Isomerism: Structural and Stereoisomerism

Isomers are molecules that share the same molecular formula but have different structural arrangements of their atoms. This seemingly simple definition conceals enormous chemical consequences: isomers can have different physical properties, different chemical reactivities, and even different biological effects.

At Edexcel A-Level, you need to understand two main categories: structural isomerism and stereoisomerism.

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Structural Isomerism

Structural isomers have the same molecular formula but different structural formulae — the atoms are connected in a different order. There are three types you need to know.

Chain Isomerism

Chain isomers differ in the arrangement of the carbon skeleton. The carbon chain can be straight or branched.

For example, C₄H₁₀ has two chain isomers:

• Butane: CH₃CH₂CH₂CH₃ (straight chain)
• Methylpropane (2-methylpropane): CH₃CH(CH₃)CH₃ (branched)

As the number of carbons increases, the number of possible chain isomers grows rapidly. C₅H₁₂ has three isomers; C₆H₁₄ has five; C₁₀H₂₂ has seventy-five.

Branched isomers tend to have lower boiling points than their straight-chain counterparts because their more compact shape reduces the surface area available for van der Waals interactions.

Position Isomerism