In which reaction, the inversion of configuration usually takes place?

The correct answer is Option (4) → S_N2 reaction

Nucleophilic substitution of alkyl halides takes place by two mechanisms, which have different stereochemical results:

• S_N2 (one step): the nucleophile attacks the carbon from the side opposite to the leaving halogen. In the transition state the carbon is bonded to both groups, and as the halide leaves, the other three groups flip over like an umbrella in the wind. An optically active halide gives a product with inverted configuration (Walden inversion).

• S_N1 (two steps): the halide first leaves to form a planar carbocation. The nucleophile can attack it from either face, so an optically active halide gives a racemic mixture.

So inversion of configuration usually takes place in the S_N2 reaction.

Why the other options are incorrect

• Option (1) S_N1 reaction — Incorrect. Attack on the planar carbocation from both sides gives racemisation, not inversion.

• Option (2) Electrophilic Substitution Reaction — Incorrect. This is typical of aromatic rings, where the substitution takes place at a planar sp² carbon, so there is no configuration to invert.

• Option (3) Elimination reaction — Incorrect. It removes H and X to form an alkene; no group replaces the halogen at that carbon.

NCERT (2026-27 edition):
Substitution Nucleophilic Bimolecular (S_N2) – “… As a result configuration is inverted, the configuration of carbon atom under attack inverts in much the same way as an umbrella is turned inside out when caught in a strong wind. This process is called as inversion of configuration.”

“… Thus, S_N2 reactions of optically active halides are accompanied by inversion of configuration.”