved in the nitrogen cycle. Identify the oxidation state of nitrogen in each species.
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Atmosphere Reserve 80 000 tonnes N2 above each hectare of land Total land area UK is 24 401 068 ha nitrogen oxides, NOX, and ammonia 20% human consumption ammonia
Figure 1 The nitrogen balance in UK agriculture (Source of gures: Royal Society 1983; MAFF 1984). Numbers to be inserted are totals for the UK in thousands of tonnes N per year
Soil reserve in top 20 cm of soil Arable land 29 tonnes N ha 1 Old grassland 1220 tonnes N ha 1 leaching
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AA3.2 THE NITROGEN BALANCE IN UK AGRICULTURE
sewage silage effluent
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AA3.3
M v ? Synoptic assessment
Revising for end of course exams
As you approach the end of the course you will need to revise most of the work you have covered. The aim of this activity is to help you do this effectively. You may wish to delay doing this activity until the most appropriate time for you but dont leave it too late. You need plenty of time to plan your revision programme well.
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An important part of the end of course examinations involves synoptic assessment. This requires you to draw together knowledge, understanding and skills learned in different parts of the course. You should feel condent about this, as many of the storylines you have studied have required you to use ideas from different areas of chemistry. Real-life contexts can rarely be fully understood through a knowledge of a single area of chemistry. However, in your revision and in thinking about how you will approach examination questions you have to acknowledge that the questions are designed to test your ability to:
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recognise the different chemical ideas you need to use to answer the question draw on your knowledge and understanding of these ideas to construct your answer.
The synoptic examination is based on the units at the end of the course, but remember that these units bring together many ideas that you have met in earlier units. Some questions will be based on applications you have not met during the course. Hopefully these will be interesting contexts and you may even enjoy applying your chemical understanding to tackling the problems you are set.
Be active
A long time ago, in Activity DF4.8, you were encouraged to be as active as possible while revising. You should now have developed your own revision strategies, based on a variety of tasks that you set yourself, but it may still be worthwhile looking back at the few suggestions given in that activity.
And nally
Finally, a few words of encouragement. Almost everyone nds written exams stressful and daunting. However, students who revise thoroughly and work to a planned programme are likely to put themselves under less pressure and achieve their best result on the day.
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AA4.1
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Dilemma over malaria
This activity provides an opportunity for you to consider the role of chemistry in ghting disease, in monitoring the environmental impact of pesticides and in decision making.
The short article reproduced on the next sheet was written by Bette Hileman and appeared in Chemical and Engineering News, published by the American Chemical Society, in September 1999. It outlines succinctly the dilemma facing those who are responsible for decisions about the use of DDT and those who determine the focus of future research. The global perspective in the article highlights the likelihood that different societies will have different views on the issue and that there are worldwide as well as local environmental concerns. The intention is that you should prepare for a group discussion on the Dilemma over malaria and that the discussion should aim to reach agreement on what you as a group of chemistry students think would be appropriate shortterm and long-term policies for dealing with malaria throughout the world.
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What you do
1 Read through the article and make a note of the factors that are important in deciding appropriate policies for dealing with malaria. 2 Decide which of these factors can be supported by scientic evidence and search for additional articles and data that are relevant to the environmental concerns and the health concerns. One way of doing this is to search the Internet for sites concerned on the one hand with DDT and on the other with malaria. It would be more efcient if some members of the group concentrated on information about malaria, its effects and its treatment, whereas others could concentrate on the environmental concerns. When extracting information from other sources try to distinguish between evidence and opinions. 3 Prepare brief presentations which you think should be considered as the group formulates its policy proposals.
AA4.1
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government insights
is still the cheapest and, in some places, the most effective way to control mosquitoes. The one Latin American country where malaria incidence has declined, Ecuador, is the only one that has increased its use of DDT to kill mosquitoes. Malaria, a serious infection of Plasmodium parasites, is one of the resurgent diseases in the world today. Each year, it causes 500 million clinical cases and kills up to 2.7 million people, many of whom are children under five. The incidence of malaria is increasing, partly because insect spraying programs have been cut back, partly because prevention programs using antimalarials have been abandoned, and partly because of global climate change. As temperatures increase at higher elevations, malaria-carrying mosquitoes extend their range, infecting people who have no natural immunity.
The inexpensive drug chloroquinine that people around the world used for years to prevent and control the disease is no longer effective because the malaria parasites have developed resistance to it. Since the alternative drugs are much more expensive, most cases of malaria go untreated. If the inside walls of a house are painted with DDT, mosquitoes are controlled for six months to a year. Some mosquitoes are resistant to DDT, but these tend to be irritated by it, so they leave the house after coming in contact with the walls. Painting the inside walls of a large house requires 400 g to 600 g of DDT. After six months, about half the original application usually akes off the walls and most of this ends up outdoors. But this amount is minuscule compared with the 800 kg used on a 100-acre cotton eld in a growing season.
WWF argues that malaria-carrying mosquitoes can be controlled by altering the environment. It advocates planting trees to dry up the soil, stocking ponds with mosquito-larvae-eating fish, and emptying water from containers so mosquitoes cannot breed. In some dry areaswhere malaria transmission rates are moderatesuch remedies may work. However, mosquitoes in some locales breed almost anywhere, in grass and wet leaves, in any small depression in a farm plot, in rain gutters and drainage ditches, in plants that hold small amounts of water. The eggs persist through droughts, waiting for the next rain or monsoon. Then, hordes of mosquitoes burst forth and often land on humans at a rate up to 100 a minute. WWF also advocates the use of bed nets soaked in pyrethroid insecticides and the limited spraying of pyrethroids inside houses to kill mosquitoes. Pyrethroids are much less persistent than DDT. However, millions of people cannot afford bed nets, and spraying pyrethroids by most accounts is about three times as expensive as painting interior walls with DDT. DDT is still used on crops in much of Africa, India, and perhaps China. To me, it seems that the best compromise for the POPs treaty would be a strict ban on agricultural uses of DDT and a total phaseout for public health use when cheap, effective alternatives are available. This might put pressure on industrialized countries to put money into research on malaria drugs and vaccines and on mosquito control. Just $84 million is spent worldwide annually on malaria research. Even the worlds best malaria drug research lab, the Walter Reed Army Institute of Research, Washington, D.C., now receives only $5 million a year for malaria drug research. No major pharmaceutical company is working on a malaria drug. If we as a society want a total ban on DDT, our government should be willing to invest a large part of the money it will take to reduce the disease burden from malaria. As travellers and as temporary inhabitants in endemic areas, we would also benet. Bette Hileman 255
AA4.2
Partition equilibrium
In this activity you will investigate the way in which a solid behaves when it can dissolve in two solvents which do not mix. Next you will plan a procedure for making quantitative measurements. You can then use data on some pesticides to look at the link between values of partition coefcients and the concentrations of pesticides in living organisms.
Requirements
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iodine crystals (2 small crystals) forceps rack and test-tubes with rubber bungs dropper pipettes cyclohexane (4 cm3) potassium iodide solution, 0.2 mol dm3 (10 cm3)
iodine
HARMFUL
cyclohexane
HIGHLY FLAMMABLE
CARE Iodine is harmful. Use forceps to pick up the iodine crystals. When you have nished, place the iodine solutions in the iodine residues container. CARE Cyclohexane is highly ammable.
Part 1: The partition of iodine between two solvents (May be a teacher demonstration)
A substance may be soluble in two solvents which do not mix, like water and cyclohexane. In this part of the activity you will investigate the way iodine partitions between these two solvents. Iodine is only sparingly soluble in water, but it dissolves readily in potassium iodide solution. In these experiments, therefore, potassium iodide solution is used instead of pure water. (When iodine dissolves in potassium iodide solution, it forms the brown complex ion, I3(aq). This is why iodine is so much more soluble in this solvent than it is in water.) Solutions of iodine in cyclohexane and aqueous potassium iodide are coloured, so you can draw qualitative conclusions about the concentration of iodine in each layer from the appearance of the two layers.
What you do
1 Take two small iodine crystals (CARE Harmful. Use forceps to pick up the crystals) of approximately the same size, and place each crystal in a test-tube. Add 2 cm3 of cyclohexane (CARE Highly ammable) to one tube and 2 cm3 of aqueous potassium iodide to the other. Stopper the tubes and shake gently until the iodine crystals dissolve. Note the colour of each solution. 2 Now add 2 cm3 of aqueous potassium iodide to the tube containing iodine dissolved in cyclohexane, and 2 cm3 of cyclohexane to the tube containing iodine dissolved in aqueous potassium iodide. Stopper the tubes, shake them for a few minutes and stand them in a rack to allow the layers to separate. 3 Using a pipette, draw off the upper, cyclohexane layer from one of the testtubes and place this solution in a clean test-tube. Investigate the effect of adding further volumes of the second solvent to each of the separated layers. 4 Record your observations, noting the colour and intensity of colour in each layer. What do your observations tell you about the way iodine partitions itself between the two solvents? You will have found that some of the iodine dissolves in one solvent, and some in the other. The partition of iodine between the two solvents is an equilibrium process. The ratio concentration of solute in solvent A concentration of solute in solvent B is a constant at a given temperature, and is called the partition coefcient. It is an equilibrium constant. You can read more about it in Chemical Ideas 7.4.
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PARTITION EQUILIBRIUM
AA4.2
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The fate of pesticides in the environment is important. They can accumulate in living organisms, then become more and more concentrated up the food chain. Measurements of this bioconcentration have been made in sh. For any compound, the bioconcentration factor is equal to concentration of compound in whole sh concentration of compound in water The bioconcentration factor does not depend much on the species of sh, or on the length of time the sh has been in pesticide-contaminated water (once the system has come to equilibrium): it is a property of the pesticide itself. Determining the bioconcentration factor is an important but expensive business. Researchers have therefore looked for ways of estimating bioconcentration factors from properties which are easier to measure, such as partition coefcients, particularly Kow. Table 1 shows lg Kow and lg(bioconcentration factor) for several pesticides.
Pesticide Organochlorines chlordane DDT DDE DDD dieldrin lindane Pyrethroids permethrin Hydrolysis products of permethrin alcohol acid Others atrazine dibenzofuran lg Kow 6.00 5.98 5.69 6.02 5.16 3.85 5.00 lg(bioconcentration factor) 4.58 4.47 4.71 4.81 4.10 2.51 3.70
Table 1 Data on bioconcentration factors for some pesticides and some of their breakdown products. Note that in both columns logarithmic values are quoted.
QUESTIONS
a Plot a graph of lg(bioconcentration factor) against lg Kow. What is the relationship between the two quantities? b The ester group in permethrin is rapidly hydrolysed in the environment (see Storyline AA4, The pyrethroid story). DDT is degraded to DDE and DDD.
Cl Cl Cl Cl Cl Cl Cl Cl DDE Cl Cl Cl
Cl DDT
Cl
Cl DDD
Cl
Refer to the values of lg Kow in Table 1. Suggest why DDT and its breakdown products DDE and DDD may persist for years, causing problems in the environment, whereas permethrin does not cause similar problems.
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AA4.3
M v ? Introduction
What makes an active pyrethroid?
In this activity you will identify features which are important for insecticidal activity in pyrethroids. This will help you to pick out common structural features in complex organic molecules.
Pyrethroids kill insects by binding to specic proteins in nerve-cell membranes. As a result the nervous system cannot function properly and the insect dies. In order for them to work, pyrethroids must have the correct physical properties so that they can move into the cell membranes. They need to have groups of the right kind in the right positions so that intermolecular forces can bind the pyrethroid molecules to proteins in the nerve cell membranes. Finally they should be easily hydrolysed so that they do not persist in the environment.
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What you do
Work in groups so that you can discuss your ideas. 1 Identify structural features which are common to the pyrethroid molecules mentioned in Storyline AA4, The pyrethroid story. These are the compounds A, B and C shown below. One way to do this is to list the structural features present in biopermethrin in a table, and tick off features which are also present in B and C.
O O O O O CN O
Cl
Cl
Cl
Cl
A Biopermethrin
B Biocypermethrin
O O
CN O Br
Br
C Deltamethrin
2 Identify three features of A, B and C which are still present in D and E. Compare the structures of D and E with biocypermethrin, structure B. Where there has been a change, what replaces the group or groups removed?
O O O CN O O CN O
Cl
D
CF3
Cl
E Fenvalerate
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O O O
Are any of the features which you identied in A, B and C still present? Do the structures have anything in common with fenvalerate, E? If you cannot see any, check with your teacher rather than spend a long time searching. You may be surprised by how few of the groups present in permethrin are essential for insecticidal activity. It appears that active pyrethroids have unsaturated groups at each end of the molecule (alkene or benzene) and that pairs of methyl groups are present. The distance between the methyl groups and the unsaturated end groups is important for activity. Overall, the compounds must be non-polar, with high Kow values (see Activity AA4.2). The presence in the centre of the molecule of a group which is easily hydrolysed means that the compounds are not persistent in soil. 4 Inspect structure H below. The substance is a rapid knock-down agent but does not kill insects. They recover from its effects. What features are missing which were present in all the other pyrethroid molecules?
O O O
N O
Does this conrm or conict with the suggestion in 3 above about features identied as important for insecticidal activity of pyrethroids?
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AA5
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Check your notes on Aspects of Agriculture
This activity helps you to get your notes in order at the end of this unit.
