Alkyl halides undergoing nucleophilic bimolecular substitution involve:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Inversion of configuration
In an SN2 (bimolecular nucleophilic substitution) reaction, the nucleophile attacks the carbon from the side opposite to the halogen, and the C–Nu bond forms as the C–X bond breaks, in one step.
The three other groups on the carbon flip over like an umbrella in the wind, so an optically active alkyl halide gives a product with the inverted configuration.
Why the other options are incorrect
Option (1) Formation of carbocation — Incorrect. A carbocation is formed in the slow first step of the SN1 reaction. SN2 is a single step with no intermediate.
Option (2) Racemic mixture — Incorrect. Racemisation goes with SN1: the planar carbocation can be attacked from either side. In SN2 the attack comes from one side only.
Option (4) Retention of configuration — Incorrect. Retention would need the nucleophile to enter on the same side as the leaving halogen; in SN2 it enters from the opposite side.
NCERT (2026-27 edition):
6.7.1 Reactions of Haloalkanes – “In case of optically active alkyl halides, the product formed as a result of SN2 mechanism has the inverted configuration as compared to the reactant. This is because the nucleophile attaches itself on the side opposite to the one where the halogen atom is present.”