Alkenes

Introduction

• Alkenes are unsaturated hydrocarbons that contain one or more carbon-carbon double bonds C=C, in their structures

• Alkenes have the general formula CnH2n

Nomenclature of alkenes and cycloalkenes

• The IUPAC names of the alkenes end with –ene and the location of the double bond takes precedence over that of the side chains in numbering the parent chain.

• The parent chain is the longest chain• For open chain alkenes, the parent chain is

numbered from the end that gives the lowest number to the first carbon of the double bond to be reached.

CH3 CH

CH2C

H2C CH3

CH3

1 2 3 4 5 6

3-methylhex-2-ene

CH3

H2C C

HCH

CH2

CH3

3-methylpent-1-ene

• For cyclohexenes, position 1 is always given to one of the two carbons at the double bond.

• The ring atoms are numbered from the carbon 1 through the double bond in whichever direction to reach a substituent first.

CH3

1

23

4

5

6

3-methylcyclohexene

Lets tryH3C C

CH3

CH2

H3C C

CH3

CH

H2C CH3

Br

CH2CH3

The existence of cis-trans isomerism

• Cis-trans isomerism have the same molecular formula and the same order of attachment of atoms but differ in arrangement of their atoms in space

• An alkene shows cis-trans isomerism if each carbon atom of the double bond has two different groups attached to it

Isomer Cis Trans

Structure

Name Cis but-2-ene Trans but-2-ene

Melting point (˚C) -139 -106

Boiling point (˚C) 4 1

C C

CH3

H

H3C

H

C C

CH3

H

H

H3C

• Trans isomer is more stable than cis-isomer because the two larger methyl groups are on the opposite side of the double bond. Hence, there is less strain on the molecule

• Cis-isomers are polar molecules because the bond moments do not cancel out. The molecule thus has a net dipole moment

• Trans-isomers are usually non polar because the bond moment cancel out each other. The molecule has no net dipole moment

• The cis-isomer will generally have a higher boiling point because of its higher polarity which leads to stronger dispersion forces between the molecules

• The cis-isomer will generally have a lower melting point because of its lower symmetry. It is more difficult to fit the cis-isomer into the crystalline structure

C C

CH3

H

H3C

H

C C

CH3

H

H

H3C

Cis but-2-ene Trans but-2-ene

Has a net dipole moment Has no net dipole moment

Exercise

• Name each compound below and show the configuration about each double bond using the cis-trans system

CH3CH2

C C

CH2CH3H

H

C C

CH2CH3H3CH2CH2C

HH

General preparation of alkenes

1. Dehydration of alcohol

Alcoholdehydration

Alkene

May be achieved by heating with excess concentrated H2SO4 or H3PO4 or Al2O3 with heat to 225 oC.

C Calumina

heatC C

H

H

H

H

+ H2O

H

H

H

H

H

OH

C

H

H

H

C

H

H

C

H

H

OH

Propan-1-ol

Concentrated H2SO4

170oC

2. Haloalkanes with KOH/ethanol• Haloalkanes undergo elimination reaction

when refluxed with purely KOH/ethanol.

R-X + KOH alkene + K-X + H2O ; X = Br, I, F, Cl

C

H

H

H

C

H

H

C

H

Br

H + KOHrefulx

C

H

H

H

C

H

C

H

H + KBr + H2O

Bromopropane Propene

C

H

H

H

C

CH3

Cl

C

H

H

H + KOHrefulx

C

H

H

H

C

CH3

C

H

H + KCl + H2O

2-chloro-2-methylpropane

Physical properties of alkenes

1. Alkenes have lower melting and boiling temperatures than alkanes

2. Weak dispersion forces between molecules. The forces are so weak that the lower alkenes (ethene, propene, butene) exist as gas at room temperature and pressure

3. The melting and boiling point increase with increasing number of carbon

Reactivity of alkenes

• The cloud of electron which forms the p bond in alkene lies above and below the plane of the molecule. In this position, the p electrons are susceptible to attack by electrophilies

• Addition reaction are the most common reactions of alkenes

• The π bond of the carbon-carbon double bond breaks in an order to release electrons to form two new σ bonds with the reactant molecule and a saturated molecule is formed

