ISOMERISMMr. Lane

Structural Isomerism These are when the atoms are

connected in different ways

Three types:a) Chain Isomerism

b) Position Isomerism

c) Functional Group Isomerism

Structural IsomerismChain Isomerism

Different arrangements of the carbon chain.

Hexane 2-Methylpentane

Structural IsomerismPosition Isomerism

Same carbon skeleton but the functional groups occupy different positions.

Butan-1-ol Butan-2-ol

Structural IsomerismFunctional Group Isomerism

Isomers have the same molecular formulae but have different functional groups and so belong to different homologous series.

Questions1. Draw and name all the chain isomers

for C6H14

2. Draw all the positional isomers for C6H13OH

Stereoisomerism In stereoisomerism, all the atoms are

connected to each other in the same way but they’re arranged differently in space

There are two types:a) Optical isomerism

(this will be covered in a future module)

b) Geometric isomerism Two different groups attached to each carbon,

either end of the double bond.

Stereoisomerism: Geometric isomers

With an alkane such as ethane, C2H6, there is free rotation about the carbon-carbon single bond.

Viewed along the carbon – carbon bond, the three hydrogen atoms of each methyl group can rotate with

respect to each other.

Stereoisomerism: Geometric isomers

In an alkene such as ethene, C2H4, the double bond prevents this rotation.

There is no rotation around the carbon-carbon double bond and the molecule is confined to a planar shape.

Stereoisomerism: Geometric isomers

OR

Stereoisomerism: Geometric isomers

OR

The geometric isomers can be described in terms of cis and

trans.Cis – Same sideTrans – Opposite sides

Stereoisomerism: Geometric isomers

Problem with cis and trans...

C CCl

H

Br

F

Which is cis, which is trans?

More sophisticated way of naming was introduced called the CIP rules (named after the chemists who developed the system, Cahn, Ingold and

Prelog.)

C CH

Cl

Br

F

CIP rules1. Look at the two groups at the end of the double

bond and rank the two groups in terms of the atomic number of the atoms concerned.

2. The atom with the higher atomic number takes precedence.

3. This is done for both ends of the double bond. 4. If the higher priority groups are on the same side of

the double bond, then it is the Z isomer (from the German zusammen which is together).

5. If they are on opposite sides then it is the E isomer( from the German entgegen which is opposite).

CIP rules

On left hand end of the double bond: C has a

higher priority than H. On right hand end of the

double bon: C has a higher priority than H. The carbons are on the same side of the double bond and so this is: (Z) -

but-2-ene

On left hand end of the double bond: C has a

higher priority than H. On right hand end of the

double bon: C has a higher priority than H.

The carbons are on opposite sides of the

double bond and so this is: (E) - but-2-ene

CIP rules

(E) - 2-bromo-but-2-ene

CIP rules

(E) - isomer (z) - isomer

CIP rules

(z) - isomer

NB:

Double bonds count as two

bonds.

CIP rules

(z) - isomer