2018 H2 Chem P1 Q25 — Halogenoalkane Solution

The answer is A (1, 2 and 3).
• Statement 1 is correct: in \(\text{S}_\text{N}2\) the nucleophile attacks from the side opposite the leaving group, in a single step, as the C-Cl bond breaks, so the configuration about the carbon is inverted.
• Statement 2 is correct: in \(\text{S}_\text{N}1\) the C-Cl bond breaks first to give a trigonal planar carbocation. The nucleophile attacks either face with equal probability, giving a 1 : 1 mixture of the two enantiomers (a racemic mixture), so racemisation occurs.
• Statement 3 is correct: in a tertiary chloroalkane the three alkyl groups are electron-donating, which disperses the positive charge and stabilises the tertiary carbocation, so \(\text{S}_\text{N}1\) is favoured. (The three bulky alkyl groups also hinder rear-side attack, disfavouring \(\text{S}_\text{N}2\).)
All three statements are correct.
• B (1 and 2 only) is wrong: it leaves out statement 3, which is correct.
• C (1 and 3 only) is wrong: it leaves out statement 2, which is correct.
• D (2 and 3 only) is wrong: it leaves out statement 1, which is correct.