Use this list as the basis of a summary of the unit by collecting together the related points and arranging them in groups. Check that your notes cover the points and are organised in appropriate ways. Most of the points are covered in the Chemical Ideas, with supporting information in the Storyline or Activities. However, if the main source of information is the Storyline or an Activity, this is indicated.
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The redox reactions involved in the interconversion of the following species in the nitrogen cycle: nitrogen gas, nitrate(V) ion, nitrate(III) ion, ammonium ion, dinitrogen oxide (N2O), nitrogen monoxide (NO), nitrogen dioxide (NO2) (Storyline AA3; Activity AA3.2). An outline of the manufacture of ammonia by the Haber Process, including essential conditions (Storyline AA3). Why the conditions in the Haber Process are chosen, including the effect the conditions have on the position of equilibrium and on the rate of reaction (Storyline AA3). The expression for the equilibrium constant, Kp, for reactions involving gases (in terms of partial pressures). How values of Kp, together with given data on partial pressures, are used to carry out calculations concerning the composition of equilibrium mixtures. The trends in reactions of the elements, and the properties of their compounds, across a period in terms of structure and bonding, including: the reactions of the elements with oxygen, chlorine and water; the acidbase character of oxides; the behaviour of chlorides towards water. The relationship between the structure and bonding of a substance and its properties. The partition equilibrium that occurs when a solute is distributed between two immiscible solvents. The design of pesticides that combine maximum efcacy with minimum environmental damage (Storyline AA4).
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The ways in which chemists can help improve food production, including providing extra nutrients, controlling soil pH and controlling pests (the Storyline in general). The effect of temperature on the rate constant of a reaction. The interpretation of silicate structures in terms of the tetrahedral silicate unit (Storyline AA2; Activity AA2.2). The interpretation of the propreties of clay minerals in terms of a simple model of layers made up of tetrahedral silicate sheets and octahedral aluminate sheets (Storyline AA2; Activities AA2.2 and AA2.3). The role of ion exchange processes in soil and the ionexchange characteristics of different soils (Storyline AA2). The principles of ion exchange. The relationship of ion exchange behaviour of anions and cations to ionic size. The effect of atomic number, charge and hydration on the size of anions and cations. The relationship between ionic size and properties.
CD1
M v ? Requirements
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In this activity you will investigate colour changes brought about in a variety of ways, and then identify the type of reaction involved in each case.
250 cm3 stoppered bottle 100 cm3 measuring cylinder alkaline solution of glucose (100 cm3)* Methylene Blue indicator Phenolphthalein and Fluorescein indicators (optional) boiling tubes, test-tubes and rack acidied solution of ammonium vanadate(V) (ammonium metavanadate, NH4VO3) (10 cm3)* zinc (granulated) zinc oxide (0.5 g) Bunsen burner lead nitrate(V) (ground to a powder) (0.5 g) potassium iodide (ground to a powder) (0.5 g) lead nitrate(V) solution, 0.5 mol dm (3 cm ) potassium iodide solution, 0.5 mol dm3 (1 cm3) teat pipettes sodium carbonate solution, 1.0 mol dm3 (1 cm3) dilute ammonia solution, 2.0 mol dm3 (5 cm3) potassium (or ammonium) thiocyanate solution, KSCN (or NH4SCN), 0.1 mol dm3 (1 cm3) potassium hexacyanoferrate(II) solution, K4Fe(CN)6, 0.005 mol dm3 (1 cm3) dilute solutions (approximately 0.1 mol dm3) containing the following ions (12 cm3 of each solution): copper(II) nickel(II) iron(III) potassium chromate(VI) solution, 0.5 mol dm3 (2 cm3) protective gloves dilute sulphuric acid, 1.0 mol dm3 (1 cm3) microscope slides and coverslips rubber bungs Universal Indicator solution sodium hydroxide solution, 1.0 mol dm3 (2 cm3) small lump of solid carbon dioxide (dry ice) and tongs, or a supply of carbon dioxide 250 cm3 beaker
dilute sulphuric acid
IRRITANT
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ammonium vanadate(V)
TOXIC IRRITANT
copper(II) ion
HARMFUL
lead nitrate(V)
TOXIC OXIDISING
nickel(II) ion
HARMFUL
potassium chromate(VI)
VERY TOXIC
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potassium hexacyanoferrate(II)
HARMFUL
potassium thiocyanate
HARMFUL
CARE Chromates(VI) irritate the eyes, the skin and the respiratory system. They are also suspected carcinogens. Avoid all skin contact and do not breathe any dust. Any spillage should be washed off at once. * See instructions for preparation in the Teachers and Technicians Guide
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What you do
Investigate the following reactions, which all involve colour changes. In each case, record your observations and try to explain as far as possible what is happening to produce the colour change. Doing all these reactions takes some time, so you will probably do only one of these. Show and explain this reaction to the other members of your class. In a similar way, go round the other members of your class and write down the observations they have made. Use textbooks to help where necessary. You will have met some of the reactions in earlier units (From Minerals to Elements, The Atmosphere and The Steel Story). 1 Place 100 cm3 of an alkaline solution of glucose (CARE Irritant) in a 250 cm3 bottle with a well-tting stopper or rubber bung. Add enough Methylene Blue indicator (CARE Harmful) to give a good dark colour to the solution. (Methylene Blue is an indicator whose colour depends on the level of oxygen dissolved in the solution.) Shake the bottle vigorously and leave it to stand. When the colour of the Methylene Blue has disappeared (this may take up to 30 minutes), shake the bottle again. You can repeat the process many times. (You can vary the colour change by repeating the experiment and adding Phenolphthalein or Fluorescein as well as Methylene Blue.) 2 Place 10 cm3 of acidied ammonium vanadate(V) solution (CARE Toxic and an irritant) into a boiling tube and add a piece of granulated zinc. Gently swirl and shake the tube until no further changes occur. You may need to heat the tube gently to speed up the reactions. 3 Heat a small sample of zinc oxide in a test-tube until there is no further change in colour. Stand the tube in a rack to cool. When cool, repeat the heating and cooling process. 4 Place a spatula-load of powdered lead nitrate(V) crystals into a test-tube. (CARE Lead nitrate(V) is toxic and oxidising. Use only a small amount and do not breathe the dust.) Add a spatula-load of powdered potassium iodide crystals. Cork the tube and shake it vigorously. Repeat using dilute aqueous solutions of these two substances. 5 Investigate what happens when you add the reagent from list A to the metal ion solution opposite it in list B in Table 1. Use a teat pipette and shake the mixture after each addition. (CARE Copper(II) and nickel(II) salts are harmful.) 6 Add a few drops of Universal Indicator solution to some water in a beaker. Add enough dilute sodium hydroxide solution to raise the pH and give a deep purple colour. Now add a small lump of solid carbon dioxide (dry ice CARE Dry ice should be handled with tongs), or bubble carbon dioxide from a generator through the solution. 7 Mix 2 cm3 of lead nitrate(V) solution with 2 cm3 of potassium chromate(VI) solution. (CARE Lead compounds are toxic and chromates(VI) are suspected carcinogens. Avoid all skin contact wear gloves.) Put a drop of the resulting mixture on a slide and observe under a microscope. Repeat the experiment but add a few drops of dilute sulphuric acid (CARE Irritant) to the potassium chromate(VI) solution before mixing.
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A sodium carbonate solution ammonia solution ammonia solution potassium hexacyanoferrate(II) solution, K4Fe(CN)6 (CARE Harmful) potassium (or ammonium) thiocyanate solution, KSCN (or NH4SCN) (CARE Harmful) Table 1
Fe3+(aq)
Summary
At the end, draw up a summary table in which you classify each change occurring as one of the following:
G G G G G
redox reaction ionic precipitation reaction acid-base reaction ligand exchange reaction polymorphic change.
(Compounds which can exist in more than one solid form, in which the ions or molecules are arranged in different ways, are said to be polymorphic.)
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Seeing colours
In this activity you can examine the nature of the light reaching your eye from coloured solutions and from coloured surfaces.
Requirements
G
hand-held direct-vision spectroscope (or diffraction grating) test-tubes and rack white light source or sunlight (an ordinary light bulb can be used, but it must be in a shade or a protective holder) aqueous solutions of a range of coloured compounds, for example: copper(II) sulphate chromium(III) chloride potassium dichromate(VI) screened Methyl Orange indicator protective gloves brightly coloured surfaces (such as exercise books or les) CARE Eye protection and gloves must be worn.
WEAR EYE PROTECTION WEAR PROTECTIVE GLOVES
G G
chromium(III) chloride
HARMFUL
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copper(II) sulphate
HARMFUL
potassium dichromate(VI)
TOXIC
G G
CARE Dichromates(VI) irritate the skin and are suspected carcinogens. Avoid all skin contact. Any spillage should be washed off at once. Wear protective gloves.
What you do
Point the spectroscope at a window or source of white light and observe the effect it has on the light. (You may wish to use a diffraction grating instead of a spectroscope.) Investigate the effect of placing a coloured solution between the white light source and the spectroscope, as shown in Figure 1. Summarise your observations in a table.
Now point the spectroscope at a well-lit brightly coloured surface such as an exercise book or le. You may want to read Chemical Ideas 6.7 to help you explain your observations.
QUESTIONS
a What effect does the spectroscope have on white light? b Describe what you observed when a coloured solution was placed between the white light source and the spectroscope. Explain why this happens. c Use your observations with coloured surfaces to help you write a short explanation of why paintings appear coloured when you look at them with the naked eye.
CD4.1
M v ? Reectance spectra
Using reflectance spectra to identify pigments
In this activity you will use reectance spectra to nd out which pigments Cima used 500 years ago to produce two different areas of blue in his altarpiece The Incredulity of S. Thomas.
The surface of a painting is not smooth and light is reected from it in all directions.
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paint surface
A reectance spectrum tells you how much light of each colour in the spectrum is reected in a particular direction. The reectance spectrum of a painting is obtained by shining light on a small area and analysing the light reected off the surface of the pigments at a particular angle. Figure 2 shows a simplied version of the apparatus used. The monochromator contains a prism which splits the white light into its component wavelengths. The prism is rotated so that the wavelength of the light focused on the painting can be varied. The apparatus also incorporates a camera which takes a photograph of the precise spot on the painting that is being examined.
painting
45
photomultiplier measures the intensity of the light reflected for each wavelength at an angle of 45 to the incident light
monochromator splits the white light into component wavelengths white light from a hot tungsten filament
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What you do
Below are the reectance spectra of two blue pigments used by Cima. They were taken from two different parts of the painting, area A and area B (see Figures 3 and 4). The horizontal axis represents the wavelengths of the light reected. The vertical axis shows the percentage of the incident light reected by the pigment.
Reflectance / %
100 90 80 70 60 50 40 30 20 10 0 400 450 500 550 600 650 700 750 Wavelength / nm
Reflectance / %
100 90 80 70 60 50 40 30 20 10 0 400 450 500 550 600 650 700 750 Wavelength / nm
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Study the reectance spectra in Figures 3 and 4 and then compare them with the reectance spectra of some traditional blue pigments which were in use at the time Cima was painting the altarpiece. These are shown on Information Sheet 1 (Traditional blue pigments).
QUESTIONS
a Which blue pigments do you think are present in the paint areas A and B? b What is the chemical composition of each of these pigments? c How would you expect the shade of blue to differ for the two pigments? (You may want to look at your Data Sheets to nd out which colours correspond to the various wavelengths in the visible spectrum.) d Look at Cimas The Incredulity of S. Thomas (Storyline CD4, Figure 13). Look carefully at the different blue areas in the picture. In which part of the painting do you think Cima might have used these pigments? e Why do you think it is important to take a photograph of the precise spot on the painting that is being subjected to measurement? f How might reectance spectra provide information on the effect of gallery lights on various pigments?
CD4.1
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Table 1 Information about some traditional blue pigments in use in the early 16th century
Reflectance / % Reflectance / %
100 (a) Smalt 90 80 70 60 50 40 30 20 10 0 400 450 500 550 600 650 700 750 Wavelength / nm
100 (b) Azurite 90 80 70 60 50 40 30 20 10 0 400 450 500 550 600 650 700 750 Wavelength / nm
Reflectance / %
Reflectance / %
100 (c) Natural Ultramarine 90 80 70 60 50 40 30 20 10 0 400 450 500 550 600 650 700 750 Wavelength / nm
100 (d) Indigo 90 80 70 60 50 40 30 20 10 0 400 450 500 550 600 650 700 750 Wavelength / nm
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CD4.2
M v ? Different types of oil
What factors affect the drying potential of an oil?
In this activity you will use information from experiments to decide what factors affect the drying potential of an oil. You can then make some deductions about the complex chemistry of the drying process.
If a drying oil is exposed to air, it eventually becomes a rubbery solid. If only a thin layer is exposed, it becomes hard. Non-drying oils do not harden in this way, but they may thicken on heating. In between the two extremes are the semi-drying oils. These thicken and form a skin when exposed to air at high temperatures. Only drying oils are suitable for mixing with pigments to make paints.
AIR AIR
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drying oil
pigment
rubbery solid
Figure 1 The action of air on an oil-based paint
Paint layers dry from the outside to give a tacky surface rst; eventually they become hard throughout. The drying process is irreversible.
