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.
