ALKANE, ALKENE & ALKYNE

<SKO3023>
<ORGANIC CHEMISTRY 1>
DR YUHANIS MHD BAKRI
DEPARTMENT OF CHEMISTRY
FACULTY OF SCIENCE AND MATHEMATICS
Also adapted from: Organic
UNIVERSITI PENDIDIKAN SULTAN IDRIS Chemistry, 9th Edition, Global
Edition. L. G. Wade, Jr. and
other resources (credited)
 ALKANE
<SKO3023>
<ORGANIC CHEMISTRY 1>
DR YUHANIS MHD BAKRI
DEPARTMENT OF CHEMISTRY
FACULTY OF SCIENCE AND MATHEMATICS
UNIVERSITI PENDIDIKAN SULTAN IDRIS Also adapted from: Organic
Chemistry, 9th Edition, Global
Edition. L. G. Wade, Jr.
 Alkanes
• General formula: CnH2n+2
• These are found in everything from natural gas to petroleum.
• The smaller alkanes have very low boiling points (b. p.); therefore,
they are gases.
CH4 C2H6 C3H8
b. p. –160 °C –89 °C –42 °C
Formulas and Physical Properties of Alkanes
Common Alkyl Groups
 Multiple Groups
• When two or more of the same substituents are present,
use the prefixes di-, tri-, tetra-, etc. to avoid having to
name the alkyl group twice.

Three methyl groups at positions 2, 5, and 7.

2,5,7-trimethyldecane
“Iso” Groups
 Complex Substituents
• Complex alkyl groups are named by using the longest
carbon chain.
• Carbon number 1 of the alkyl group is the carbon
attached to the main chain.
Degree of Alkyl Substitution
 Boiling Points of Alkanes
As the number of carbons in an alkane increases, the
boiling point increases due to the larger surface area and
the increased van der Waals attractions.
 Melting Points of Alkanes
• Melting points increase as the carbon chain increases.
• Alkanes with an even number of carbons have higher melting
points than those with an odd number of carbons.
 Alkane Sources

• Alkanes are obtained from

petroleum and petroleum
by-products.
• Fractional distillation will
separate the crude oil into
mixtures of alkanes with a
range of boiling points.
Catalytic Cracking and
Hydrocracking
 Cycloalkanes: CnH2n

Cycloalkanes contain rings of carbon atoms.

 Cycloalkane Nomenclature

• Cycloalkane is the main chain; alkyl groups attached to the

cycloalkane will be named as alkyl groups.
• If only one alkyl group is present, then no number is necessary.
 Cycloalkane Nomenclature

• If there are two or more substituents, number the

main chain to give all substituents the lowest
possible number.
 Cycloalkanes As Substituents

• The cycloalkane becomes a substituent when the acyclic

portion of the molecule contains fewer carbons than the
cyclic part or when there is a more important functional
group in the molecule.
SYNTHESIS OF ALKANES
REACTIONS OF ALKANES
Initiation Step: Formation of Chlorine Atom

A chlorine molecule splits homolytically into chlorine

atoms (free radicals).
 Propagation Step: Carbon Radical

The chlorine atom collides with a methane molecule and abstracts

(removes) an H, forming another free radical and one of the
products (HCl).
Propagation Step: Product Formation

The methyl free radical collides with another chlorine molecule,

producing the organic product (methyl chloride) and regenerating
the chlorine radical.
Overall Reaction
 Termination Steps

• A reaction is classified as a termination step when any two free

radicals join together, producing a nonradical compound.
• Combination of a free radical with a contaminant or collision with
a wall are also termination steps.
 ALKENE
<SKO3023>
<ORGANIC CHEMISTRY 1>
DR YUHANIS MHD BAKRI
DEPARTMENT OF CHEMISTRY
FACULTY OF SCIENCE AND MATHEMATICS
UNIVERSITI PENDIDIKAN SULTAN IDRIS Also adapted from: Organic
Chemistry, 9th Edition, Global
Edition. L. G. Wade, Jr.
 IUPAC Nomenclature

• Find the longest continuous carbon chain that includes

the double-bonded carbons.
• Ending -ane changes to -ene.
• Number the chain so that the double bond has the
lowest possible number.
• In a ring, the double bond is assumed to be between
carbon 1 and carbon 2.
IUPAC and New IUPAC
 Ring Nomenclature
In a ring, the double bond is assumed to be between
carbon 1 and carbon 2.
 Multiple Double Bonds

• Give the double bonds the lowest numbers possible.