Iodine numbers
Natural oils can be classied according to their iodine number. This is calculated on the basis of how much iodine(I) chloride (ICl) will react with the oil. The more ICl reacts with the oil, the higher the iodine number. Table 1 gives the iodine numbers for some natural oils.
Oil linseed poppy walnut almond olive castor cotton-seed maize sesame Drying potential drying drying drying non-drying non-drying non-drying semi-drying semi-drying semi-drying Iodine number 170195 140158 140150 <100 <100 <100 103111 117130 100120
3 1 2
10
3
curve b
1
curve a 10 20 30 40 50 60 70 Time / days
No signicant increase in mass was observed if the linseed oil samples were kept in an atmosphere of nitrogen.
Salters Advanced Chemistry 2000 see Copyright restrictions
CD4.2
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QUESTIONS
a How is the drying potential of an oil related to its iodine number? b Look at the structures of some of the carboxylic acid components of the triesters found in natural oils. You can nd these in Chemical Ideas 13.6. With which structural feature of these molecules do you think ICl will react? What type of reaction is involved? c Which parts of the triester molecules in natural oils do you think are involved in the drying process? d What other substance seems to be involved in the drying reaction? Explain your answer. e Suggest what type of chemical processes might be involved when natural oils harden. f What experiments would you want to do to conrm your suggestions? g Explain how the drying of an oil paint is different from the drying of watercolours.
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CD4.3
M v ? The problem
Investigating paint media
In this activity you will analyse gas-liquid chromatograms to nd out which oil was used as the binding medium for Cimas paints in The Incredulity of S. Thomas.
Oils are triesters of glycerol with long-chain carboxylic acids (see Chemical Ideas 13.6). Different oils contain different amounts of each acid, and the ratio of the acids can be characteristic of a particular oil. The trouble is that the cross-linking process which causes paint to dry involves the unsaturated carboxylic acid chains, and this polymerisation is irreversible. So, when chemists analyse samples of paint, they measure the ratio of two saturated carboxylic acids, since these are not involved in the crosslinking. The acids chosen are palmitic acid and stearic acid. Their ratio is unchanged by the drying process.
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glycerol
The groups R, R' and R" vary according to the carboxylic acid present (for palmitic acid, for instance, R is CH3(CH2 )14 ). The mixture is now acidied with hydrochloric acid to liberate the free carboxylic acids. These will be simple fatty acids as well as acids of the polymeric cross-linked material. Stearic acid and palmitic acids are among the fatty acids produced:
CH3(CH2)14COOH palmitic acid CH3(CH2)16COOH stearic acid
The free acids are not very volatile and are difcult to analyse using g.l.c. (They take a long time to emerge from the column and do not produce sharp peaks.) The next stage is therefore to convert the acids to their more volatile methyl esters. Because of the very small quantities involved, a powerful methylating agent called diazomethane is used:
RCOOH + CH2N2 diazomethane RCOOCH3 + N2
It is this mixture of methyl esters which is injected onto the column of the gas-liquid chromatograph. (You can nd out how a gas-liquid chromatograph works in Chemical Ideas 7.6.) Figure 1 shows the chromatogram obtained when a small sample of paint from Cimas altarpiece was treated in this way.
CD4.3
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100
Recorder response
Stearate
50
Palmitate
v
0 0 10 20 30 Time/min
Figure 1 Gas-liquid chromatogram of a sample made from paint from Cimas The Incredulity of S. Thomas.
What you do
Use the information on Information Sheet 2 (Gas-liquid chromatograms) to help you analyse the chromatogram in Figure 1. (The same conditions were used to obtain all the chromatograms.) First, identify the peaks due to the palmitate and stearate esters, and then work out the palmitate : stearate ratio in the sample. The peaks are very sharp so you can assume that the peak height is proportional to the amount of compound present. (Strictly speaking, you should measure the area under each peak.) Now work out the palmitate : stearate ratios for each of the reference samples, from paint made up in known oils. Compare these ratios with the value you obtained for the sample from Cimas paints.
QUESTIONS
a Which oil do you think was used to bind Cimas paints in 1504? Explain your answer. b Explain why the palmitate:stearate ratio is the one chosen for analysis. c Suggest why methyl esters are more volatile than the free carboxylic acids. d Give another method for converting palmitic acid to its methyl ester. Write a balanced equation for the reaction.
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0 0 10 20 30 Time / min (c) Sample made from paint bound with poppy seed oil
0 0 10 20 30 Time / min (d) Sample made from paint bound with linseed oil
0 0 10 20 30 Time / min (e) Sample made from paint bound with walnut oil
0 0 10 20 30 Time / min (f) Sample made from paint bound with egg tempera
Figure 2 Gas-liquid chromatograms of: (a) pure methyl palmitate; (b) pure methyl stearate; (cf) samples made from paint bound with poppy seed oil, linseed oil, walnut oil and egg tempera, respectively.
CD4.4
M v ?
Identifying a pigment
In this activity you can combine evidence from scientic and historical sources to identify the yelloworange pigment Cima used for the robe of S. Peter in his painting The Incredulity of S. Thomas.
What you do
Your task is to identify the yellow-orange pigment used for the robe of S. Peter. You will use the kind of scientic results and historical information that scientists and art historians at the National Gallery in London had at their disposal when analysing the painting. (S. Peter is the Apostle with the white hair and a beard, standing in the foreground next to Christ on the right-hand side of the picture.)
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Historical evidence
Information Sheet 3 (Background information on some yellow pigments) gives some historical information about the use of yellow and orange pigments. These extracts are taken from a standard textbook on painting materials. Such information is always used in addition to the results of any scientic analysis.
Summary
Write a short report summarising your ndings. Which pigment do you think Cima used? Give reasons for your decision.
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IDENTIFYING A PIGMENT
CD4.4
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250
245
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Si Si Ta Au
Zn
240
237
W Fe
Te Te Fe
235
230
As Be
Ba
Ba
228
225
223
As Cd Ni Co
Cd
Cu
Cu
Figure 1 A small section (from 223 nm to 252 nm) of the reference spectrum for use with LMA emission spectra of pigments. (The upper spectrum is a spark spectrum; the lower one an arc spectrum.)
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CD4.5
M v ?
Finding a perfect match
In this activity you will use reectance spectra to nd modern replacements for two blue pigments used in Cimas The Incredulity of S. Thomas.
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What you do
Imagine you are a scientist working in the Scientic Department at the National Gallery. You have just received a memo from the Conservation Department asking you to recommend the best blue pigments for use in the restoration of two areas of Cimas altarpiece:
G G
the blue-green areas of the ceiling the dark blue mantle of the Apostle on the extreme left of the group.
You have already supplied the Department with the identity of the original pigments used by Cima (look back to Activity CD4.1). Now you need to nd the best modern substitutes. The reectance spectra of some modern blue pigments are shown on Information Sheet 5 (Some modern blue pigments). Use these spectra to help you construct a reply to the Conservation Department. Outline the reasons for your recommendations.
CD4.5
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Reflectance / %
100 (c) Manganese Blue 90 80 70 60 50 40 30 20 10 0 400 450 500 550 600 650 700 750 Wavelength / nm
(d) Monastral Blue 100 90 80 70 60 50 40 30 20 10 0 400 450 500 550 600 650 700 750 Wavelength / nm
Reflectance / %
100 (e) Prussian Blue 90 80 70 60 50 40 30 20 10 0 400 450 500 550 600 650 700 750 Wavelength / nm
(f) French Ultramarine 100 90 80 70 60 50 40 30 20 10 0 400 450 500 550 600 650 700 750 Wavelength / nm
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CD5
M v ? Requirements
G G G G G G G G G G G G G G G
Comparing hydrocarbons
The purpose of this activity is to compare the behaviour of three liquid hydrocarbons cyclohexane (an alkane), cyclohexene (an alkene) and methylbenzene (an arene) with a series of chemical reagents.
test-tubes and rack bungs boiling tube teat pipettes cyclohexane (2 cm3) cyclohexene (2 cm3) methylbenzene (2 cm ) bromine in cyclohexane solution (3 cm3) glass rod concentrated ammonia solution (2 cm ) bromine water (6 cm3) concentrated nitric acid (2 cm3) concentrated sulphuric acid (fresh) (7 cm3) dilute sulphuric acid, 1.0 mol dm3 (20 cm3) dilute potassium manganate(VII) solution, 0.02 mol dm3 (3 cm3) potassium manganate(VII) crystals (0.5 g) solid sodium carbonate (0.05 g) sodium disulphite(IV) (metabisulphite) solution, 1.0 mol dm3 (5 cm3) 250 cm3 beaker 10 cm3 measuring cylinder 100 cm3 conical ask 0 110 C thermometer source of hot water crushed ice (50 g) methyl benzoate (2.5 cm3)
methylbenzene
HARMFUL HIGHLY FLAMMABLE
bromine
TOXIC CORROSIVE
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bromine water
IRRITANT
G G G
cyclohexane
HIGHLY FLAMMABLE
G G G G G G G
cyclohexene
HIGHLY FLAMMABLE
CARE Cyclohexane, cyclohexene and methylbenzene are highly ammable liquids. Keep bottles stoppered when not in use and well away from naked ames. Avoid skin contact and do not breathe the vapours. CARE Bromine is corrosive, causes severe burns and gives off a toxic vapour. Handle the bromine solution with care. Measure out in a fume cupboard using a marked pipette. CARE Solid potassium manganate(VII) is a powerful oxidising agent. It causes staining of skin and clothes. Wear protective gloves if necessary.
methyl benzoate
HARMFUL
sodium carbonate
IRRITANT
HARMFUL
COMPARING HYDROCARBONS
CD5
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Introduction
Arenes have characteristic properties which are very different from those shown by alkanes and alkenes. In this activity you will compare the reactions of cyclohexane (an alkane), cyclohexene (an alkene) and methylbenzene (an arene). (Benzene itself is toxic and has carcinogenic properties, so it cannot be used.)
CH3
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cyclohexane cyclohexene methylbenzene
What you do
For each of the test-tube reactions, use just a few drops of each of the hydrocarbons. In each case, think rst what you expect to happen and why. Then compare this with what you actually observe. Describe what happens in each case. Write equations where appropriate, and name the products of any reactions. It may be best to draw up tables for your results. You may wish to consult Chemical Ideas Chapter 12 to help you interpret your observations. Classify the type of reaction occurring, choosing words from the list below: substitution addition oxidation nucleophilic electrophilic radical
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CD5
COMPARING HYDROCARBONS
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HNO3
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NO2
is trinitrotoluene (TNT; toluene is the old name for methylbenzene). A safer compound to nitrate is methyl benzoate:
O C OCH3 O C c. H2SO4 OCH3
+ methyl benzoate
HNO3
H2O
5 Measure 2.5 cm3 of methyl benzoate (CARE Harmful) into a small conical ask and then add 5 cm3 of concentrated sulphuric acid (CARE Corrosive). When the liquid has dissolved in the acid, cool the mixture in ice. Prepare the nitrating mixture by carefully adding 2 cm3 of concentrated sulphuric acid (CARE Corrosive) to 2 cm3 of concentrated nitric acid (CARE Corrosive; oxidising agent). Cool the mixture in ice during the addition. Now add the nitrating mixture drop by drop from a teat pipette to the solution of methyl benzoate while cooling. (Do not allow the nitrating mixture to get into the rubber teat.) Stir the mixture with a thermometer and keep the temperature below 10 C. When the addition is complete, allow the mixture to stand at room temperature for another 15 minutes. After this time, pour the reaction mixture onto about 25 g of crushed ice and stir until the ice has melted.
QUESTIONS
a You used cyclohexane and cyclohexene as examples of a typical alkane and alkene respectively. Why werent simpler hydrocarbons such as ethane and ethene used? b In the reactions with bromine water and acidied potassium manganate(VII) solution, why was it necessary to shake the tubes thoroughly? c How would you expect benzene to react with an acidied solution of potassium manganate(VII)? Explain your answer. d i In the nitration of methyl benzoate, what precautions were taken to help prevent further nitration to a dinitro-derivative? ii Give the names and structural formulae of two nitro-compounds that are likely to contaminate the crystals of methyl 3-nitrobenzoate that you made. iii Suggest how you could purify your crystals and conrm that they are crystals of methyl 3-nitrobenzoate.
CD6
M v ?
Making azo dyes
In this activity you can investigate the range of colours which can be obtained by making azo dyes with different coupling agents.
Requirements
G G G G G G G G G G G G G G G G
boiling tubes test-tube and rack bungs protective gloves 10 cm3 measuring cylinder phenylamine (1.5 cm3) ethyl 4-aminobenzoate (benzocaine) (2 g) dilute hydrochloric acid, 1.0 mol dm3 (10 cm3) 250 cm3 beaker ice-salt mixture 0 110 C thermometer sodium nitrite (nitrate(III)) (2 g) glass rod starch-iodide paper dilute sodium hydroxide solution, 2.0 mol dm3 (6 cm3) small quantities of each of the following coupling agents: phenol methylphenol (any isomer) naphthalen-2-ol
sodium hydroxide solution
CORROSIVE
methylphenol
TOXIC CORROSIVE
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HARMFUL
phenol
TOXIC CORROSIVE
phenylamine
TOXIC
sodium nitrite
TOXIC OXIDISING
CARE Work in a fume cupboard when measuring out phenylamine. CARE Phenol and methylphenols are toxic and corrosive. Wipe up any crystals that get spilt. CARE Only small amounts of the azo dyes should be made. They should be disposed of promptly by ushing down the sink with lots of water and detergent.
