2023 H2 Chem P1 Q18 — Introduction to Organic Solution

The answer is C (5).
\(\text{C}_5\text{H}_{10}\) has one degree of unsaturation. Since ring structures are excluded, each isomer must contain one C=C double bond, so all are alkenes.
The five acyclic constitutional isomers (shown in the diagram) are pent-1-ene, pent-2-ene, 2-methylbut-1-ene, 3-methylbut-1-ene and 2-methylbut-2-ene.
cis-pent-2-ene and trans-pent-2-ene are stereoisomers, not constitutional isomers, so pent-2-ene is counted once.
• A is wrong: 2 counts only the unbranched alkenes (pent-1-ene and pent-2-ene) and leaves out the three branched ones.
• B is wrong: 3 counts only the branched alkenes (2-methylbut-1-ene, 3-methylbut-1-ene and 2-methylbut-2-ene) and leaves out the two unbranched ones.
• D is wrong: 6 counts cis- and trans-pent-2-ene separately, but they have the same connectivity and are stereoisomers, not constitutional isomers.