C C + X Y C C

YX

Addition reactions of alkenes

1. Addition of Hydrogen (Hydrogenation)• The addition of hydrogen to an alkenes can

be carried out by passing hydrogen and the alkenes, both in the gaseous state, over a metal catalyst

C C + H H C C

HH

Ni / Pt

150 oC 5 atm

• Example hydrogenation

C C

H

H

H

H

+ H H C C

H H

H

HH

H

Ni / Pt

150 oC 5 atm

ethene ethane

C C + H H C C

H H

CH3

HH

H3C

Ni / Pt

150 oC 5 atm

ethene ethane

H3C

H H

CH3

Exercise

• Write down the structure of the products, if any, of the following reactions

CH3CH CHCH3 + H2Ni

150oC, 5 atm

+ H2

Ni

150oC, 5 atm

2. Addition of halogen (Halogenation)• Both chlorine, Cl and bromine, Br, add rapidly

to the double bond at room temperature without the need of any catalyst

C C + C C

XX

room temperatureX2

X2 = Cl2 or Br2

• For example

C C

H

H

H

H + Cl2 C C

H

H

H

H

Cl Cl

Ethene 1,2-dichloroethane

+ Br2

Br

Br

cyclohexene 1,2-dibromocyclohexane

room temp

• The mechanism of addition of bromine to ethene is as follows:The mechanism is described as electrophilic addition. Because the bromine molecule acts as an electrophile

when it is attracted to the electron-rich carbon-carbon double bond

When a bromine molecule approaches an ethene molecule, the p electron cloud of ethene interacts with the approaching bromine molecule, causing a polarisation of the Br-Br bond.

C C

H

H

H

H

Br

Br

The double bond of ethene induces a

dipole on Br2

Electron move from the double bond towards the partially positively charge (Brδ+) and at the same time electrons in the Br-Br bond are repelled to the partially negatively charged bromine atom (Brδ-). This results in the formation of two ions, a carbocation and a bromide ion, Br-. A carbocation is a species that contains a carbon atom with only three bonds to it and bearing a positive charge.

C C

H

H

H

H

Br

Br

slowC C

H

H

H

H

Br

+ Br-

electrophilic attack carbocation bromide ion

The carbocation is very unstable and quickly combines with the bromide ion, Br- to form 1,2-dibromoethane.

C C

H

H

H

H

BrBr-

fastC C

H

H

H

H

Br Br

1,2-dibromoethane

Exercise

• Name and draw structural formulae for the products of the following halogenation process

CH3CH CHCH3 + Br2

+ Br2

3. Reaction with hydrogen halides• Hydrogen halide readily add to the carbon-

carbon double bond in the cold to give haloalkanes

• The rate of addition decreases in the order H-I > H-Br > H-Cl because the bond energy of the hydrogen halides increases from H-I to H-Cl

ethene Haloalkane

C C

H

H

H

H C C

H

H

H

H

H X

+ H X cold

• Addition halide to unsymmetrical alkenes– When propene reacts with a hydrogen halide such

as hydrogen bromide, HBr, there are two possible products

C C

H

H3C H

H

H Brcold

C C

H

H3C H

H

H Br

1-bromopropane (minor)

C C

H

H3C H

H

Br H

2-bromopropane (major)

cold

– Markovnikoff’s rule• In the addition of hydrogen halide to a carbon-carbon

double bond in an unsymmetrical alkene, the hydrogen atom of the hydrogen halide adds to the carbon atom of the double bond with the greatest number of hydrogen atoms.

C C

H

H3C H

H

Carbon atom with the greatest number of hydrogen atoms

Unsymmetrical double bond

– Example of Markovnikoff’s rule

H3CH2C C

HCH2 + HBr H3C

H2C

HC CH3

Br

not H3CH2C

H2C CH2

Br

CH3

+ HCl

CH3

Cl

CH3

Cl

not

– Stability of carbocation• When the double bond in propene attacks the

hydrogen in the acid, two different carbocations can be formed. The initial step in this reaction involves the formation of the carbocation. Two different carbocation can be formed.

C C

H

H3C H

H

+ H Br C C

H

H3C H

H

H

+ Br-

C C

H

H3C H

H

H

primary carbocation (less stable)

Secondary carbocation (more stable)

+ Br-

– The secondary carbocation are more stable than the primary. This is because in secondary there are two alkyl groups pushing electrons onto the positively charged carbon atom.

