Isomerism

This lesson provides a comprehensive treatment of isomerism in organic chemistry, covering structural isomerism (chain, position, and functional group isomerism), E/Z (geometric) isomerism with the Cahn-Ingold-Prelog priority rules, and optical isomerism (chiral centres, enantiomers, racemic mixtures, and polarimetry). Isomerism is a unifying theme that connects nomenclature, bonding, reaction mechanisms, and biological activity. A secure understanding of all types of isomerism is essential for A-Level Chemistry and is tested explicitly in both short-answer and extended-response questions.

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What Is Isomerism?

Key Definition: Isomers are molecules that have the same molecular formula but a different arrangement of atoms. This different arrangement can affect their physical properties, chemical reactivity, and biological activity.

Isomerism is divided into two broad categories:

1. Structural isomerism — the atoms are bonded together in a different order.
2. Stereoisomerism — the atoms are bonded in the same order but are arranged differently in space.

Diagram description: Imagine a branching tree. At the top is 'Isomerism'. This branches into 'Structural isomerism' (left) and 'Stereoisomerism' (right). Structural isomerism then branches into 'Chain', 'Position', and 'Functional group'. Stereoisomerism branches into 'E/Z (geometric)' and 'Optical'.