In optically active alkyl halides, reaction is accompanied by a racemisation. The

carbocation formed in the slow step is -lybbridised in planar structure and attack of nucleophile may take place from elther side resulting in a mixture of products, one with the same configuration and other with inverted configuration. The isomer corresponding to inversion of configuration is present in slight excess as also depends upon the degree of shielding of reacting carbon from the front side.