Return to Contents Chapter 8: Nucleophilic Substitution Ch 8 contents
Overall a nucleophilic susbtitution can be represented as follows:
Nucleophilic substitution at sp3 C
There are two fundamental events in a nucleophilic substitution reaction:
  1. formation of the new σ bond to the nucleophile
  2. breaking of the σ bond to the leaving group
Depending on the relative timing of these events, two different mechanisms are possible: 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 SN1 or SN2, though they may be differences in regiochemistry and / or stereochemistry  (which can provide some evidence as to which mechanistic path is occuring).


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.

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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary