Question

Out of ‘X’ and ‘Y’, which one will undergo racemisation and why?

Hint:

Racemisation = $S_N1$ (two-step reaction). Walden Inversion = $S_N2$ (one-step reaction).

Answer 1

Step 1: Understanding the Concept:
Racemisation is a characteristic of the \( S_N1 \) mechanism due to the formation of a planar carbocation intermediate.
Step 2: Detailed Explanation:
In an \( S_N1 \) reaction (like that of compound X), the first step is the formation of a carbocation.
A carbocation is \( sp^2 \) hybridized and planar.
The nucleophile can attack this planar intermediate from either side (front or back) with equal probability.
This results in a 50:50 mixture of two enantiomers, leading to a racemic mixture.
Although X (tert-butyl bromide) is achiral, if an optically active \( 3^\circ \) halide reacted similarly, it would undergo complete racemisation.
Step 3: Final Answer:
Compounds reacting via the \( S_N1 \) mechanism (like X) undergo racemisation because of the planar carbocation intermediate.