CARE Wear gloves throughout this experiment. Avoid all skin contact with the reagents and the azo dyes produced.
Azo dyes
Azo dyes are produced in a diazo coupling reaction. For example,
+ N
Cl + HR'Z
coupling agent
R'Z + HCl
diazonium salt
azo dye
where R and R' are arene groups and Z is a functional group such as OH or NH2. When Z is an OH group, the coupling agent is prepared in alkaline solution. Many modern azo dyes are formed directly on the bres. First the cotton material is dipped into a solution of the coupling agent. The material is almost colourless at this stage. Next the cotton is treated with an ice-cold solution of a diazonium salt made from an arylamine. The insoluble dye is trapped in the bres.
What you do
Your group will work as a development team. You will prepare a series of azo dyes with different R'Z groups to investigate the range of colours that can be produced.
Salters Advanced Chemistry 2000 see Copyright restrictions
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CD6
M v ?
This means that you will use the same diazonium salt in each reaction and vary the coupling agent. You can select your amine RNH2 to make the diazonium salt, and the coupling agents (HR'Z) from those in the table below.
Amines (RNH2) Coupling agents (HR'Z)
NH2 phenylamine
OH phenol
*
COOC2H5 ethyl 4-aminobenzoate (benzocaine) NH2 OH 3-methylphenol
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* *
CH3
OH naphthalen-2-ol
QUESTIONS
a Write out equations for the formation of the dyes you have made. b Why is it essential to keep the diazonium salt solution below 5 C? c When phenol dissolves in sodium hydroxide solution, sodium phenoxide is formed. Write down its formula. Suggest why this is used in the coupling reaction rather than phenol itself.
CD7.1
M v ? Requirements
G
In this activity you will dye cotton cloth with a brereactive dye and with a direct dye, and then compare the fastness to washing.
samples of untreated white cotton cloth (about 5 g each) (2) protective gloves 400 cm3 beakers (2) Durazol Red 2B solution (a direct dye) (250 cm3)* Procion Red MX-5B solution (a reactive dye) (200 cm3)* sodium chloride (13 g) sodium carbonate (0.5 g) stirring rods tongs soap powder strip chromatography paper beaker to use as chromatography tank Bunsen burner, tripod and gauze CARE Avoid all skin contact with the dyes. Gloves must be worn. CARE Eye protection must be worn.
WEAR EYE PROTECTION
G G G G G G G G G G G G
Durazol Red 2B
IRRITANT
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sodium carbonate
IRRITANT
What you do
You are given instructions for dyeing cotton cloth with two different dyes, a bre-reactive dye and a direct dye. Dye samples of white cotton with the two dyes and then compare their fastness to washing. While you are dyeing the cotton samples, you can investigate the hydrolysis of the reactive dye in the dyebath using chromatography.
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Fastness testing
5 Devise a procedure for testing your dyed samples for fastness to washing. Remember that when clothes are washed, it is important that the colours do not fade, and also that colours are not transferred from one garment to another. The instructions on a packet of washing powder will give you an idea of the conditions suitable for testing fastness to washing.
Results
6 Write a short report summarising your ndings. It should include the following points:
G G G
How did the appearance of the two samples of dyed cloth compare? What are the advantages and disadvantages of each dyeing process? How did fastness to washing of the two dyes compare?
Be prepared to make an oral presentation of your results. Compare your ndings with those of other groups.
CD7.2
M v ? Requirements
G G G G G G
This activity demonstrates how a knowledge of the structure of dyes and bres enables chemists to dye successfully a wide variety of fabrics.
protective gloves 400 cm3 beaker stirring rod tongs dye mixture (10 cm3)* small samples (about 20 cm2) of white cotton, polyester and nylon apparatus for paper chromatography chromatography solvent (95% butanol:5% water)
dye mixture
IRRITANT
butanol
HARMFUL
FLAMMABLE
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CARE Use the butanol solvent in a fume cupboard or well-ventilated area. Keep away from naked ames. * See instructions for preparation in the Teachers and Technicians Guide CARE Avoid skin contact with the dye solution. Gloves must be worn.
What you do
You are provided with a mixture of three dyes. First, you will use the dye mixture to dye three types of fabric as described below. Different bres have different structures and bind dyes to different extents. You can then investigate the composition of the dye mixture using chromatography. Finally, you will use your observations, together with your knowledge of bres and how dyes bind to bres, to assign structures to each of the dyes in the mixture.
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NH
4 Use your knowledge of how dyes bind to bres, together with your observations, to assign structures X, Y and Z to each of the three dyes in the mixture. Explain your reasoning. Remember that paper is a cellulose polymer made up of glucose units, like cotton. You made need to look back to Designer Polymers for the structure of nylon and polyester bres. Polyesters are relatively non-polar. Water does not penetrate the bres well and the fabric is best dyed with small, relatively nonpolar molecules. The main attractive forces between the dye molecules and the polyester bres are instantaneous dipoleinduced dipole forces. Nylon too has non-polar sections in its bres, but also has NH3+ groups at the end of the chains in acid solution.
CD9
Check your notes on Colour by Design
This activity helps you get your notes in order at the end of this unit.
Use this list as the basis of a summary of the unit by collecting together the related points and arranging them in groups. Check that your notes cover the points and are organised in appropriate ways. Most of the points are covered in the Chemical Ideas, with supporting information in the Storyline or Activities. However, if the main source of information is the Storyline or an Activity, this is indicated.
G
An outline of the process of oxidative cross-linking by which unsaturated oils harden; the relationship of this process to their use as media in oil-based paints. What arenes and arene derivatives (aromatic compounds) are. The structure of benzene. How the characteristic properties of aromatic compounds arise from the delocalisation of electrons. The following electrophilic substitution reactions of arenes: halogenation of the ring, nitration, sulphonation, Friedel-Crafts alkylation and Friedel-Crafts acylation. The formation of azo dyes by coupling reactions involving diazonium compounds. The structure of a dye molecule in terms of its various components: chromophore, groups which modify the chromophore, groups which made the dye more soluble in water and groups which attach the dye to the bre (Storyline CD5CD7). Ways in which dyes attach themselves to fabrics: weak intermolecular forces, hydrogen bonds, ionic attractions and covalent bonding (Storyline CD7; Activity CD7.2). The relationship between the colour of a dye and the presence of a chromophore, and groups that modify the chromophore, in the dye molecule. The relationship between colour in materials and transitions between electronic energy levels.
The absorption of ultraviolet light and visible light in terms of transitions between electronic energy levels. The use of ultraviolet (u.v.) and visible spectroscopy to help identify unsaturated organic molecules. Colour changes associated with the following types of chemical changes: acidbase (indicators), ligand exchange, redox, precipitation and polymorphism (different crystal structures). The relationship between the properties of pigments (colour shade, colour intensity, fastness) to relevant properties (Storyline CD2 and CD3). The general principles of gas-liquid chromatography (g.l.c.). The techniques used to identify the materials used in a painting, including the use of g.l.c., atomic emission spectroscopy, and visible spectroscopy (reection and transmission) (Storyline CD3 and CD4; Activities CD4.14.5). The nature of fats and oils as mixed esters of propane1,2,3-triol with varying degrees of unsaturation.
CD
Harcourt Education Ltd 2004 Salters Advanced Chemistry These pages have been downloaded from www.heinemann.co.uk/science
O1.1
M v ? Requirements
G
What is the relationship between a solvent and the substances that dissolve in it?
In this activity you investigate the solubilities of four substances in three different solvents. You can draw some general conclusions about solubility from the data you collect, and explain your observations in terms of chemical ideas you have learned during the course. You may need to revise ideas about chemical bonding (Chemical Ideas 3.1 and 5.1), intermolecular forces (Chemical Ideas 5.3, 5.4 and 5.6), and the enthalpy and entropy changes during solution (Chemical Ideas 4.5).
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the following solids, in powdered form in stoppered specimen tubes (about 2 g of each): anhydrous sodium chloride anhydrous calcium chloride glucose (or sucrose) grated candle wax (2 g) test-tubes (3) and rack hexane (12 cm ) propanone (12 cm3) distilled water (12 cm3)
3
calcium chloride
IRRITANT
G G G G G
hexane
HARMFUL HIGHLY FLAMMABLE
propanone
HIGHLY FLAMMABLE
CARE Hexane and propanone are highly ammable liquids. Keep bottles stoppered when not in use and well away from naked ames. Avoid skin contact and do not breathe the vapours. Return residues to the correct residues bottle. Do not pour them down the sink.
What you do
1 Test the solubilities of the solids in the different solvents using the method described in Activity M1.3 of From Minerals to Elements. (CARE Hexane and propanone are highly ammable. Extinguish all ames before using these liquids. Avoid skin contact and do not breathe vapours. Return residues to the correct residue bottle. Do not pour them down the sink.) It may be best for each pair of students to investigate the solubility of one solid in the range of solvents, and then join with other pairs to produce a full table of results. 2 Use the information contained in your results table to work through the points which follow.
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O1.1
WHAT IS THE RELATIONSHIP BETWEEN A SOLVENT AND THE SUBSTANCES THAT DISSOLVE IN IT?
M v ?
c Fill in Table 1, and use it to explain why solutions form or do not form in each case.
Solid Principal interactions in solid Solvent Principal interactions in solvent Principal interactions in solution Enthalpy change which would accompany the formation of a solution as a result of these changes in interactions Is the solid likely to dissolve?
hexane
v
NaCl propanone
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water
hexane
CaCl2
propanone
water
hexane
glucose
propanone
water
hexane
wax
propanone
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O1.2
M v ? Requirements
G
This activity looks in more detail at the changes which occur when ionic solids dissolve. It provides evidence to support and test the explanations you put forward in Table 1 in Activity O1.1. It may be best to share the work so that each student or pair of students chooses one solid to study in both parts of the activity.
the following solids, in powdered form in stoppered specimen tubes: anhydrous sodium chloride (12 g) anhydrous calcium chloride (23 g) anhydrous iron(III) chloride (33 g) distilled water (300 cm3) polystyrene cups (or insulated beakers) 0 110 C thermometer 50 cm3 measuring cylinder burettes (2) weighing bottles with stoppers rubber bungs to t burettes small funnel access to balance
calcium chloride
IRRITANT
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G G G G G G G G G
iron(III) chloride
HARMFUL IRRITANT
What you do
Note Calcium chloride and iron(III) chloride each take in water from the air. It is important to use fresh anhydrous samples and to work quickly. and invert it several times to mix the contents. Continue adding the solid until it has all dissolved. 9 Record the volume of the solution. 10 Repeat the procedure in steps 79 for the other two solids. 11 Calculate the volume of the solid you added in each case, using its mass and the density given below.
Solid sodium chloride calcium chloride iron(III) chloride Density/g cm3 2.2 2.5 2.8
12 Use the relationship volume change = (volume of solution) (volume of solid + volume of water) to work out in each case the change in volume when the solid dissolves. 13 Draw up a table to show your results.
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O1.3
M v ?
What factors affect the enthalpy change of formation of an ionic compound?
In this activity you will see how the formation of an ionic compound from its constituent elements can be imagined to take place in several stages, each with its own enthalpy change. The values of these enthalpy changes determine the overall value for the enthalpy change of formation of the compound.
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You can imagine the stages involved as: (1) turning the elements Na(s) and Cl2(g) into gaseous atoms (2) turning the atoms into gaseous ions (3) bringing the gaseous ions together to form NaCl(s) The following equations describe the stages: (a) Na(s) Na(g) (b) Na(g) Na+(g) + e (c) Cl2(g) Cl(g) (d) Cl(g) + e Cl(g) (e) Na+(g) + Cl(g) NaCl(s) 1 Draw up a table and match the equations (a) to (e) with the stages (1) to (3) above. 2 Each of the equations (a) to (e) denes a standard enthalpy change. Match the enthalpy changes (i) to (v) below with the equations (a) to (e). (i) lattice enthalpy of sodium chloride DHLE! (NaCl(s)) (ii) rst electron afnity of chlorine DHEA! (Cl(g)) (iii) standard enthalpy change of atomisation of sodium DHat! (Na(s)) (iv) standard enthalpy change of atomisation of chlorine DHat! ( Cl2(g)) (v) rst ionisation enthalpy of sodium DHi! (1) (Na(g)) 3 Draw out the enthalpy cycle showing how you get from the elements in their standard states, Na(s) and Cl2(g), to the compound, NaCl(s), using the stages dened by the equations (a) to (e). Label each stage with the appropriate symbol taken from (i) to (v). This type of enthalpy cycle for a simple ionic compound is known as a Born-Haber cycle. 4 Apply Hesss Law to the cycle and obtain an equation that shows how DHf! (NaCl(s)) depends on the other enthalpy changes. 5 Set up a spreadsheet that will calculate DHf! for any Group 1 halide. The only data input required will be the values for the enthalpy changes (i) to (v).
QUESTION
How would you need to adapt your enthalpy cycle and spreadsheet to cope with the halides of Group 2 elements?
O3.1
M v ?
The enthalpy change of vaporisation of water
The rst part of this activity is optional. It introduces you to some practical techniques which you have not previously encountered in the course, and allows you to measure a value for the enthalpy change of vaporisation (DHvap) of water. You can then compare this with values for some other liquids and use the ideas you have learned in Chemical Ideas 4.4 to interpret the results.
