Electrophilic substitution reactions are chemical reactions in which an electrophile displaces a functional group in a compound, which is typically, but not always, a hydrogen atom. Electrophilic aromatic substitution reactions are characteristic of aromatic compounds, and are important ways of introducing functional groups of benzene rings. The other main type of electrophilic substitution reaction is an electrophilic aliphatic substitution reaction.
Electrophilic aromatic substitution
In electrophilic substitution in aromatic compounds, an atom appended to the aromatic ring, usually hydrogen, is replaced by an electrophile. The most important reactions of this type that take place are aromatic nitration, aromatic halogenation, aromatic sulfonation and acylation and alkylating Friedel-Crafts reactions. It further consists of alkylation and acylation.
Electrophilic aliphatic substitution
In electrophilic substitution in aliphatic compounds, an electrophile displaces a functional group. This reaction is similar to nucleophilic aliphatic substitution where the reactant is a nucleophile rather than an electrophile. The four possible electrophilic aliphatic substitution reaction mechanisms are SE1, SE2(front), SE2(back) and SEi (Substitution Electrophilic), which are also similar to the nucleophile counterparts SN1 and SN2. In the SE1 course of action the substrate first ionizes into a carbanion and a positively charged organic residue. The carbanion then quickly recombines with the electrophile. The SE2 reaction mechanism has a single transition state in which the old bond and the newly formed bond are both present.
Electrophilic aliphatic substitution reactions are:
- Nitrosation
- Ketone halogenation
- Keto-enol tautomerism
- aliphatic diazonium coupling
- carbene insertion into C-H bonds
- Carbonyl alpha-substitution reactions
References
- March, Jerry (1985). Advanced Organic Chemistry (5th ed.). Wiley.