Previous Year NEET Question- Haloalkanes L-1

Question: In an $S_N 1$ reaction on chiral centres, there is#

A) inversion more than retention leading to partial racemisation

B) $100 %$ retention

C) $100 %$ inversion

D) $100 %$ racemisation.

Answer: inversion more than retention leading to partial racemisation#

Sol:#

In case of optically active alkyl halides, $\mathrm{S}{\mathrm{N}} 1$ reaction is accompanied by racemisation. The carbocation formed in the slow step being $s p^2$ hybridised is planar and attack of nucleophile may take place from either side resulting in a mixture of products, one having the same configuration and other having inverted configuration. The isomer corresponding to inversion is present in slight excess because $\mathrm{S}{\mathrm{N}} 1$ also depends upon the degree of shielding of the front side of the reacting carbon.