Nucleophilic substitution reactions are of two types
( a) SN1 type( Unimolecular nucleophilic reactions proceed in two steps:
Rate, r = k [ RX). It is a first order reaction.
Reactivity order of alkyl halide towards SN1 mechanism 3 ° > 2 ° > 1 °
Polar solvents, low concentration of nucleophiles and weak nucleophiles favour SN1 mechanism.
In SN1 reactions, partial racemisation occurs due to the possibility of frontal as well as backside attack on planar carbocation.