• To explain the nucleophilic substitution reaction of alkyl halides, the time interval between bond breaking and bond making steps can be considered.
• When the breaking of the C-X bond and the formation of the new bond to the nucleophile takes place simultaneously, the nucleophilic substitution reaction of the alkyl halide takes place as a one step reaction.
Accordingly, the one step reaction can be presented as follows:
• When the formation of the new bond to the nucleophile takes place after the breaking of the C-X bond the nucleophlic substitution reaction of the alkyl halide takes place as a two step reaction.
Accordingly, the reaction that takes place by two steps can be presented as follows.
The reaction that takes place by two steps goes through an intermediate carbocation. On considering the stability of the carbocation formed, the tertiary alkyl halides (R1, R2, R3 = alkyl) which are able to form a more stable tertiary carbocations undergo nucleophilic substitution in two steps. The primary alkyl halides (R1, R2 = H, R3 = H or alkyl) undergo nucleophilic substitution reactions in one step as they are unable to form a stable intermediate carbocation.
The pathway taken by the secondary alkyl halides (R1 = H, R2, R3 = alkyl) depend on the reaction conditions.
Vinyl and phenyl carbocations are unstable and therefore, they do not react by the two step pathway. They also do not react by the one step path because the C-X bond is stronger than in alkyl halides due to its double bond character. This can be shown by resonance.
