(1) SN1 reaction:
Consider the alkaline hydrolysis of tert butyl bromide to give tert butanol.
CH3C(CH3)2−Br+NaOHΔ−→CH3C(CH3)2−OH+NaBr
The
rate of the reaction is proportional to the concentration of tert butyl
bromide only and is independent of the concentration of hydroxide ion.
Rate =K[CH3C(CH3)2−Br]
The
order of the reaction is 1. In the rate determining step, carbon to Br
bond breaking is followed by the carbon to hydroxide ion bond formation.
Hence, the reaction is unimolecular as only tert butyl bromide is
involved in the rate determining step.
Thus SN1 reaction is unimolecular nucleophilic substitution reaction.
(2) SN2 reaction
Consider the alkaline hydrolysis of methyl bromide to give methanol.
CH3−Br+NaOHΔ−→CH3−OH+NaBr
The rate of the reaction is proportional to the concentration of methyl bromide as well as the concentration of hydroxide ion.
Rate =K[CH3−Br][OH−]
The
order of the reaction is 2. In the rate determining step, the carbon to
hydroxide ion bond formation and carbon to Br bond breaking are
simultaneous processes. Hence, the reaction is bimolecular as both
methyl bromide and NaOH are involved in the rate determining step.
Thus SN2 reaction is bimolecular nucleophilic substitution reaction.