Ch 8: Nucleophilic Substitution

Chapter 8: Nucleophilic Substitution

Overview
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 σ bond to the nucleophile
breaking of the σ 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 hybridised in order for these reactions to occur.
This is important since students often want to make use of nucleophilic substitution reactions of vinyl (C=C-LG) or aryl (Ar-LG) 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 favourable.