2021 O Level Chem P1 Q35 — Organic Chemistry / Isomerism Solution

Answer: B. Isomers have the same molecular formula but a different structural arrangement. Butane is C₄H₁₀, so an isomer of butane must also be C₄H₁₀.

The correct structure is 2-methylpropane, (CH₃)₃CH — four carbons and ten hydrogens in a branched chain, with only single bonds. It has the same molecular formula as butane but a branched skeleton, so it is an isomer.

The other structures are not isomers of butane: a four-carbon ring (cyclobutane) is C₄H₈, and any structure containing a C=C double bond is an alkene, C₄H₈ — both have two fewer hydrogens than butane.