2019 O Level Chem P1 Q33 — Organic Chemistry / Isomerism Solution

Answer: C (the diagram showing n-butane and 2-methylpropane). Butane, C₄H₁₀, has exactly two structural isomers: the straight chain n-butane, CH₃–CH₂–CH₂–CH₃, and the branched 2-methylpropane, where a propane backbone carries a methyl branch on the middle carbon.

A correct diagram must show these two genuinely different structures, not the same molecule drawn twice in different orientations. The diagram showing the unbranched chain alongside the branched structure is the one with two different connectivities, both C₄H₁₀, so it is correct.

The distractors show the same molecule drawn twice — for example n-butane bent into an L-shape next to n-butane drawn straight, or 2-methylpropane with the branch flipped to the other side. Bending a chain or flipping a branch does not change which atom is bonded to which, so those are identical compounds, not isomers. Only the diagram with two different structures (option C) shows the two isomers of butane.