Electrophilic substitution reactions arechemical reactions in which anelectrophile displaces afunctional group in acompound, which is typically, but not always, aromatic. Aromatic substitution reactions are characteristic ofaromatic compounds and are common ways of introducing functional groups intobenzene rings. Somealiphatic compounds can undergo electrophilic substitution as well.

In electrophilic substitution inaromatic compounds, an atom appended to thearomatic ring, usually hydrogen, is replaced by an electrophile.The most important reactions of this type that take place arearomatic nitration,aromatic halogenation,aromatic sulfonation andacylation and alkylatingFriedel-Crafts reactions.It further consists of alkylation and acylation.

In electrophilic substitution inaliphatic compounds, an electrophile displaces a functional group. This reaction is similar tonucleophilic aliphatic substitution where the reactant is a nucleophile rather than an electrophile. The four possible electrophilic aliphatic substitution reaction mechanisms areS_E1,S_E2(front),S_E2(back) andS_Ei (SubstitutionElectrophilic), which are also similar to the nucleophile counterpartsS_N1 andS_N2. In the S_E1 course of action the substrate first ionizes into acarbanion and a positively charged organic residue. The carbanion then quickly recombines with the electrophile. The S_E2 reaction mechanism has a singletransition 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

• March, Jerry (1985).Advanced Organic Chemistry (5th ed.). Wiley.