2024 H2 Chem P1 Q20 — Alkene Solution

The answer is B (1 only).
HBr adds to propene by electrophilic addition through a carbocation. \(\text{H}^+\) can add to either carbon of the C=C:
• adding to the terminal \(\text{CH}_2\) gives the secondary carbocation \(\text{CH}_3\overset{+}{\text{C}}\text{HCH}_3\), which then gives 2-bromopropane;
• adding to the middle carbon gives the primary carbocation \(\text{CH}_3\text{CH}_2\overset{+}{\text{C}}\text{H}_2\), which would give 1-bromopropane.
• Statement 1 is correct: the secondary carbocation has two electron-donating methyl groups that disperse its positive charge, so it is more stable than the primary carbocation and forms faster. This is why 2-bromopropane is the main product.
• Statement 2 is incorrect: the carbocation is planar and steric hindrance plays little part here; the product ratio is decided by carbocation stability. Any steric effect of the methyl group would hinder attack at the middle carbon, which works against forming 2-bromopropane.
• Statement 3 is incorrect: an alkyl group is electron-donating, not electron-withdrawing. An electron-withdrawing group would destabilise the secondary carbocation.
• A is wrong: it includes statement 2, which is incorrect.
• C is wrong: statements 2 and 3 are both incorrect.
• D is wrong: statement 3 is incorrect, and statement 1, which is correct, is left out.