– The secondary carbocation combine with Br- to form 2-bromopropane.

C C

H

H3C H

H

H

+ Br-C C

H

H3C H

H

HBr

2-bromopropane

– The order of stability carbocations is as follows:

R C

R

R

> R C

R

H

> R C

H

H> H C

H

H

TertiaryCarbocation

SecondaryCarbocation

PrimaryCarbocation

MethylCarbocation

# The R group is alkyl group

increasing stability

Example

• Arrange the following alkenes in order of increasing reactivity on addition of hydrogen bromide.

– The relative stabilities are directly related to the stabilities of the intermediate carbocations formed.

CH3CH CHCH3H2C CH2

ethene(CH3)2C CH2

2-methylpropene but-2-ene

– 2-methylpropene is the most reactive because it forms the tertiary carbocation, the most stable carbocation

– But-2-ene forms the secondary carbocation

– Ethene forms the primary carbocation, thus it is the least reactive

– The order of increasing reactivity is ethene <but-2-ene < 2-methylpropene

(CH3)2CCH3

CH3CHCH2CH3

CH3CH2

Exercise

• When but-1-ene,

reacts with hydrogen bromide, what are the structures of the two possible intermediate carbocations formed?

• Which one of these two ions is more stable? Explain your answer.

• What is the major product?

H3CH2C C

HCH2

Exercise

• Name and draw the structural formula for the major products

CH3CH CHCH3 + HI

H3C C

CH3

CH2 + HBr

CH2

+ HCl

4. Reaction with bromine water• An alkene reacts readily at room temperature

with an aqueous solution of bromine to produce a mixture of products, bromoalcohol and dibromoalkane. The bromine water would be decolourised.

C C + 2 Br2 + H2O C C

Br OH

+ C C

Br Br

+ HBr

Bromoalcohol Dibromoalkane

• The addition reaction occurs in the way that yields more stable carbocation. Follow the Markovnikoff’s rule.

C C C C

Br OH

C C

Br Br

Bromoalcohol

Dibromoalkane

+ Br2 C C

Br

+ H2O

+ Br-

Exercise

• Write and overall equation for the reaction of propene with bromine water

CH3CH CH2 + 2Br2 + H2Oroom conditions

5. Reaction with concentrated suphuric (VI) acid• An alkene undergoes addition reaction with

concentrated sulphuric (VI) acid in the cold to give alkyl hydrogensulphate.

C C + H2SO4 C C

H

O

S

O

O O-H

Alkyl hydrogensulphate

cold

• When added to the water and warmed, the alkyl hydrogensulphate is converted to an alcohol. This is a hydrolysis reaction. Breaking up a compound by reacting it with water.

• The addition of concentrated sulohuric acid to unsymmetrical alkenes follows Markovnikoff’s rule

C C

H OH

+ H2O + H2SO4

C C

H

O

S

O

O O-H

Alkyl hydrogensulphate

warm

Alcohol

6. Reaction with stem (hydration)• When steam and ethene passed over a

catalyst, ethanol is produced. A temperature of 330 oC and a pressure of 60 atm are used in the presence of phosphoric acid catalyst.

• The hydration of alkenes follow Markovnikoff’s rule

H2C CH2 + H2OH3PO4

330 oC, 60 atmH3C

H2C OH

• For example, the hydration of propene

H3C C H3PO4

330 oC, 60 atm

C + H2O

H H

H H3C C C

H H

H

HOH

not

H3C C C

H H

H

H OH

Exercise

• Name and draw structural formula for the major products of these addition reactions of alkenes

CH3CH CHCH3 + H2OH3PO4

330 oC, 60 atm

H3C C

CH3

CH2+ H2O H3PO4

330 oC, 60 atm

7. Reaction with potassium manganate (VII) (Oxidation)

• An alkene react with dilute potassium manganate in an acidic or alkaline solution to give a diol

+ H2OC C + [O]dilute MnO4

- / H+

room temp C C

OH OH

8. Reaction with oxygen• Like all hydrocarbon, alkenes burns in air to

produce carbon dioxide and waterAlkene + O2 CO2 + H2O

Thank you..