Alkane and Alkyl Halides PP5

Resolution of a Racemate
Enantiomers are alike in every respect except in their action on polarized light. They are therefore very difficult to separate,
hence a racemate is difficult to separate. However, using chemical means, a racemate can be resolved (separated) by 1 st
converting it to diastereomeric mixture which can then be separated using chromatographic method: Consider an acidic
racemate [A(+),(-)]; react the racemic acid with a pure optically active organic base of known optical activity e.g [B(+)] to
form diastereomeric salt: B + A B
+ B A
A
Diastereomeric Salts, which can be separated by:

(i) Fractional distillation
(ii) Fractional crystalisation
(iii) Chromatography
A B
A B
H3O
H3O
NOTE A + HB
A meso compound cannot be resolved A + HB
Determination of Absolute configuration
Polarimeter- is used to detect if a compound is optically active or not and to check if positive or negative. However the
absolute configuration of an optically active compound can determined using R/S notation as proposed by Cahn Ingold and
Prelog:
R/S Notation By Cahn Ingold Prelog Rule

i. Arrange the 4 groups around the chiral C-atom in order of increasing priority based on atomic mass
ii. Visualize the molecule with the least priority group directed away from the viewer
iii. View the remaining groups starting from the highest priority group to the least of the 3groups
iv. Clockwise movement of view designate R configuration (+ve isomer); anticlockwise movement of view means S
configuration (-ve isomer)
Note
In this exercise, horizontal bonds are assumed pointing out of plane of paper, while vertical bonds are assumed pointing inside
the plane of paper.
Assignment
Assign E or Z configuration
3. Identify the chiral counter(s) in 1-bromo-1,2- dihydroxylpropane.

4. Determine the absolute configuration of the chiral centers in 3 above.
Chemistry of Hydrocarbon
Alkanes
Homologous series
Characteristics of Homologous series
As we go down the series, the molecular weight of the member increase down the group, density, boiling point and
melting point also increases.
A member differs from the immediate neighbor by -CH2.

Method of Preparation
1. Alkanes can be obtained from petroleum by the process of cracking or fractional distillation of higher members.
2. Grignard synthesis: R-MgX alkyl magnesium halide
Aprotic solvent i.e solvent that does not contain active hydrogen is used in Grignard’s synthesis. In
presence of active H, (e.g water) R-MgCl formed gets hydrolysed:

R H + MgClOH
R MgCl + H OH
Alkane
3. Hydrogenation of unsaturated hydrocarbon: If you react alkene in the presence of

hydrogenating catalyst (Pt, Ni or Paladium- Pd)
4. Wurtz reaction: 2 molecules of alkyl halides (identical or different) react in the presence of metallic sodium to give
alkanes. This two molecules may be identical or different. H C CH + 2 NaCl
2 CH3Cl + 2 Na 3 3
2 Na + 2 NaCl
CH3Cl H3C C2H5
+ C2H5Cl
CH3 CH2 CH3

Reduction of alkyl halide using appropriate reagent such as LiAlH4 (lithium aluminium tetrahydride)

Reducing agents: NaBH4, Na/NH3, Zn/HCl, LiAlH4

4 CH3CH2Cl 4CH3CH3 + LiCl + AlCl3

Properties of Alkanes
Alkanes are non- polar molecule and they are soluble only in non-polar solvent.
Hybridization of alkane is sp3 .

Chemical Properties in Alkanes
1. Halogenation reaction: Alkanes react with chlorine and bromine in the presence of sunlight energy to give chloro and
bromo derivatives respectively.
2. Combustion reaction: Hydrocarbon or alkanes react in the presence of oxygen to give CO2 and H2O as the only product; If
you use excess O2you are going to have CO2 and H2O. 3(n+1) O n CO2 + (n+1) H2O
CnH2n+2 + 2
2
Sometimes, coke and CO may also be formed depending on the proportion of O 2 to hydrocarbon. In the presence of
insufficient O2; coke and CO mayCbe formed thus:
+ 2(n+1) O2 n CO + (n+1) H2O
nH2n+2
2
2(n+1) O2 n C n CO n CO + (n+1) H2O

CnH2n+2 + + +
2 3 3 3
3. Pyrolysis: it is the same thing as cracking. It is the decomposition of hydrocarbon by reaction of heat and sometimes
in the presence of catalyst.
In Petroleum industry, it is called ‘thermal’ or ‘catalytic cracking’. It usually takes place at high temperature in the
‘absence of air’. The benefit include:
Production of fuel for internal combustion engine
Production of lower hydrocarbons for the preparation of large number of chemicals in the petroleum industry.
4. Isomerization: Formation of isomers of organic compounds:
Alkyl Halides
An alkyl group have the general formula of CnH2n+1 and it can be represented by R.
Properties of Alkyl Halide
A. Physical properties
1. They have higher boiling point than corresponding alkane e.g CH3Cl > CH4, because of the molecular weight. The boiling
or melting point of alkyl halides increases with the weight of the halogen.
2. Despite their polarity, they are insoluble in H2O but soluble in organic solvent.
Alkyl halides are denser than water.
B. Chemical Properties
1. They undergo nucleophilic substitution reaction with reagent that can generate nucleophile such as

2. Dehydrogenation- removal of HX to form unsaturated molecule.

When you have 2 unsymmetrical compound, you will have 2 possible result which leads to the use of Saytzeff’s rule.
3. Preparation of Grignard reagent: alkyl-halide reacts with magnesium
dry ether in the presence of dry ether to give grignard
reagent. R X Mg R MgX

Preparation of Alkyl halide
1. From Alcohol (ROH): Alcohol react with appropriate halogenation reagent to produce alkyl halide. There are various
halogenation reagent which include:
(i) acid halide (HX);
(ii) Phosphorous trihalide or pentahalide (PCl3 / PCl5)
+ R X + H2O
(iii) Thionyl chloride (SOCl2) R OH HX
R X + SO2
+ HCl
R OH + SOCl2
PBr3 3 R Br + H3PO3
R OH +
PBr5 R Br + POBr + HCl

R OH +

2. Halogenation of some hydrocarbon: e.g

(i)
(ii)

3. Addition of hydrogen halide to alkene (unsaturated compound). Example:

4. Addition of halogens to alkene or alkyne

Alkane and Alkyl Halides PP5

Uploaded by

Copyright:

Available Formats

Alkane and Alkyl Halides PP5

Uploaded by

Document Information

Original Description:

Original Title

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

Alkane and Alkyl Halides PP5

Uploaded by

Copyright:

Available Formats

Resolution of a Racemate

Diastereomeric Salts, which can be separated by:

R/S Notation By Cahn Ingold Prelog Rule

3. Identify the chiral counter(s) in 1-bromo-1,2- dihydroxylpropane.

3. Hydrogenation of unsaturated hydrocarbon: If you react alkene in the presence of

CH3 CH2 CH3

2(n+1) O2 n C n CO n CO + (n+1) H2O

2. Dehydrogenation- removal of HX to form unsaturated molecule.

PBr5 R Br + POBr + HCl

4. Addition of halogens to alkene or alkyne

You might also like