Haloalkanes and Haloarenes

The correct statement regarding nucleophilic substitution reaction in a chiral alkyl halide is ;

1. Retention occurs in SN1 reaction and inversion occurs in SN2 reaction.
2. Racemisation occurs in SN1 reaction and retention occurs in SN2 reaction.
3. Racemisation occurs in both SN1 and SN2 reactions.
4. Racemisation occurs in SN1 reaction and inversion occurs in SN2 reaction.

Answer

(4) Racemisation occurs in SN1 reaction and inversion occurs in SN2 reaction.

Solution

SN1: planar (sp²) carbocation is formed

∵ Nu- attacks both faces equally ⇒ (+) and (−) forms ⇒ racemisation

SN2: one step, Nu- attacks from the back side (opposite to X)

∵ the three groups flip like an umbrella ⇒ inversion (Walden inversion)

Answer: (4)