Overview

Chapter 8: Nucleophilic Substitution

Overall a nucleophilic susbtitution can be represented as follows: Nucleophilic substitution at sp3 C

There are two fundamental events in a nucleophilic substitution reaction:

formation of the new s bond to the nucleophile
breaking of the s bond to the leaving group

Depending on the relative timing of these events, two different mechanisms are possible:

Bond breaking to form a carbocation preceeds the formation of the new bond : S_N1 reaction
Simultaneous bond formation and bond breaking : S_N2 reaction

A third possibility, where the nucleophile adds then the leaving group departs can't occur because it would require that the electrophilic C become pentavalent.

However, it is also useful to appreciate that the overall outcome of the transformation (i.e. the substitution) is often the same regardless of whether it is S_N1 or S_N2, though they may be differences in regiochemistry and / or stereochemistry (which can provide some evidence as to which mechanistic path is occuring).

IMPORTANT

In general terms the C bearing the LG needs to the sp³ hybridized in order for these reactions to occur.
This is important since students often want to make use of nucleophilic substitution reactions of vinyl or aryl systems which are not generally effective.
The reasons for this are that the adjacent p bonds are electron rich and will repel the Nu^- and / or that the vinyl and phenyl carbocations are not very favorable.