Explain stereochemistry of `S_(N)1` reaction with suitable example.

Explanation

In `S_(N)1` reaction, if the substrate i.e., alkyl halide is optically active, then it will resuly in partial racemisation. The carbocation is obtained as an intermediate product has different groups and is planar and thus, the nucleophile can attack from either side. If the nucleophile attacks from same side of leaving groups, the retention of configuration takes place and if it attacks from opposite side of leaving group, inversion of configuration takes place. The attack from opposite side is more favourable and thus enantiomers are obtained in unequal amount. If the enantiomers are obtained in 50 : 50 proportion, it is called recemic mixture and the product is optically inactive. The process is known as racemisation. Racemic mixture is denoted as `(pm)`.

The hydrolysis of optically active 2-bromobutane results in formation of `(pm)` -Butan-2-ol.