2022 H2 Chem P1 Q22 — Halogenoalkane Solution

The answer is B (1 only).

One enantiomer of (1-chloroethyl)benzene gives a 50 : 50 (racemic) mixture of the two enantiomers of 1-phenylethanol, so the reaction goes by the \(\text{S}_\text{N}\text{1}\) mechanism. The C-Cl bond breaks to give the carbocation \(\text{C}_6\text{H}_5\text{CH}^+\text{CH}_3\), which is trigonal planar at the positive carbon, and \(\text{OH}^-\) attacks either face with equal chance.

• Statement 1 is correct: the trigonal planar carbocation intermediate has a plane of symmetry (the plane containing the positive carbon and its three bonded atoms).
• Statement 2 is incorrect: there is only one intermediate. Because it has a plane of symmetry, its mirror image is superimposable on it, so it is not a pair of mirror-image intermediates. The two products come from attack on its two faces.
• Statement 3 is incorrect: \(\text{S}_\text{N}\text{2}\) goes by back-side attack in one step and gives only the product with inverted configuration, not a racemic mixture.

• A is wrong: statements 2 and 3 are false.
• C is wrong: statement 2 is false, and statement 1, which is true, is left out.
• D is wrong: the racemic product rules out \(\text{S}_\text{N}\text{2}\).