Electrophilic substitution

Electrophilic substitution reactions involve an electrophile taking the place of a functional group within a molecule. While this often happens in aromatic compounds, it is not limited to them. For aromatic systems, such reactions are a standard way to attach new groups to a benzene ring, with a hydrogen atom usually being the one replaced. Key examples include nitration, halogenation, sulfonation, and the Friedel-Crafts alkylation and acylation processes.

In aliphatic compounds, the same basic idea applies: an electrophile swaps in for a functional group. This mirrors nucleophilic aliphatic substitution, except the attacking species is an electrophile instead of a nucleophile. The possible mechanisms are SE1, SE2 (front), SE2 (back), and SEi, which parallel the SN1 and SN2 mechanisms of nucleophilic substitution.

In the SE1 pathway, the substrate first splits into a carbanion and a positively charged organic fragment; the carbanion then quickly combines with the electrophile. The SE2 mechanism proceeds through a single transition state where both the breaking and forming bonds are present. Examples of electrophilic aliphatic substitution include nitrosation, ketone halogenation, keto-enol tautomerism, aliphatic diazonium coupling, carbene insertion into C-H bonds, and carbonyl alpha-substitution reactions.