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12 V d.c. supply joulemeter electrical heating coil (see Figure 1) apparatus for boiling and condensing water (see Figure 1) small beakers or specimen tubes (2) access to balance CARE Eye protection must be worn.
WEAR EYE PROTECTION
G G
What you do
1 Assemble the apparatus shown in Figure 1, for boiling and condensing water. Use water close to boiling in the side-arm boiling tube. 2 Position one small beaker as shown in the diagram; have a second weighed beaker ready.
6 12 V d.c. supply water out Detail of insulation using three pieces of expanded polystyrene sheet
joulemeter
water expanded polystyrene insulation (or cotton wool lagging in a tall beaker)
thick copper wire sealed into glass tube water in solder beaker coil of about 3 cm 28 s.w.g. nichrome wire
3 Switch on the power. Allow the water to warm up and then boil for a few minutes until it drips at a slow steady rate from the condenser into the unweighed small beaker. Then change over the beakers and simultaneously reset the joulemeter. 4 Allow about 3 cm3 of water to distil over, then exchange the beakers once more, reading the joulemeter at the same instant. 5 Record the mass of beaker plus water and calculate the mass of water collected.
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O3.1
M v ?
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Table 1 Enthalpy changes of vaporisation (DHvap ) and boiling points (Tb ) for some compounds
i Water has a high DHvap value compared with the other substances in Table 1. Use your knowledge of intermolecular forces to explain this observation. ii Ethanol also has an unusually high DHvap value of +38.5 kJ mol1. Explain how the changes which occur during boiling are similar for ethanol and water. e All the DHvap values are highly endothermic. Nevertheless, the compounds are volatile and can easily be boiled. What other change which favours the process accompanies vaporisation? f i At the boiling point, a liquid and its vapour are in equilibrium. What can you say about the total entropy change for a system which is at equilibrium? ii What is the relationship between the entropy change on vaporisation of a liquid (DSvap) and the entropy change in the surroundings (DSsurr) at the boiling point? iii DSsurr during an endothermic change can be calculated from the thermal energy supplied and the temperature at which the change occurs. For boiling, the relationship is: DSsurr = DHvap Tb
Use this relationship to calculate DSsurr values for the ve compounds in Table 1. g i Use your answers for f ii and f iii to calculate DSvap values for the ve compounds in Table 1. ii What general observation can you make about the values of DSvap for the four organic compounds in Table 1? iii Explain this observation in terms of the molecular-kinetic model which chemists use to describe the structures of liquids and vapours. iv How does the value of DSvap for water compare with the value you found for the four organic liquids? Explain why there is a difference. h i Convert the values for DHvap in Table 1 into kJ kg1. ii These gures should show you that, for the transport of a given mass of vapour through the atmosphere, water carries much more energy with it than the other substances. Explain why this is important for the climate of the Earth.
Salters Advanced Chemistry 2000 see Copyright restrictions
03.2
M v ?
What crystals form when a solution is cooled?
Freezing and crystallisation cause density changes in the oceans and are among the factors that drive ocean currents. This activity uses copper(II) sulphate, which is easy to distinguish from ice crystals, to investigate the effect of cooling solutions containing different concentrations of an ionic compound. The questions, which help you to interpret your observations, also provide an opportunity for you to reinforce what you have learned in Chemical Ideas 4.4 about entropy changes and their relationship to enthalpy changes and temperature.
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Requirements
G G G G G G
saturated copper(II) sulphate solution (4 cm3) 250 cm3 beaker test-tubes (2) and rack crushed ice table salt 10 C110 C thermometer CARE Eye protection must be worn.
WEAR EYE PROTECTION
copper(II) sulphate
HARMFUL
What you do
1 Make an icesalt freezing bath by mixing crushed ice and table salt in a beaker. 2 Place about 3 cm3 of saturated copper(II) sulphate solution in one test-tube. Add 1 cm3 of saturated copper(II) sulphate solution to 2 cm3 of water in the other test-tube. 3 Place the test-tubes into the freezing bath and allow the solutions to cool. Make a note of the temperature at which crystals form in each solution, the appearance of the crystals, and whether they oat or sink. 4 Before you throw away the contents of the freezing bath, make a note of its temperature and appearance. g The enthalpy change (DH) when pure water freezes to produce ice is the same as the enthalpy change when copper(II) sulphate solution freezes to produce ice. However, ice crystals do not form from copper(II) sulphate solution until the temperature has fallen below 273 K. This is because the entropy changes (DSsys) are different for the two processes. i For which process will the entropy change (DSsys) be greater: pure water ice at its freezing point; or copper(II) sulphate solution ice at its freezing point? Explain your answer. ii At the freezing point, ice is in equilibrium with the water or solution which is freezing. Therefore DStotal must be zero. You have made decisions about DSsys in g i. How must DSsurr compare for the two processes: pure water ice at its freezing point; and copper(II) sulphate solution ice at its freezing point? iii The entropy change in the surroundings (DSsurr) is related to DH and the temperature at which freezing occurs. Use this relationship to explain why ice forms from copper(II) sulphate solution at a temperature below 273 K. The same idea explains why it is possible to have water, ice and salt present together at temperatures well below 273 K. So you can melt ice on roads in winter, and make it run off, by adding salt. You can also make freezing baths like the one you used in this activity.
What it means
a When a solution of copper(II) sulphate cools, what determines whether it is water or copper(II) sulphate that crystallises out rst? b Describe how you think the density of a copper(II) sulphate solution changes when: i copper(II) sulphate crystals form ii ice crystals form. c The salt concentration in sea water is typically about 0.5 mol dm3. How might ocean currents be affected if the sea were saturated with salt? d The enthalpy change when water freezes to form ice is 6010 J mol1. What will be the gain in entropy of the surroundings when water freezes at 273 K? e At 273 K the entropy of ice is lower than the entropy of water by 22.0 J K1 mol1. Explain why the entropy of ice is lower. f Comment on the total entropy change of the system and surroundings that accompanies the freezing of water at 273 K.
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O4.1
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Finding out more about weak acids
You can compare some of the properties of weak and strong acids by using ethanoic acid as a typical weak acid and hydrochloric acid as a typical strong acid. The measurements you make can be interpreted in terms of the theory in Chemical Ideas 8.2. By the end of the activity you should have seen that pH is less sensitive to changes in weak acid concentration than strong acid concentration. You should then appreciate one of the reasons why the thousandfold change in atmospheric carbon dioxide concentration that has occurred during the Earths history has not caused a major change in the pH of the oceans.
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300
Requirements
G G G
pH meter 100 cm3 beaker solutions of hydrochloric acid of the following concentrations (about 50 cm3 of each solution): 0.1 mol dm3 0.03 mol dm3 0.01 mol dm3 0.003 mol dm3 0.001 mol dm3 solutions of ethanoic acid of the same concentrations as above (about 50 cm3 of each solution) CARE Eye protection must be worn.
WEAR EYE PROTECTION
What you do
1 Pour sufcient 0.001 mol dm3 hydrochloric acid into a beaker to allow you to measure the pH. Record the pH and return the solution to the stock bottle. 2 Repeat the measurements with the other hydrochloric acid solutions. If you work from the lowest to the highest concentration you can use the same beaker, and there is no need to rinse it out or clean the glass electrode between readings. 3 Now repeat steps 1 and 2 using the ethanoic acid solutions. Before you start, make sure the glass electrode is rinsed with distilled water to remove all traces of hydrochloric acid. 4 Present all your results in the form of a table. iii How does your answer to b i compare with the behaviour you would expect of a strong acid? d i Explain what you understand by the term weak acid. ii Write an expression for the acidity constant (Ka) of ethanoic acid. (You can abbreviate the formula to HA for convenience.) iii It is reasonably accurate to regard [H+(aq)] and [A(aq)] as being equal in the solutions of ethanoic acid used in this activity. Explain why we can do this. iv Another reasonably accurate assumption is to regard [HA(aq)] as equal to the amount of acid used to make 1 dm3 of each solution. For example, we can say [HA] = 0.1 mol dm3 in ethanoic acid solution of concentration 0.1 mol dm3. Explain why this is an accurate assumption. v Rewrite Ka for ethanoic acid solutions of concentrations 0.1 mol dm3 and 0.001 mol dm3, using the assumptions discussed in d iii and iv. You should be left with expressions for Ka which involve only [H+(aq)] and numbers. vi How does the value for [H+(aq)] change when a 0.1 mol dm3 solution of ethanoic acid is diluted by a factor of 100, to 0.001 mol dm3? vii You have just worked out how you would expect [H+(aq)] and [HA(aq)] to be related for ethanoic acid an example of a weak acid. How does the observation you made in b ii compare with this expected behaviour?
Discussion of results
a i Explain what you understand by the term pH. ii Explain how changes in pH and changes in [H+(aq)] are related. b From your results, what appears to be the pH change associated with: i a 10-fold change in the concentration of hydrochloric acid? ii a 100-fold change in the concentration of ethanoic acid? c i Explain what you understand by the term strong acid. How is this different from a concentrated acid? ii Explain why you would expect a 10-fold change in the concentration of a strong acid to lead to a 10-fold change in [H+(aq)].
O4.2
M v ? Requirements
G
In this activity you make pH measurements of some buffer solutions and see what happens when you add water, acid and alkali to them. You then compare their behaviour to a solution of similar pH that is not a buffer. Interpreting the results should reinforce the ideas you learned about in Chemical Ideas 8.3.
T
G G G G G G G
0.5 mol dm3 solutions of the following: ethanoic acid (75 cm3) potassium (or sodium) ethanoate (75 cm3) methanoic acid (25 cm3) potassium (or sodium) methanoate (25 cm3) ammonium chloride (25 cm3) ammonia solution (25 cm3) hydrochloric acid (15 cm3) potassium (or sodium) hydroxide (15 cm3) 25 cm3 measuring cylinder 10 cm3 measuring cylinder distilled-water wash bottle 100 cm3 beakers (7) pH meter 1 104 mol dm3 nitric(V) acid (50 cm3 ) glass rod
What you do
As you work through this activity, ll in your results in Table 1. 1 Make up three buffer solutions by mixing 25 cm3 portions of the 0.5 mol dm3 solutions in the pairs listed below. (Use 100 cm3 beakers to hold the buffer solutions and stir each one with a glass rod.) Buffer A Buffer B Buffer C ethanoic acid + potassium ethanoate methanoic acid + potassium methanoate ammonium chloride + ammonia solution
Measure the pH of each buffer using a pH meter. Rinse the glass electrode with distilled water between measurements. 2 Remove 5 cm3 of Buffer A and add it to 45 cm3 of distilled water in a fourth beaker. This is Buffer D. Record its pH. a How are the pH values of Buffer A and Buffer D related? b Does the pH of a buffer depend on the total amounts of acid and salt present or on the ratio of their amounts? Explain your answer. 3 Make up two more buffers using 0.5 mol dm3 ethanoic acid and 0.5 mol dm3 potassium ethanoate as follows: Buffer E Buffer F 10 cm3 ethanoic acid + 40 cm3 potassium ethanoate 40 cm3 ethanoic acid + 10 cm3 potassium ethanoate
Record the pH values. c Use Le Chateliers principle to explain the way the pH values vary among the Buffers A, E and F.
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M v ?
The measurements you have made show that buffers can have pH values spread over a wide range. There are two steps to designing a buffer with a particular pH:
G G
coarse tuning to select the pH region ne tuning to adjust the pH to the actual value required.
the chemical system chosen the proportions of acid and salt used.
d Which of these two factors would you make use of in i the coarse tuning ii the ne tuning? 4 Place 50 cm3 of 1 104 mol dm3 nitric(V) acid into another beaker. This is Solution G; it has a similar pH to the ethanoic acid + potassium ethanoate mixtures, but it is not a buffer solution. Add 3 cm3 of 0.5 mol dm3 hydrochloric acid to solutions A, D and G. Record the pH changes. e Explain the different effects of adding the same amount of hydrochloric acid to these three solutions. 5 Add 3 cm3 of 0.5 mol dm3 hydrochloric acid or potassium hydroxide to buffers B, C, E and F. You are free to choose which solutions to add to which buffers. Record the pH changes. f Explain the effects of adding acid or alkali to the buffers.
System pH DpH + 3 cm3 HCl DpH + 3 cm3 KOH Was the solution an effective buffer?
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Solution
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O5
Check your notes on The Oceans
This activity helps you get your notes in order at the end of this unit.
Use this list as the basis of a summary of the unit by collecting together the related points and arranging them in groups. Check that your notes cover the points and are organised in appropriate ways. Most of the points are covered in the Chemical Ideas, with supporting information in the Storyline or Activities. However, if the main source of information is the Storyline or an Activity, this is indicated.
G
Calculation of entropy changes for a reaction given entropies of reactants and products. Comparison of the following properties of water with those of other liquids, and other hydrides of Group 6 elements, and the relationship of these properties to molecular structure: specic heating capacity, enthalpy change of vaporisation, and density changes on melting. The inuence of oceans on climate in terms of the characteristic properties of water (Storyline O3). The meaning and use of the following terms: strong acid, strong base, pH. The ionic product of water, Kw. Calculation of the pH of solutions of strong acids and strong bases. The meaning and use of the following terms: weak acid, acidity constant Ka, pKa. Calculation of the pH of solutions of weak acids. How buffer solutions work, and their applications. Calculation of the pH of a buffer solution. The meaning and use of the term solubility product for simple ionic compounds of formula Xn+Yn. The use of solubility products to perform calculations concerning dissolving and precipitation processes. Acidbase and precipitation processes in the oceans in terms of Ka and Ksp (Storyline O4). The global inuence of the processes occurring when carbon dioxide dissolves in water (Storyline O4).
