a)In $SN^1$ reaction there is considerable charge on the carbon while there is little or no charge on the carbon in $SN^2$ reaction.
b)In $SN^1R^+$ reacts with solvent rather than stronger nucleophiles may be
c)In $SN^1$ as solvent polarity increases intimate ion pair attraction increases and so inversion increses
d)As nucleophilicity of solvent decreses raceimisation increses
