2023 O Level Chem P1 Q38 — Alkenes / Esters Solution

Answer: A. The stem gives the alkene as \(\text{CH}_2\text{CHCH}_2\text{CH}_3\) — i.e. but-1-ene, \(\text{CH}_2{=}\text{CHCH}_2\text{CH}_3\) (the double bond is on C1–C2, not on C2–C3).

Adding steam (H₂O) across the double bond with an acid catalyst can give either regiochemistry at O Level:

• Markovnikov addition — OH to the more-substituted carbon — gives butan-2-ol, \(\text{CH}_3\text{CH(OH)CH}_2\text{CH}_3\).
• Anti-Markovnikov addition — OH to the terminal carbon — gives butan-1-ol, \(\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH}\).

Esterifying either alcohol with propanoic acid (\(\text{CH}_3\text{CH}_2\text{COOH}\)) gives a propanoate ester with the propanoyl group on the acid side. Structure 1 (n-butyl propanoate, from butan-1-ol) and structure 2 (sec-butyl propanoate, from butan-2-ol) are both valid possibilities for Q.

Structure 3 has the wrong carbon chain length on the alcohol side — it doesn't come from a four-carbon alcohol, so it cannot be Q. Only 1 and 2 are correct — option A.