Isomerism

Isomerism is the existence of two or more compounds that share the same molecular formula but have different arrangements of their atoms. Each such compound is called an isomer. Malay: keisomeran · Chinese: 同分异构.

At SPM level the arrangement that differs is the way the carbon chain and any functional group are joined, which is called structural isomerism. The molecules contain exactly the same number of each kind of atom, so their molecular formula is identical, but because the atoms are connected in a different order the compounds are genuinely different substances with different names and, often, different physical properties such as boiling point. As the number of carbon atoms in a molecule grows, the number of possible isomers grows quickly, which is one reason carbon forms so many compounds. A branched isomer usually has a lower boiling point than its straight-chain partner, because its more compact shape gives weaker forces between molecules.

Example. Butane and 2-methylpropane both have the molecular formula C₄H₁₀. In butane the four carbon atoms lie in a straight chain, while in 2-methylpropane three carbons form a chain with the fourth as a branch. Same formula, different structures, so they are structural isomers.

Confusion to avoid. Do not treat isomers as the same compound just because they share a molecular formula, and do not confuse isomerism with members of a homologous series. Isomers have the identical molecular formula; successive members of a homologous series differ by CH₂ and so have different molecular formulae.

Isomerism explains why a single molecular formula can name more than one compound, an idea that supports the naming and structural-formula work throughout Carbon Compounds and appears in the structure questions of SPM Chemistry.