An $${S_N}2$$  reaction at an asymmetric carbon of a compound always gives

Solution :
$${S_N}2$$  reactions proceed with inversion of configuration. Since the attacking nucleophile is not necessarily the same as that of leaving group, the product cannot be enantiomer of the substrate and thus necessarily will not have opposite optical rotation. Moreover since only one product is obtained, we can not obtain diastereomers.