JEE Main 2024 · 27 January · morning · Single correct · +4 / −1
The correct statement regarding nucleophilic substitution reaction in a chiral alkyl halide is ;
- (a) Retention occurs in S_N 1 reaction and inversion occurs in S_N 2 reaction.
- (b) Racemisation occurs in S_N 1 reaction and retention occurs in S_N 2 reaction.
- (c) Racemisation occurs in both S_N 1 and S_N 2 reactions.
- (d) Racemisation occurs in S_N 1 reaction and inversion occurs in S_N 2 reaction.
Answer and solution
Answer (d) Racemisation occurs in S_N 1 reaction and inversion occurs in S_N 2 reaction.
Solution
Chemistry ma’am · 4 steps
Ask where the nucleophile can attack: a flat carbocation is open on both faces, SN2 only from the back.
- SN1 path. The halide leaves first and gives a carbocation. Its carbon is sp² and planar, so the nucleophile can attack from either face with nearly equal chance.
- SN1 outcome. Attack from the two faces gives both enantiomers, so the product is a (largely) racemic mixture. This is racemisation.
- SN2 path. There is no intermediate. The nucleophile attacks from the side directly opposite the leaving halide, in one step. The other three groups flip over like an umbrella in the wind, so the product has the opposite configuration. This is Walden inversion.
Mechanism Key species Direction of attack Stereochemical result S_N1 planar carbocation either face racemisation S_N2 single transition state back side only inversion - So. A chiral alkyl halide racemises in S_N1 and inverts in S_N2, which is option (d).
The idea to keep. A flat carbocation can be attacked from both faces, so SN1 gives racemisation. One-step backside attack in SN2 always gives inversion.
Answer from the NTA final answer key. Solution by PrepWiser.