Factors that determine the relative solubility of a solute in aqueous and non-aqueous solvents. The meaning and use of the terms: enthalpy change of solution, lattice enthalpy, enthalpy of solvation (hydration). The solution of an ionic solid in terms of an enthalpy cycle involving enthalpy change of solution, lattice enthalpy and enthalpies of solvation (hydration) of ions. The effect of atomic number, charge and hydration on the radii of anions and cations. The relationship between ionic size and properties. The Born-Haber cycle for simple ionic compounds. Entropy as a measure of the number of ways that molecules and their associated energy quanta can be arranged. The process of dissolving in terms of energy and entropy factors. The tendency of a process to occur in terms of entropy changes in the system ( Ssys ) and surroundings ( Ssurr ), and the requirement that the total entropy change ( Stotal ) should be positive. Calculations of entropy changes using the expression: Stotal = Ssys + Ssurr.
Harcourt Education Ltd 2004 Salters Advanced Chemistry These pages have been downloaded from www.heinemann.co.uk/science
MD1.1
M v ?
Aldehydes and ketones
This activity considers the formation and reactions of aldehydes and ketones.
Requirements
G G G G G G G G G G G G G G
potassium dichromate(VI) solution, 0.1 mol dm3 (2 cm3) protective gloves dilute sulphuric acid, 2 mol dm (10 cm ) propan-1-ol (3 drops) propan-2-ol (3 drops) 2-methylpropan-2-ol (3 drops) test-tubes and rack propanal (3 drops) propanone (3 drops) angled glass tubing and rubber bung Bunsen burner, tripod and gauze 400 cm3 beaker (for hot water bath) Fehlings solution 1 (2 cm3) Fehlings solution 2 (2 cm3)
propanone
HIGHLY FLAMMABLE
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propanal
IRRITANT
HIGHLY FLAMMABLE
CARE Propanal, propanone and the alcohols are highly ammable liquids. Keep bottles stoppered when not in use and well away from naked ames. Avoid skin contact and do not breathe the vapours. CARE Dichromates(VI) irritate the skin and are suspected carcinogens. Avoid all skin contact. Any spillage should be washed off at once. Wear protective gloves.
sulphuric acid
CORROSIVE
What you do
1 Place about 1 cm depth of 0.1 mol dm3 potassium dichromate(VI) solution (CARE Toxic. Avoid skin contact. Wear gloves) in a test-tube. Add 2 mol dm3 sulphuric acid (CARE Corrosive) until the tube is half full. Then divide this mixture as equally as possible between ve test-tubes. You are going to investigate the effect of the oxidising mixture on various oxygen-containing compounds starting with propan-1-ol, propan-2-ol and 2-methylpropan-2-ol. 2 Add 3 drops of one of the alcohols to the oxidising mixture in one of the tubes. Be careful not to add too much alcohol. (CARE Alcohols are highly ammable. Keep the bottle well away from naked ames.) Carefully warm the contents of the tube until they just begin to boil. (CARE Do not continue to boil the liquid in case alcohol vapour catches re.) 3 Label the tube and leave it to stand. Repeat the procedure in step 2 with each of the other two alcohols. 4 Make a note of any changes of appearance of the mixtures in the tubes. Work out what has happened in each case, and present your results in the form of a table showing the structural formulae of the alcohols and any products which are formed.
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5 Where reaction has occurred, distil out a few drops of liquid from each tube using the apparatus shown (Figure 1).
HEAT
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6 The products which you have distilled over should be propanal and propanone (CARE Highly ammable liquids). Use these liquids, or the liquids from stock bottles of propanal and propanone, and repeat steps 2 and 3 with the two liquids separately. 7 Prepare a hot water bath. A suitable arrangement is a 400 cm3 beaker half full of water on a tripod and gauze over a Bunsen ame. Transfer about 1 cm3 of one of the distillates (or reagents from the stock bottle) to a test-tube. Add about 1 cm3 of Fehlings solution 1 followed by 1 cm3 of Fehlings solution 2. (CARE Fehlings solution 2 contains sodium hydroxide and is corrosive.) Place the test-tube in the hot water bath and observe any colour changes. Now repeat the experiment with the second liquid.
QUESTIONS
a Which of the alcohols, propan-1-ol, propan-2-ol and 2-methylpropan-2-ol reacted readily with potassium dichromate(VI) solution? b Account for the change of colour of the mixtures, when one occurs, in the reactions above. c Write down the full structural formulae of the organic products. d One of the compounds formed from the reactions above reacts further with potassium dichromate(VI) solution. Write down the structural formulae of this compound and the organic product of this further reaction. e The red precipitate formed on reaction with Fehlings solution is Cu2O (the reagent contains Cu2+(aq) ions). i Suggest what has happened to the organic reagent in the tube when reactions have occurred. ii Compare this with the process that occurs with the organic reagent and potassium dichromate(VI) solution. f Propanal and propanone can be reduced to alcohols using the reagent sodium tetrahydridoborate(III), NaBH4. Give the names and structural formulae of the alcohols that will be produced. g Propanal reacts with hydrogen cyanide, as shown in the equation below:
H C O + HCN C2H5 H C CN OH
Write a corresponding equation for the reaction of propanone with hydrogen cyanide.
MD1.2
M v ?
BAC determination using gasliquid chromatography
By carrying out this activity you will learn more about the use of g.l.c. for measuring blood-alcohol concentrations. The technique can easily be adapted, and is used to analyse many other kinds of mixture. This activity illustrates ideas about g.l.c. which you learned in Colour by Design. You may need to refer to Chemical Ideas 7.6 before you begin.
Introduction
A sample which is a mixture of several similar compounds will produce a g.l.c. trace showing separate peaks for each compound. In general, for compounds of the same chemical type, more volatile compounds have shorter g.l.c. retention times. amount of propan-1-ol is added. A measured sample of this mixture is then analysed by g.l.c. The gas chromatogram consists of two peaks, corresponding to ethanol and propan-1-ol. The area of each peak is proportional to the amount of compound in the sample, but the instruments sensitivity may vary from run to run. Using a propan-1-ol standard overcomes this error the detector will give a high reading for all propan-1-ol peaks if its sensitivity is high, or a low reading for all peaks if the sensitivity is low. In either case, the ratio of the ethanol and propan-1-ol peak areas will be the same. It is this ratio which is used to calculate the BAC. For fairness, at least two determinations are made, and the equipment is calibrated periodically using an ethanol solution of known concentration. Commercial instruments calculate peak areas electronically. However, it is possible to regard a peak as a triangle. The area is then the height of the triangle multiplied by half the length of the base.
Recorder response
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What you do
Use the following information to investigate how g.l.c. can be used to estimate blood-alcohol concentration (BAC). 1 The chromatogram illustrated in Figure 1 was produced by a mixture of the rst ve straight-chain primary alcohols. a Measure the retention times for the ve peaks. Record these in a table, together with the name and formula of the alcohol responsible for each peak. b Estimate a retention time for hexan-1-ol. 2 When blood is analysed for its alcohol content, an exactly measured sample is diluted with water and a standard
2 1
4 5
Figure 1 Chromatogram of a mixture of the rst ve straight-chain primary alcohols: 1, methanol; 2, ethanol; 3, propan-1-ol; 4, butan-1-ol; 5, pentan-1-ol.
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Figure 2 shows g.l.c. traces for three blood samples. Trace I corresponds to a BAC of 80. Note: In practice the peaks are much narrower, as in Figure 1, and the areas under the peaks are found using special computer programs built into the recorders attached to the gas chromatograph. Figure 2 is drawn to enable you to calculate the areas yourself. Peak area is height times half-width of the triangle obtained by extrapolating the lines of the peak.
Figure 2 The g.l.c. traces of three blood samples each containing a standard amount of propan-1-ol
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Recorder response
Trace I
1 Recorder response
Retention time/min
Trace II
Retention time/min
Recorder response
Trace III
Retention time/min
c Calculate the ratio of peak areas (ethanol peak : propan-1-ol peak) in trace I. d Calculate the peak area ratios in traces II and III. Hence calculate BAC values for the other two blood samples. Was either over the limit? e Suggest a reason why propan-1-ol is chosen for the standard rather than any of the other alcohols represented in Figure 1.
MD3.1
M v ? What you do
Making a toolkit of organic reactions
This activity will help you to revise and use some of the organic reactions you have met in the course.
The idea is to organise some of the reactions you have met in the course, together with a few new ones, into a toolkit of reactions which you can use to design organic syntheses. 1 First make sure that you are familiar with the main reactions of the functional groups that you have met throughout the course. These are summarised in Chemical Ideas 14.2. 2 Read the Reference section below about useful synthetic reactions. This gives you some hints about making new CC bonds as well as some extra reactions which are often useful in synthesis. You are not expected to remember these extra reactions, but you should be able to use them correctly if you are given them. 3 Then use the new reactions in the reference section, together with the more familiar ones you have met in earlier units, to complete the two ow sheets, Chart A and Chart B. These two ow sheets make up your toolkit. To complete them, you should write the reaction conditions over each arrow and, where possible, the reaction type (substitution, oxidation, acylation, etc.) under the arrow. 4 Use the toolkit to answer the questions on page 315.
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These reactions are very useful because they provide a method of building sidechains onto a benzene ring. A good way of extending a carbon chain is to use the reaction of cyanide ions, CN, with halogenoalkanes. The cyanide ion is a powerful nucleophile and will displace the halogen atom in much the same way as OH. For example:
CH3CH2Br + CN CH3CH2CN propanenitrile + Br
The reaction is carried out by reuxing the halogenoalkane with a solution of sodium cyanide in ethanol and water. The product is a nitrile.
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M v ?
Nitriles themselves have few direct uses, but they are very important as synthetic intermediates. The important thing is that a new carboncarbon bond has been made, and the nitrile group can then undergo further reactions. For example, when nitriles are hydrolysed with dilute acids, carboxylic acids are formed.
CH3CH2CN H+(aq) / H2O reflux CH3CH2COOH
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Sometimes it is necessary to reduce aldehydes and ketones back to alcohols. This does not take place readily and requires a powerful reducing agent. A complex metal hydride is used, called sodium tetrahydridoborate(III), NaBH4:
RCHO NaBH4 RCH2OH
R' C R O
NaBH4
R' CH R OH
A different reducing agent, tin in the presence of concentrated hydrochloric acid, is used to convert nitro-groups into amino-groups:
NO2 Sn + c. HCl reflux NH2
One nal reaction. You will nd that acyl chlorides are very useful synthetic intermediates. These can be made from carboxylic acids by reuxing with a reactive liquid called sulphur dichloride oxide (SCl2O). The reaction mixture must be completely dry.
CH3COOH + SCl2O CH3COCl + SO2 + HCl
Note You should be familiar with Friedel-Crafts alkylation and acylation reactions (see Chemical Ideas 12.4 and 14.2) and the reduction of aldehydes and ketones (see Chemical Ideas 13.7 and 14.2). You are not expected to remember the rest of the reactions in this reference section, but you should be able to use them correctly if you are given them.
primary alcohol
alkene
halogenoalkane
O R R carboxylic acid (reacts as above) CH2 COOH amine CH2 NH2 R C NH R secondary amide
CH3
alkane
Notes
The formation of a nitrile from a halogenoalkane is a carboncarbon bond-forming reaction. The carboxylic acid formed from the nitrile has an extra carbon atom in the side-chain. All other reactions are simple functional group interconversion.
The halogenoalkane shown will only be a minor product of the reaction from the alkene. The main product will be the isomer with the Br atom attached to the second carbon atom. 3 You may wish to add other reactions to this toolkit, for example, the formation of a secondary alcohol from an alkene, RCH CHR, and a ketone from a secondary alcohol.
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O NH2 primary amide
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313
Br
Cl
SO2OH NO2 2
1, 2 1, 2
O C R
Notes 2
The Friedel-Crafts reactions are carboncarbon bond-forming reactions. All the substitution reactions of arenes are electrophilic.
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NH2
314
MD3.1
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QUESTIONS
a Use the toolkit to design some simple two-step syntheses. In each case, write out the synthesis in the form of a owchart and write the reagents and conditions on the arrows. i CH3CH2CH2OH ii
CH2Br CH3CHBrCH3 CH2COOH
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iii CH3CH2COOH iv
CH3CH2CONH2 CH2Cl
v CH3CH2OH
CH3CH2NH2
b Now, see whether you can work out routes for the following conversions. You should be able to carry out each conversion in no more than three steps. You have a supply of methanol in addition to the starting material, but no other organic compounds. i
CH2OH COOCH3
ii CH3CH
CHCH3
CH3
CH
CH2CH3
COOH
NH2
c Starting from ethene, work out a synthetic route for the preparation of the amide CH3CH2CONHCH2CH3 (six steps). You may not use any other organic compound.
