SN1 reaction:
In the case of optically active alkyl halides, SN1 reactions are accompanied by racemisation. This is because the attack of the nucleophile may be accomplished from either side resulting in a mixture of products, one having the same configuration and the other having opposite configuration.

SN2 reaction:
In the case of optically active alkyl halides, there is inversion of configuration. This is because the nucleophile attaches itself on the side opposite to the one where the halogen atom is present
