7.1.1 Optical Isomerism

Introduction to Isomerism

• Structural isomerism: Isomers differ in the connectivity of atoms. It includes positional, chain, and functional isomers.
• Stereoisomerism: Isomers differ in the spatial arrangement of atoms. It includes geometrical (cis-trans) and optical isomers. Structural isomerism can be classified into three types: positional isomerism, chain isomerism, and functional isomerism.

Stereoisomerism is divided into two types: geometrical isomerism and optical isomerism.

The diagram below provides a summary of the different types of isomerism:

Optical Isomerism Overview

Optical isomers are molecules that exist as non-superimposable mirror images and are not identical to one another.

• Optical isomerism is a form of stereoisomerism that arises due to chirality. This type of isomerism occurs in molecules with a chiral carbon, also called an asymmetric carbon, which is a carbon atom bonded to four distinct functional groups.

• Chirality occurs in molecules that have a chiral centre, typically a carbon atom bonded to four different groups.

• Molecules with a single chiral centre will exist as optical isomers or enantiomers.

• Enantiomers are non-superimposable mirror images of each other. They differ in their interaction with plane-polarised light but are chemically and physically identical in all other aspects.

Chirality and the Asymmetric Carbon

• A chiral centre is typically a carbon atom attached to four different groups. This makes the molecule asymmetric and unable to be superimposed on its mirror image.
• For example, consider a molecule with the following groups attached to carbon: W | X - C - Y | Z

Here, if W, X, Y, Z are all different, the carbon atom is chiral.

The two mirror-image forms of this molecule cannot be made to overlap perfectly, no matter how you rotate them. This property is what gives rise to optical isomerism.

Enantiomers and Plane-Polarised Light

Enantiomers rotate plane-polarised light in different directions:

• One enantiomer will rotate light in a clockwise direction, called dextrorotatory (denoted as (+)-isomer).
• The other enantiomer will rotate light in a counterclockwise direction, called laevorotatory (denoted as (-)-isomer).
• A polarimeter is used to measure the direction and degree of light rotation caused by a sample of an enantiomer.

Identifying and Drawing Enantiomers

To draw enantiomers:

1. Identify the chiral centre.
2. Arrange the four different groups around the chiral centre in a tetrahedral geometry.
3. Draw a mirror image of this structure to represent the second enantiomer. Here's an example for 2-chlorobutane:

• One enantiomer will have a particular orientation of groups (e.g., Chlorine pointing upward).
• The mirror image will have chlorine pointing downward, with all other groups reversed.

Optical Activity in 2-Chlorobutane

Among the isomers of $C_4H_9Cl$, 2-chlorobutane exhibits optical isomerism because it contains a chiral centre (the second carbon in the chain is attached to four distinct groups: $-Cl$,$-H$, $-CH_3$, and $-CH_2CH_3$).

The Importance of Optical Isomerism in Biochemistry

• Enzymes, drugs, and other biological molecules are often chiral and stereospecific.
• In many cases, only one enantiomer of a chiral molecule will be biologically active or will interact correctly with a target molecule in the body.

Optical Activity

Optical isomers have the same chemical and physical properties as each other, except for their effect on plane polarised light.

A light beam consists of waves that vibrate in all planes. Some substances have the ability to remove all the light waves except those vibrating in a certain plane - producing plane polarised light.

Plane polarised light: light in which all the waves vibrate in the same plane. This diagram shows you how:

Optical isomers can rotate plane polarised light. They are described, therefore, as being optically active. The 2 enantiomers of an optically active molecule will rotate the plane polarised light in opposite directions.One enantiomer rotates it in a clockwise direction, and the other rotates in an anticlockwise direction.