• Use di-, tri-, or tetra- before the ending -ene to specify
how many double bonds are present.
Alkenes As Substitutents
 Cis-Trans Isomers

• Also called geometric isomerism

• If there are similar groups on same side of the double
bond, alkene is cis.
• If there are similar groups on opposite sides of the
double bond, alkene is trans.
• Not all alkenes show cis-trans isomerism.
 E-Z Nomenclature
• Use the Cahn–Ingold–Prelog rules to assign priorities to
groups attached to each carbon in the double bond.
• If high-priority groups are on the same side, the name is
Z (for zusammen).
• If high-priority groups are on opposite sides, the name is
E (for entgegen).
Example

• Assign priority to the

substituents according
to their atomic number
(1 is highest priority).
• If the highest priority
groups are on opposite
sides, the isomer is E.
• If the highest priority
groups are on the same
side, the isomer is Z.
Commercial Uses of Ethylene
 Heat of Hydrogenation
• Combustion of an alkene and hydrogenation of an alkene can
provide valuable data as to the stability of the double bond.
• The more substituted the double bond, the lower its heat of
hydrogenation.
 Physical Properties of Alkenes

• Low boiling points, increasing with mass

• Branched alkenes have lower boiling points.
• Less dense than water
• Slightly polar
• Pi bond is polarizable, so instantaneous dipole–dipole
interactions occur.
• Alkyl groups are electron-donating toward the pi bond, so they
may have a small dipole moment.
 Zaitsev’s Rule
• If more than one elimination product is possible, the most-
substituted alkene is the major product (most stable).

major product
(trisubstituted)
 Substitution or Elimination?

• The strength of the nucleophile determines the order:

Strong nucleophiles or bases promote bimolecular
reactions.
• Primary halides usually undergo SN2.
• Tertiary halides are a mixture of SN1, E1, or E2. They
cannot undergo SN2.
• High temperature favors elimination.
• Bulky bases favor elimination.
 Dehydration of Alcohols

• Use concentrated H2SO4 or H3PO4 and remove low-boiling alkene

as it forms to shift the equilibrium and increase the yield of the
reaction.
• E1 mechanism
• Rearrangements are common.
• The reaction obeys Zaitsev’s rule.
 Dehydration Mechanism: E1
Step 1: Protonation of the hydroxyl group (fast equilibrium)

Step 2: Ionization to a carbocation (slow; rate limiting)

 Dehydration Mechanism: Step 3

Step 3: Deprotonation to give the alkene (fast)

 Solved Problem
Propose a mechanism for the sulfuric acid-catalyzed dehydration of t-butyl alcohol
Solution
The first step is protonation of the hydroxyl group, which converts it to a good leaving group.

The second step is ionization of the protonated alcohol to give a carbocation.

Abstraction of a proton completes the mechanism.

 Reactivity of the Carbon–Carbon
Double Bond

• The most common reactions of double bonds

transform the pi bond into a sigma bond.
Types of Alkene Reactions
Types of Additions
 Addition of HX to Alkenes

• Step 1 is the protonation of the double bond.

• The protonation step forms the most stable
carbocation possible.
• In step 2, the nucleophile attacks the carbocation,
forming an alkyl halide.
• HBr, HCl, and HI can be added through this
reaction.
 Mechanism of Addition of HX
Step 1: Protonation of the double bond

Step 2: Nucleophilic attack of the halide on the carbocation

 Regioselectivity
• Markovnikov’s rule: The addition of a proton to the
double bond of an alkene results in a product with
the acidic proton bonded to the carbon atom that
already holds the greater number of hydrogens.
• Markovnikov’s rule (extended): In an electrophilic
addition to the alkene, the electrophile adds in such
a way that it generates the most stable
intermediate.
 Markovnikov’s Rule

The acid proton will bond to carbon 3 in order to produce

the most stable carbocation possible.
 Markovnikov’s Rule (Continued)

• The bromide anion then adds to the carbocation.

Example
 Solved Problem
Show how you would accomplish the following synthetic conversions.
(a) Convert 1-methylcyclohexene to 1-bromo-1-methylcyclohexane.

Solution
This synthesis requires the addition of HBr to an alkene with Markovnikov
orientation. Ionic addition of HBr gives the correct product.
More Examples
 Hydration of Alkenes

• Addition of water to the double bond forms an alcohol.

• The addition follows Markovnikov’s rule.
• This is the reverse of the dehydration of alcohol.
• It uses dilute solutions of H2SO4 or H3PO4 to drive equilibrium
toward hydration.
 Mechanism for Hydration
Step 1: Protonation of the double bond forms a carbocation.

