chemrevise.org 20/08/2013

1

Optical Isomerism

N GoalbyChemrevise.org

Types of isomerism

Isomerism

Structural isomerism

Stereoisomerism

Geometric

isomerism

Optical isomerism

chemrevise.org 20/08/2013

2

Optical Isomerism

Optical isomerism occurs in carbon compounds with 4 different groups of atoms attached to a carbon (called an asymmetric carbon).

H

H H

H

H

HH

H

H

H H

H

H

HH

H

Mirror images Non superimposable

Compounds that show optical isomerism are called chiral compounds.

mirror

not isomers (same compound)

key

Optical Isomers

H

H H

H

H

HH

H

Can be rotated and are superimposable

chemrevise.org 20/08/2013

3

C

H

C

H

C

H

H

H

C

H H

H

O

H

H

These four groups are arranged tetrahedrally around the carbon.

CH5C2

CH3H

OH

CC2H5

CH3H

OHThis causes two different isomers that are not superimposable to be formed. They are mirror images key

Chiral Carbons

The central asymmetric carbon is called the chiral centre.

Optical isomers have similar physical and chemical properties, but they rotate plane polarised light in different directions.

One optical isomer will rotate light clockwise and the other isomer will rotate light in an anti-clockwise direction.

key

Polarising filter

Unpolarised light

Light source

Plane polarised light Sample

solution of chiral substance

Plane polarised light after rotation

Angle of rotation of plane polarised light

α

Analysing filter

chemrevise.org 20/08/2013

4

One optical isomer will rotate plane-polarised light clockwise (+)(called dextrorotatory). The other will rotate it anticlockwise(-)(called laevorotatory).

A mixture of equal amounts of the two optical isomers will not rotate plane-polarised light. The mixture is called a racemic mixture or a racemate

CH3C CN

H

OH

mirror

These stereoisomers are called enantiomers.

key

CH3C CN

H

OH

Brief method•first put water in sample tube and rotate analyser to block out light•replace water with sample. If the sample is chiral light will be seen•rotate analyser again to block out light. The amount the analyser is rotated is the amount the light has been rotated.

Method for using polarimeter

chemrevise.org 20/08/2013

5

Chemical Reactions and Optical Isomers

Formation of a racemate:

A racemate will often be formed in a reaction when a triagonal planar reactant or intermediate is approached from both sides by an attacking species

:CNNC:

C

CH3

O

H

e.g. the reaction of ethanal and HCN

The carbonyl group is planar and can be attacked from both sides

key

:CNNC:

C

CH3

O

H

C

CH3 CN

OH

H

C

NC CH3

OH

H

There is an equal chance of either enantiomer forming so a racemate forms. No optical activity is seen

Chemical Reactions and Optical Isomers

key

chemrevise.org 20/08/2013

6

CH2 CH CH2 CH3

H Brδ+ δ-

CH3

C+

CH2

CH3

H :Br -

C+

C CH2 CH3

H

HH

H

:Br -

C

CH3

Br

H

CH2CH3

C

CH3

Br

H

CH2CH3

CH2 CH2 CH2 CH3

Br

The bromide can attack this planar carbocation from both sides leading to a racemate

If the alkene is unsymmetrical, addition of hydrogen bromide can lead to isomeric products.

Addition of HBr to But-1-ene

Nucleophilic substitution mechanismBromoethane [Primary haloalkane] + aqueous hydroxide ions

δδδδ+ δδδδ-CH3

H

BrC

H

-OH

CH3

H

OHC

HBr

-

This is the mechanism we learnt in module 2. There are, however, two major types of nucleophilic substitution and the products formed by the different mechanisms can show different optical activity.

The two mechanisms are called SN1 and SN2

EXTRA

chemrevise.org 20/08/2013

7

C

CH2CH3

H

CH3 Br

Nucleophilic substitution mechanism, SN1

2-bromobutane [Secondary haloalkane] + aqueous hydroxide ions

δδδδ+ δδδδ-

-OH

Br-

CH3

H

C

C2H5

+

EXTRA

The Br breaks off leaving a planar carbocation intermediate

The OH- ion can then attack from either side resulting in different enantiomers and a racemate forms

C

C2H5CH3

H

OH

C

CH3H5C2

H

OH

or

C

CH2CH3

H

CH3 Br

Nucleophilic substitution mechanism, SN2Bromoethane [Primary haloalkane] + aqueous hydroxide ions

δδδδ+ δδδδ-

-OH Br

-

CH3

C

H H

BrHO

EXTRA

In the SN2 mechanism no intermediates are formed and the reaction occurs via a transition state.

If the reactant was chiral then during the reaction the opposite enantiomer would form. The animation on the next slide shows this happening.

C

CH2CH3

H

OH CH3

chemrevise.org 20/08/2013

8

CCH3

H

H3CH2C

Br

CCH3

H

H3CH2C

HO Br

C

CH3

H

H3CH2C

HO Br

C

CH3

H

CH2CH3

HO Br

-HO:

Br-If the reactant was chiral then during the reaction the opposite enantiomer would form.

CCH3

H

CH2CH3

HO

Ibuprofen

CH

C

CH

CH

C

CH

CH2

CH

CH3

CH3

CH

CH3

C

OH

O

Drug action and optical isomersDrug action may be determined by the stereochemistry of the molecule. Different optical isomers may have very different effects

chemrevise.org 20/08/2013

9

Thalidomide

One enantiomer of thalidomide causes birth defects in unborn children whilst the other had useful sedative problems. Unfortunately is was given in a racemic mixture when first used.

C

C

C

C

C

C

C

C

O

O

N C

C

C C

C

O

ON

HH

H

H

HH

HH

H

H

C

C

C

C

C

C

C

C

O

O

N C

C

C C

C

O

O

N

H

H

H

H

HH

H HH

H

Where is Chiral carbon?

R thalidomide (dangerous drug) S thalidomide (effective drug)