Nucleophilic Substitution

Nucleophilic Substitution

Substitution reactions are defined as a displacement reaction where a functional group of one chemical compound is replaced by another group of a second compound. Nucleophilic substitution reactions are an important class of organic reactions that involve a molecule that can donate a pair of electrons. The general reaction scheme is shown below:

There are two main mechanisms in substitution reactions of alkylhalides, namely the unimolecular substitution (SN1) and bimolecular substitution (SN2) mechanisms. Below are reaction schemes that illustrate the difference between the two mechanisms.

Unimolecular substitution (SN1)

 

Bimolecular substitution (SN2)

Below are general rules that govern whether a substitution reaction between an alkyl halide and a nucleophile undergoes SN1 or SN2 mechanism:

1. The SN1 mechanism dominates when the reaction site is crowded due to steric effects.
2. Tertiary alkyl halides can only undergo substitution reaction via the SN1 mechanism.
3. The SN2 mechanism is favoured in neutral solution
4. The SN2 mechanism is not favoured in the presence of a strong base as a nucleophile.
5. Primary alkyl halides can only undergo substitution reactions via the SN2 mechanism.
6. Secondary alkyl halides can undergo either SN1 or SN2 mechanism, and strongly depends on the crowding at the reaction site of the alkyl halide.