Step 2: Nucleophilic attack of water

Step 3: Deprotonation of the alcohol

 Orientation of Hydration

The protonation follows Markovnikov’s rule: The proton

adds to the less substituted end of the double bond, so
the positive charge appears at the more substituted end
(most stable carbocation).
 Rearrangements Are Possible

Methyl Shift:

• A methyl shift after protonation will produce the more

stable tertiary carbocation.
 Hydroboration of Alkenes

• H. C. Brown of Purdue University discovered that diborane

(B2H6) adds to alkenes with anti-Markovnikov orientation to
form alkylboranes, which, after oxidation, give anti-
Markovnikov alcohols.
• Brown received the Nobel Prize in Chemistry in 1979 for his
work in the field of borane chemistry.
 Addition of Halogens

• Cl2, Br2, and sometimes I2 add to a double bond to

form a vicinal dihalide.
• This is an anti addition of halides.
 Mechanism of
Halogen Addition
 Mechanism of Halogen Addition to
Alkenes continued.

• The intermediate is a three-membered ring called the

halonium ion.
 Catalytic Hydrogenation
of Alkenes

• Hydrogen (H2) can be added across the double bond in a

process known as catalytic hydrogenation.
• The reaction only takes place if a catalyst is used.
 Syn Hydroxylation
of Alkenes

• Alkene is converted to a syn-1,2-diol

• Two reagents:
– Osmium tetroxide, OsO4, followed by hydrogen
peroxide or
– Cold, dilute solution of KMnO4 in base
 Permanganate Dihydroxylation

• A cold, dilute solution of KMnO4 also hydroxylates alkenes

with syn stereochemistry.
• The basic solution hydrolyzes the manganate ester, liberating
the glycol and producing a brown precipitate of manganese
dioxide, MnO2.
 Oxidative Cleavage
with KMnO4

• If the solution is warm or acidic or too concentrated,

oxidative cleavage of the glycol may occur.
• Disubstituted carbons become ketones.
• Monosubstituted carbons become carboxylic acids.
• Aldehydes, if made, are oxidized to carboxylic acids
under these conditions.
Examples
 Ozonolysis

• Ozone will oxidatively cleave (break) the double bond to

produce aldehydes and ketones.
• Ozonolysis is milder than KMnO4 and will not oxidize
aldehydes further.
• A second step of the ozonolysis is the reduction of the
intermediate by zinc or dimethyl sulfide.
Comparison of Permanganate
Cleavage and Ozonolysis
 Solved Problem
Ozonolysis–reduction of an unknown alkene gives an equimolar mixture of
cyclohexanecarbaldehyde and butan-2-one. Determine the structure of the original
alkene.

Solution
We can reconstruct the alkene by removing the two oxygen atoms of the carbonyl groups
(C=O) and connecting the remaining carbon atoms with a double bond. One uncertainty
remains, however: The original alkene might be either of two possible geometric isomers.
 ALKYNE
<SKO3023>
<ORGANIC CHEMISTRY 1>
DR YUHANIS MHD BAKRI
DEPARTMENT OF CHEMISTRY
FACULTY OF SCIENCE AND MATHEMATICS
UNIVERSITI PENDIDIKAN SULTAN IDRIS Also adapted from: Organic
Chemistry, 9th Edition, Global
Edition. L. G. Wade, Jr.
 Introduction

• Alkynes contain a triple bond.

• Their general formula is CnH2n–2.
• There are two elements of unsaturation for
each triple bond.
• Some reactions resemble the reactions of
alkenes, like addition and oxidation.
• Some reactions are specific to alkynes.
 Nomenclature: IUPAC

• Find the longest chain containing the triple

bond.
• Change -ane ending to -yne.
• Number the chain, starting at the end
closest to the triple bond.
• Give branches or other substituents a
number to locate their position.
 Examples of Nomenclature

• All other functional groups, except ethers and halides,

have a higher priority than alkynes.
 Common Names

Named as substituted acetylene

CH3 C CH
methylacetylene
(terminal alkyne)
CH3 CH3
CH3 CH CH2 C C CH CH3
isobutylisopropylacetylene
(internal alkyne)
 Physical Properties

• Nonpolar, insoluble in water

• Soluble in most organic solvents
• Their boiling points are similar to an alkane of
the same size.
• Less dense than water
• Up to four carbons, gas at room temperature
 Ethyne

• Commonly called acetylene

• It is used in welding torches.
• The oxyacetylene flame reaches temperatures
as high as 2800 °C.
• It is thermodynamically unstable. When it
decomposes to its elements, it releases 234 kJ
(56 kcal) of energy per mole.
 Synthesis of Acetylene from
Natural Gas

• Methane forms acetylene when it is heated for a very

short period of time.
 Solved Problem
Show how to synthesize 3-decyne from acetylene and any necessary alkyl halides.

