Chirality

Full Notes

What Is Optical Isomerism?

Optical isomerism is a form of stereoisomerism where molecules have the same structural formula but different spatial arrangements.

If a carbon atom is bonded to four different atoms or groups it is called a chiral carbon (giving a chiral centre in a molecule).

There are two possible ways the atoms or groups can be arranged giving two possible stereoisomers. The isomers are non-superimposable mirror images of one another and are called enantiomers.

Example 2-hydroxypropanoic acid (Lactic Acid)

Chiral carbon: CH₃CH(OH)COOH

Two enantiomers exist as mirror images.

Identifying Chiral Centres

To identify a chiral centre:

• Look for a carbon atom bonded to four different groups.
• These groups must differ in what they are also connected to, not just whether they are the same element (for example CH₃ and CH₂CH₃ are different groups if bonded to the same carbon).

For Example 2-hydroxypropanenitrile

The middle carbon is chiral: bonded to H, OH, CH₃, and CN.

Summary

• A chiral centre is a carbon attached to four different groups.
• Molecules with a chiral centre exist as non-superimposable mirror images called enantiomers.
• Check connectivity carefully when deciding if four groups are different.