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MD3.2
M v ? What you do
Classifying reactions
This activity is based on the toolkits produced from Activity MD3.1 and will enable you to familiarise yourself with them.
Complete the table below by listing, for each type of reaction, one or more homologous series that undergo this reaction. Draw the functional group and give a balanced equation for an example of the reaction.
Type of reaction Name of homologous series i ii iii Esterication Elimination Acylation i i i ii iii iv Addition electrophilic nucleophilic Substitution radical electrophilic nucleophilic Oxidation i ii iii i ii Reduction i ii i ii Ester Functional group
O
Example of reaction
H+
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Hydrolysis
OR
QUESTIONS
a Name each of the homologous series in your table that can act as an acid. b Name a homologous series that can act as a base. c What types of organic compound have hydrogen bonding as the main intermolecular force? d Give two different reducing agents used in organic reactions and give examples of these reactions. e Both sodium cyanide and hydrogen cyanide are extremely toxic. Why are they still used in organic synthesis?
MD3.3
M v ? What you do
Using the toolkit to synthesise medicines
In this activity you will use your knowledge of organic reactions to devise ways of synthesising some complex organic molecules. You will also see how spectroscopy can be used to monitor the chemical reactions under investigation.
In Parts 1 and 2, you will use the toolkit of organic reactions from Activity MD3.1 to help you suggest synthetic routes for the preparation of two medicines, paracetamol and ibuprofen. In Part 3, you will use spectra to identify some of the organic molecules used in the synthesis of ibuprofen, and in Part 4, you will study the structure of some sex hormones.
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NHCOCH3
QUESTIONS
a Write out the full structural formula of the target molecule. b For each step in your synthesis, choose a word from the list below to describe the type of reaction occurring: substitution oxidation esterication elimination reduction hydrolysis addition ethanoylation polymerisation c Paracetamol is a white crystalline solid which melts at 169 C. It is fairly soluble in hot water but insoluble in cold water. Explain how you could purify your product. d Describe one way in which you could test whether your sample was pure paracetamol.
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e Give the structure and name of the halogenoalkane needed for this Friedel-Crafts alkylation reaction. f Classify this reaction by stating the type of reagent involved and the type of reaction.
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C H3C O
h What is the name and structure of the acyl chloride needed for this Friedel-Crafts acylation? i What catalyst should be used?
C H3C O
H3C
C H
COOH
What carboncarbon bond-forming reaction could be used to introduce a new functional group which could easily be changed to a carboxylic acid group?
k What key intermediate must be prepared from the ketone before this reaction can take place? l What functional group must rst be obtained from the ketone before the key intermediate can be prepared?
m Now write a total synthesis of the target molecule, giving the reaction conditions necessary for each step above the arrow linking the intermediates. Where possible, classify the reactions according to reagent and reaction type beneath the arrow. n The synthesis of a new medicine must be accompanied by the correct stereochemistry. This may be crucial to the medicines action in the body. Place an asterisk (*) against any chiral carbon atom present in the target molecule.
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CH3 CH3
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benzene
step 1
step 2
compound A
C H3C O compound B
H3C
COOH
H ibuprofen
The i.r., n.m.r. and mass spectra of compounds A, B, C and ibuprofen are given in Figures 1, 2, 3 and 4, respectively. You will need to refer to these to answer the questions below.
QUESTIONS
o For compound A: i Why are there several peaks at around 3000 cm1 in Figure 1(a)? ii Identify the hydrogen atoms responsible for each of the signals in Figure 1(b). iii Identify the ions responsible for the peaks at mass 134 and 91 in Figure 1(c). Why has the peak at mass 135 an abundance of about 10% of the peak at mass 134? p Identify and explain the main changes that have occurred to the i.r., n.m.r. and mass spectra during step 2 of the synthesis, i.e. the conversion of compound A, in Figures 1(a), 1(b) and 1(c), to compound B, in Figures 2(a), 2(b) and 2(c). q i By comparing the spectra for compound C, in Figures 3(a), 3(b) and 3(c), with those of compound B, deduce the structure of compound C. ii What type of reaction is involved in step 3, the conversion of B to C? r The spectra for ibuprofen, in Figures 4(a), 4(b) and 4(c), are quite complex. Identify, with reasons, as many of the main features of these spectra as possible.
319
100 80 60 40 20 0 800 600 4000 3500 3000 2500 2000 1500 Wavenumber / cm
Figure 2(a) The i.r. spectrum of B (in solution)
100
80
60
Transmittance / %
20
Transmittance / %
40
0 4000
3000
Wavenumber / cm
6H 6H
Absorption
Absorption
TMS 2H 1H 3 0 2 1 10 9 8 2H 2H 7
5H
10
6 5 4 Chemical shift
6 5 4 Chemical shift
100
Intensity / %
Intensity / %
91
80
60
92
40 134
20
43
65
43 77
91 105
119 134 177 100 120 140 Mass 160 180 200
20
40
60
80 Mass
100
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1000 500 2H 3H 3 1H 2 1 0 TMS 161 176
320
80
60
Transmittance / %
20
1500
Transmittance / %
40
Wavenumber / cm 1
6H 3H TMS 2H 1H 1H 2 1 0 10 9 8 3 2H 2H 7 3H
Absorption
Absorption
2H
2H
1H
10
6 5 4 Chemical shift
6 5 4 Chemical shift
100 163
80
Intensity / %
60
Intensity / %
43
91
119
20
57
77
121 135
105
40
60
80
100
120 Mass
140
140
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1000 500 6H TMS 2H 1H 3 1H 2 1 0 163 206 207 160 180 200 220
MD3.3
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CH3 OH C O OH
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O testosterone O progesterone
HO oestradiol
The male sex hormone, testosterone, is secreted in the testes and controls the development of secondary sexual characteristics at puberty and sexual activity in the adult. Oestradiol is one of the principal female sex hormones controlling secondary sexual characteristics. It belongs to a group of female sex hormones called oestrogens. Progesterone is a second type of female steroid hormone. Its main function is to prepare the wall of the uterus for implantation of a fertilised ovum. Contraceptive pills usually contain a combination of an oestrogen and progesterone. High levels of these two hormones suppress normal monthly ovulation; this is the natural mechanism used by the body to suppress ovulation during pregnancy.
QUESTIONS
s Is the alcohol functional group in oestradiol primary, secondary or tertiary? t Explain how a simple test-tube reaction using FeCl3 solution could be used to distinguish between testosterone and oestradiol. u As a synthetic chemist you wish to modify the progesterone structure in the hope of nding safer, more effective birth-control pills. Use your toolkit to show how you could make the following compounds from progesterone. i
CH3 CHOH
HO
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MD3.3
M v ?
ii
CH3 CH O
O C CH3
O H3C C O
Synthesise the molecule above from your product in u i in one step. iii
CH3 CH COOH
Transmittance / %
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HOOC
Synthesise the molecule above from your product in u i in three steps. v A steroid compound was known to be a female sex hormone. It was thought to be either progesterone or oestradiol. The infrared spectrum of the steroid is shown in Figure 5. Use the chart of i.r. absorption frequencies in the Data Sheets to help you decide on the possible identity of the hormone. Give reasons for your decision.
100
80
60
40
20
0 4000
3000
2000 Wavenumber /
1600 cm1
1200
800
Figure 5 The i.r. absorption spectrum of the unknown steroid (in CCl4 solution)
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MD3.4
M v ?
Manufacturing salbutamol (Optional extension)
This activity looks in more detail at the reaction sequence which is used to make salbutamol, and at how much the chemicals used in its synthesis contribute to its cost. The infrared, nuclear magnetic resonance and mass spectra of these compounds are also analysed. There is more about salbutamol in Chemical Storylines MD3.
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G G G
The starting material should be cheap and readily available. The route should involve as few steps as possible, for speed and because every step involves some loss of material. Yields should be high for each step. Inexpensive and safe reagents and solvents should be used. Purication should be easy medicines must not contain contaminants.
A possible synthesis of salbutamol begins with aspirin and involves ve steps. The yields for these steps are high compared with many reactions in organic chemistry. Although the nal step has a yield of 30% it should be remembered that 30% of the inactive isomer is also formed. The reaction sequence contains no unusual materials.
Reaction sequence
Step 1
O HO O H3C O aspirin CH3OH H2SO4 H 3C O O O compound A O
Solvent methanol 10% w/v solution Other reagent sulphuric acid 10 g per kg aspirin
H3 C
Step 2
O H 3C O O O compound A AlCl3 compound B
(w/v means weight (mass) of solute : volume of solvent. A solution containing 100 g of solute in 1 dm3 of solvent would be a 10% w/v solution. In this case, the solute is aspirin and the solvent is methanol.)
H3C
Step 3
O H 3C compound B Br2 O HO compound C O Br
Solvent trichloromethane 10% w/v solution Other reagent bromine 1 mole Br2 per mole of B
Step 4
O H3C O HO compound C O Br (CH3)3CNH2 H 3C O HO compound D O O H N
MANUFACTURING SALBUTAMOL
MD3.4
M v ?
Step 5
O H 3C O HO compound D O H N LiAlH4 HO HO salbutamol (plus an equal amount of its optical isomer)
Solvent Other reagent
HO
H N
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Costs
Solvents methanol nitrobenzene trichloromethane ethoxyethane Cost/ dm3 4.20 9.30 10.70 6.00 Reagents
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Economics of process
Table 1 gives the yields and relative molecular masses of the organic compounds in the reaction sequence. 1 Copy out Table 1 and complete the other entries to show the masses and amounts of these compounds which could be made starting with 1 kg of aspirin.
Compound aspirin A B C D salbutamol Mr 180 194 194 273 265 239 Yield/% 85 60 75 55 30 Mass produced/g 1000 Amount produced/mol 5.56
2 Use information contained in the reaction sequence and in Table 1 to complete a copy of Table 2 to show the costs of the reagents and solvents used to convert 1 kg of aspirin into salbutamol.
Reagent/solvent aspirin methanol sulphuric acid nitrobenzene aluminium chloride trichloromethane bromine 2-amino-2-methylpropane ethoxyethane lithium tetrahydridoaluminate Quantity required 1 kg Cost/
Total cost
Table 2
3 You now know the cost of converting 1 kg of aspirin into salbutamol. What is the cost of synthesising 1 kg of salbutamol?
QUESTIONS
a What other costs will be involved in the production of salbutamol? b What percentage of the cost of making salbutamol is from the use of solvents? Suggest how this cost may be reduced.
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QUESTIONS
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For some of the following questions, you will need to refer to the tables of characteristic i.r. absorption frequences and proton n.m.r. chemical shifts in the Data Sheets. c For the starting material in this synthesis, aspirin (2-ethanoylhydroxybenzoic acid): i identify the bonds responsible for the broad peak around 3000 cm1, and the sharp peaks at 1750 cm1 and 1690 cm1 in Figure 1(a). ii identify the hydrogen atoms responsible for the signals at chemical shifts 2.2. and 13.1 in Figure 1(b). The hydrogen atoms responsible for the cluster of signals in the chemical shift range 7.5 0.5 are indicated as w, x, y and z on the following structure of aspirin.
O
w
HO O H3C O
z
iii identify the ions responsible for the peaks at mass 180, 163, 120 and 43 in Figure 1(c). d Compare Figures 1(a)1(c) and Figures 2(a)2(c). Identify and explain the main changes that have occurred to the i.r., n.m.r. and mass spectra during the conversion of aspirin into compound A in step 1. e i Compound B and compound A both have molecular ion peaks of mass 194. How are compounds B and A related? ii With the help of the relevant spectra, Figures 3(a)3(c), deduce the structure of compound B. f i Compare Figure 3(b) and Figure 4(b). Identify and explain the main change that has occurred to the n.m.r. spectrum in the conversion of compound B into compound C in step 3. ii In the mass spectrum of compound C, Figure 4(c), there are two molecular ion peaks of equal intensity, at mass 272 and 274 respectively. Explain this observation. g The n.m.r. spectrum of salbutamol is given in Figure 5. This also shows the integrated trace, which goes upward in steps. The height of each step in the trace is proportional to the number of hydrogen atoms absorbing at the chemical shift. Suggest which signals correspond to which hydrogen atoms in the salbutamol molecule and give reasons for your choice.
100 80
100
80 60
Transmittance / %
Transmittance / %
60 40
3500
3000
2500
2000
Wavenumber / cm
Figure 2(a) The i.r. spectrum of compound A (in the gas phase)
3H Absorption
Absorption
1H 4 3 2 1 0 13 12
1H 1H 1H 1H TMS
1H 1H 1H 1H 11 10 9 8 7 6 5 Chemical shift 4 3 2 1
13
12
11
10
7 6 5 Chemical shift
Intensity / %
Intensity / %
100
120
100 80
80 138
60
43
60 40 20 43 92 163 0 194 0 20 40 60 80
Figure 2(c) The mass spectrum of compound A
40
92
20
40
60
80
MANUFACTURING SALBUTAMOL
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3H 3H TMS 0 120 152 140 160 180 200
MD3.4
327
100
100
80
80
Transmittance / %
Transmittance / %
60
60
40
40
3500
3000
2500
2000
1500
1000
500
3500
3000
2500 2000
Wavenumber / cm
Wavenumber / cm
Figure 4(a) The i.r. spectrum of compound C (in solution)
3H Absorption
3H 2H 1H 1H 1H 11 10 9 8 7 6 5 Chemical shift 4
Absorption
1H 4 13 3 2 1 0 12
1H 1H 1H 1H
TMS
13
12
11
10
8 7 6 5 Chemical shift
Intensity / %
162
194
Intensity / %
100 147
80
60 179
40
20
43
63 79 91
119
20
40
60
80
20 40 60 80 100 120 140 160 180 200 220 240 260 280 Mass
Figure 4(c) The mass spectrum of compound C
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1500 1000 500 3H TMS 3 2 1 0 179 272 274 241 243
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MANUFACTURING SALBUTAMOL
MD3.4
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Absorption
9H
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2H 1H 1H 10 9 8 1H 1H 1H 7 6 5 4 Chemical shift 3 2 1 1H 2H
TMS
0
Figure 5 The n.m.r. spectrum of salbutamol
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MD5.1
M v ? Requirements
G
In this activity you can use your skills in handling organic chemicals to prepare a semi-synthetic penicillin. You can then test your product for bacterial activity.
