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 .