Optical isomers and chirality

In a nutshell

Optical isomers are a type of stereoisomer. They contain a chiral centre and they rotate plane-polarised light.

Properties of optical isomers

Optical isomers are a type of stereoisomer. Some of their defining properties are:

They have a chiral centre - an atom with four different atoms or group of atoms attached to it.
They are mirror images of each other - they cannot be superimposed.
They are optically active - they rotate plane polarised light.
They are also known as enantiomers.

Optically active

As optical isomers are optically active they rotate plane-polarised light. This means that light vibrates in one direction, unlike normal light which vibrates in all directions. One enantiomer rotates in the clockwise direction, whilst the other rotates in an anti-clockwise direction.

Racemate

A racemate is a mixture of enantiomers in equal amounts. Racemates do not display optical activity because the enantiomers cancel the light rotating effect of each other.

Reaction products of planar molecules give rise to a racemate. The planar molecules must be unsymmetrical for a racemic mixture to arise. A symmetrical planar molecule will not display optical isomerism.

Example

Chemistry; Organic chemistry II; KS5 Year 12; Optical isomers and chirality

Drawing optical isomers

procedure

1.	Identify the chiral centre.
2.	Draw one of the enantiomers in a tetrahedral shape.
3.	Draw the mirror image next to it.
4.	Draw a dashed line in-between the two enantiomers.

Chemistry; Organic chemistry II; KS5 Year 12; Optical isomers and chirality

Multiple chiral centres

Some molecules have more than one chiral centres. Molecules with more than one chiral centre will have more than two optical isomers. The maximum number of optical isomers can be found by using the formula:

$Total \,number\, of \,optical\, isomers = 2^n \\where \,n \,is\,the \,number \,of\,chiral\,centres$

Example

Determine the total number of isomers for $2$ -methylpentan- $4$ -ol.

Firstly, find the number of chiral centres. For this example:

$n = 2$

Then, use the number of chiral centres to find the total number of optical isomers:

$Total \,number\, of \,optical\, isomers = 2^n = 2^2 = \underline4$

The diagram below displays all the optical isomers for $2$ -methylpentan- $4$ -ol.

Chemistry; Organic chemistry II; KS5 Year 12; Optical isomers and chirality