100 cm3 well-stoppered bottle (or conical ask) 6-aminopenicillanic acid (6-APA) (1.0 g) 25 cm3 measuring cylinders (2) 10 cm3 measuring cylinder sodium hydroxide solution, 1 mol dm3 (5 cm3 ) teat pipettes benzoyl chloride (0.5 cm ) propanone (5 cm3 ) test-tubes 100 cm3 beakers (2) ethyl ethanoate (15 cm3 ) glass rod (or magnetic stirrer) pH meter (or Universal Indicator paper) dilute hydrochloric acid, 1 mol dm3 (10 cm3 ) 50 cm3 separating funnel saturated sodium hydrogencarbonate solution (25 cm3 ) 4 agar plates impregnated with Bacillus subtilis* cork borer (57 mm) ethanol (for sterilisation) beaker of disinfectant protective gloves adhesive tape small amounts of control solution for testing bacterial activity: 6-APA solution (made by dissolving 0.13 g 6-APA in a solution of 0.15 g sodium hydrogencarbonate in 250 cm3 water; take 10 cm 3 of this solution and dilute to 100 cm3) sodium benzoate solution (made by dissolving 0.13 g sodium benzoate in 250 cm3 water; take 10 cm3 of this solution and dilute to 100 cm3 with water) CARE 6-APA can act as a sensitiser by inhalation or skin contact. Wear protective gloves and do not inhale the dust. CARE If you are allergic to penicillins, you should not do this activity.
WEAR PROTECTIVE GLOVES
G G G G G G G G G G G G G G G G G G G G G G
benzoyl chloride
CORROSIVE IRRITANT
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ethanol
3
HIGHLY FLAMMABLE
ethyl ethanoate
HIGHLY FLAMMABLE
propanone
HIGHLY FLAMMABLE
CARE Benzoyl chloride is corrosive and lachrymatory (it is a severe eye irritant) and must be used in a fume cupboard. It gives off fumes of hydrogen chloride gas in moist air. Wear gloves when measuring out, and use a pre-marked teat pipette. CARE Propanone, ethyl ethanoate and ethanol are highly ammable liquids. Keep bottles stoppered when not in use, and well away from naked ames. CARE Consult your teacher before handling the bacterial culture and follow the safety instructions carefully. Wear a lab coat and gloves all the time. Cover any skin cuts with effective waterproof dressings and wash your hands thoroughly at the end of the session. Report any spillages immediately. Any material which has come into contact with the bacterial culture must be sterilised before disposal, or before returning to stock cupboards. The sealed plates must be sterilised in a pressure cooker or autoclave before disposal.
MD5.1
M v ?
v v
O C
S N
O C
H N N O C O O Na+ S
+
Cl O
H N N O C O OH S
phenylpenicillin
Figure 1 Reaction scheme for the synthesis of phenylpenicillin
Note that benzoyl chloride is a relatively unreactive acyl chloride and can be used in aqueous solution. The 4-membered lactam ring is easily destroyed by strong acids and by alkalis. To reduce this hydrolysis reaction to a minimum, the pH of the solution is kept in the range pH 58 during the preparation. When you acidify the reaction mixture with hydrochloric acid during the purication procedure, the pH of the solution falls to pH 2, so you must work quickly at this stage. As you go through the stages of the synthesis on page 332, use the column on the right-hand side to keep track of the changes taking place. Write the structure of the product where this has changed and a brief comment about what has happened at that stage. When you carry out the extraction with ethyl ethanoate to purify your product, make sure you know what is in each layer.
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M v ?
What you do
Part 1: Making and purifying the penicillin
1 Weigh out 1.0 g of 6-APA (CARE Wear protective gloves and do not inhale the dust) and mix it with 10 cm3 of distilled water in a stoppered bottle (or conical ask). 2 Add 1 mol dm3 of sodium hydroxide (CARE Irritant) drop by drop until a just-clear solution is obtained. This should take about 5 cm3 of the sodium hydroxide solution. 3 In the fume cupboard, dissolve 0.5 cm3 of the benzoyl chloride (CARE Corrosive and a severe eye irritant) in 5 cm3 of propanone (CARE Highly ammable) in a clean, dry test-tube. Add this solution drop by drop, with swirling, to the dissolved 6-APA in the bottle. Stopper the bottle rmly and shake the mixture gently for about 10 minutes. (CARE You may need to release the pressure once or twice by easing off the stopper in a fume cupboard.) 4 Transfer the reaction mixture to a 100 cm3 beaker and add 10 cm3 of ethyl ethanoate (CARE Highly ammable). Using a pH meter (or pH paper) acidify the mixture with stirring, using 1 mol dm3 hydrochloric acid. Add the acid until the pH of the solution falls to pH 2. (Any unreacted 6-APA forms a water-soluble hydrochloride. Phenylpenicillin is more soluble in organic solvents than water.) 5 Transfer both layers to a separating funnel and shake the mixture well. Separate into two 100 cm3 beakers. Keep both layers. (The density of ethyl ethanoate is 0.90 g dm3. Make sure you know which layer is which.) 6 Return the aqueous layer to the funnel and add a further 5 cm3 of ethyl ethanoate. Shake the mixture and separate it into the two beakers. You can now discard the aqueous layer down the fume-cupboard sink. (CAUTION Do not discard the wrong layer!) 7 Now add 10 cm3 of water to the organic layer in the beaker. Adjust the pH to 67 by adding saturated sodium hydrogencarbonate solution. Transfer the mixture to the separating funnel and shake it well, taking care to release any build-up of pressure. This time run the lower aqueous layer into a clean 25 cm3 measuring cylinder. Add water to adjust the volume in the measuring cylinder to 25 cm3 and stir well. This solution contains the phenylpenicillin you have made. Notes
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MD5.1
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10 Use the sterilised cork borer to make a well in the centre of an agar plate impregnated with Bacillus subtilis, by pressing the borer into the agar and then lifting out the cut plug of agar using a sterile spatula. (Flame the spatula in the same way as the cork borer.) Place the agar plug straight into a beaker of disinfectant. Re-ame the cork borer and spatula after use. Almost ll the well in the agar with the diluted penicillin solution. 11 Cover the plate and seal using small pieces of adhesive tape, as shown in Figure 2. Do not totally seal round the rim as this may create anaerobic conditions and encourage the growth of harmful bacteria. Label the plate with your initials, the name of the micro-organism and the date. Write something to indicate the treatment given to the plate. (Do not lick the labels.) 12 Now set up three control plates in the same way, to compare with your penicillin sample. Fill the well in the agar of the rst plate with 6-APA solution, and the well in the second plate with sodium benzoate solution: you are provided with these two solutions. (Their concentrations have been adjusted to about 50 g cm3, comparable to that of the penicillin solution). Leave the well in the third plate empty. Cover, seal and label the plates as before. 13 Take care not to tip the plates. Leave them on the bench at room temperature (20 C) for 2448 hours. Do not leave them where other people can interfere with them (a secure corner of the prep room is probably best). 14 Make sketches of the four plates at the end of this time. (CARE Do not open the plates once they have been sealed.) The agar will appear cloudy in the areas where bacteria are growing. (You will see this more clearly if you hold the plates up to the light.) Using a ruler, measure the size of any inhibition of bacterial growth. Did your penicillin solution show any antibacterial activity? CARE Any material which has come into contact with the bacterial culture must be sterilised before disposal, or before returning to stock cupboards. The sealed plates must be sterilised in a pressure cooker or an autoclave before disposal.
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Figure 2
QUESTIONS
a Explain why the penicillin you made is called a semi-synthetic penicillin. b Why was NaOH(aq) added to the 6-APA in step 2, before treatment with benzoyl chloride? c What product other than penicillin is formed when 6-APA reacts with benzoyl chloride? What type of bond is formed in this reaction? d Explain how the penicillin you produced was puried. e Why was it necessary to have the three control plates when testing for antibacterial activity?
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MD5.2
M v ?
A closer look at the structure of penicillins (Optional extension)
In this activity you will make a model of the penicillin nucleus, 6-APA, and investigate its stereochemistry. You will then investigate the effect of the structure of the side-chain on the antibacterial activity of different penicillins.
Requirements
G
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334
What you do
Work in small groups to carry out this activity. This will speed up the modelbuilding, and allow you to discuss the answers to the questions while you are looking at the models.
a What functional group is present in the b-lactam ring? b Suggest why the b-lactam ring is so susceptible to attack by acids and alkalis, and reacts readily to form open-chain compounds. 2 Now convert your b-lactam model into a model of 6-amino penicillanic acid (6-APA). This is the pencillin nucleus common to all penicillins. Before you do this, look at the formula of 6-APA very carefully. The molecule has a very precise stereochemistry:
H 2N N O COOH S 6-APA
c How many chiral carbon atoms are there in a molecule of 6-APA? Mark these on the diagram with an asterisk (*). 3 If your model kit is large enough, you could add an acyl group, and convert your model of 6-APA into a model of a penicillin. For example, substituting
O O CH2 C
d The other optical isomers of penicillin V are much less active against bacteria. Explain why the correct stereochemistry is crucial. e The structure of penicillin V was worked out in the 1940s, but it was not synthesised chemically until 15 years later. Why do you think totally synthetic penicillins have never been produced commercially on a large scale?
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MD5.2
M v ?
CH3CH2CH HO CH3(CH2)6
CHCH2 CH2
natural
v
Penicillin X
natural
v
Penicillin K natural not used commercially
Penicillin G
CH2
natural
Penicillin V
CH2
O
Methicillin
CH3
semi-synthetic controlling resistant Staphylococcus
CH3
Flucloxacillin
F N O CH3
Cl
semi-synthetic controlling resistant Staphylococcus
Ampicillin
CH NH2 HO CH NH2
semi-synthetic
Amoxycillin
semi-synthetic
Carbenicillin
CH COOH
semi-synthetic
pneumonia, burns
f g
Explain the distinction between natural and semi-synthetic penicillins. i Look at the R group in the side-chains of methicillin and ucloxacillin. What structural feature of penicillins appears to be important in resisting attack by the b-lactamase enzyme? (Hint Look at the groups attached to the rst carbon in the side-chain. Remember that large groups affect the size and shape of a molecule.) ii Suggest how penicillins such as methicillin and ucloxacillin are able to resist attack by the b-lactamase enzyme. iii Penicillins act by inhibiting a bacterial enzyme that helps to make the bacterial cell wall. How might the active site of this enzyme differ from the active site of b-lactamase?
335
MD6
M v ?
Check your notes on Medicines by Design
This activity helps you get your notes in order at the end of this unit.
Use this list as the basis of a summary of the unit by collecting together the related points and arranging them in groups. Check that your notes cover the points and are organised in appropriate ways. Most of the points are covered in the Chemical Ideas, with supporting information in the Storyline or Activities. However, if the main source of information is the Storyline or an Activity, this is indicated.
G
The role of computer modelling techniques in the design of medicines (Storyline MD4). The identication of functional groups within a polyfunctional molecule, as a way of making predictions about its properties. How to devise synthetic routes for preparing organic compounds. The use of the following terms to classify organic reactions: hydrolysis, oxidation, reduction, condensation and elimination. The classication of organic reactions according to their reaction mechanisms: nucleophilic substitution, electrophilic addition, electrophilic substitution, nucleophilic addition and radical. The use of a combination of spectroscopic techniques (m.s., i.r., n.m.r. and u.v. and visible) to elucidate the structure of organic molecules.
v v
336
The chemical principles behind methods which can be used to detect ethanol in the body (g.l.c. and i.r. spectroscopy) (Storyline MD1; Activity MD1.2). The following reactions involving aldehydes and ketones: formation by oxidation of alcohols, oxidation to carboxylic acids, reduction to alcohols and reaction with hydrogen cyanide (Activity MD1.1). The mechanism of the nucleophilic addition reaction between an aldehyde or a ketone and hydrogen cyanide. The meaning of the terms: drug, medicine, molecular recognition, pharmacological activity, pharmacophore, receptor site, agonist, antagonist, lead compound (Storyline in general). The structure of a pharmacologically active material in terms of its functional components: pharmacophore and groups which modify the pharmacophore (Storyline MD3). The action of biologically active chemicals and how this relates to their interaction with receptor sites. The factors affecting the way that species interact in three dimensions: size, shape, bond formation and orientation (Storyline MD4). The role of chemists in designing and making new compounds for use as pharmaceuticals (Storyline MD3, MD4 and MD5).