Solution
Another name for 3-decyne is ethyl n-hexylacetylene. It can be made by adding an ethyl group and a
hexyl group to acetylene. This can be done in either order; we begin by adding the hexyl group.
 Synthesis of Alkynes by Elimination
Reactions

• Removal of two molecules of HX from a vicinal or

geminal dihalide produces an alkyne.
• Dehydrohalogenation of a geminal or vicinal
dihalide gives a vinyl halide.
• Under strongly basic conditions, a second
dehydrohalogenation may occur to form an alkyne.
 Reagents for Elimination

• Molten KOH or alcoholic KOH at 200 °C favors an internal

alkyne.
• Sodium amide, NaNH2, at 150 °C, followed by water, favors
a terminal alkyne.
 KOH Elimination

The KOH elimination tends to give the most stable,

most highly substituted alkyne.
 Catalytic Hydrogenation
of Alkynes

• Two molecules of hydrogen can add across the triple bond

to form the corresponding alkane.
• A catalyst such as Pd, Pt, or Ni needs to be used for the
reaction to occur.
• Under these conditions the alkyne will be completely
reduced; the alkene intermediate cannot be isolated.
 Hydrogenation with
Lindlar’s Catalyst

• The catalyst used for the hydrogenation reaction is partially

deactivated (poisoned).
• The catalyst used is commonly known as Lindlar's catalyst; it is
composed of powdered barium sulfate, coated with palladium
poisoned with quinoline.
• The reaction produces alkenes with cis stereochemistry.
 Reduction of Alkynes with
Metal Ammonia

• To form a trans alkene, two hydrogens must be

added to the alkyne anti stereochemistry, so this
reduction is used to convert alkynes to trans
alkenes.
 Addition of 1 Mole of Halogen

• Cl2 and Br2 add to alkynes to form vinyl dihalides.

• They may add syn or anti, so the product is a mixture
of cis and trans isomers.
• It is difficult to stop the reaction at dihalide.
 Addition of 2 Moles of Halogen

• Two moles of halogen can add to the triple

bond, forming a tetrahalide.
 Addition of HX

• One mole of HCl, HBr, and HI adds to alkynes to form vinyl

halides.
• If 2 moles of HX are added, the product is a geminal
dihalide.
• The addition of HX is Markovnikov and will produce a
geminal dihalide.
HX Addition Examples
 Anti-Markovnikov Addition of
Hydrogen Bromide to Alkynes

• By using peroxides, hydrogen bromide can be added to a

terminal alkyne anti-Markovnikov.
• The bromide will attach to the least substituted carbon,
giving a mixture of cis and trans isomers.
 Hydration of Alkynes

• Mercuric sulfate in aqueous sulfuric acid adds

H—OH to one pi bond with a Markovnikov
orientation, forming a vinyl alcohol (enol)
that rearranges to a ketone.
• Hydroboration–oxidation adds H—OH with
an anti-Markovnikov orientation, and
rearranges to an aldehyde.
 Oxidation of Alkynes

• Similar to oxidation of alkenes

• Dilute, neutral solution of KMnO4 oxidizes
alkynes to a diketone.
• Warm, basic KMnO4 cleaves the
triple bond.
• Ozonolysis, followed by hydrolysis, cleaves the
triple bond.
 Permanganate Oxidation of
Alkynes to Diketones

• Under neutral conditions, a dilute potassium permanganate

solution can oxidize a triple bond into a diketone.
• The reaction uses aqueous KMnO4 to form a tetrahydroxy
intermediate, which loses two water molecules to produce the
diketone.
 Permanganate Oxidation of Alkynes
to Carboxylic Acids

• If potassium permanganate is used under basic conditions or if

the solution is heated too much, an oxidative cleavage will take
place and two molecules of carboxylic acids will be produced.
 Ozonolysis

• Ozonolysis of alkynes produces carboxylic acids

(alkenes gave aldehydes and ketones).

• Either permanganate cleavage or ozonolysis can be

used to determine the position of the triple bond in
an unknown alkyne.