In an reaction on chiral centres, there is [2015]
(1)
In case of optically active alkyl halides, reaction is accompanied by racemisation. The carbocation formed in the slow step being 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 also depends upon the degree of shielding of the front side of the reacting carbon.