Electrophilic aromatic substitution
This chapter describes the role played by nitrogen in the bromination of acetanilide, illustrates one principle of great importance in electrophilic aromatic substitution: the rate of reaction is strongly dependent upon the nature of the substituent(s) in the aromatic nucleus. The relative ease of substitution at different positions in an aromatic compound is also determined by the nature of the substituent(s): in the case of acetanilide, the order of reactivity at the nuclear carbons is para > ortho >> meta. Both the directive effects and the rate-controlling effects of substituents are of great importance in synthetic applications of electrophilic aromatic substitutions. The effects are conveniently discussed in: monosubstituted benzenes, di- and polysubstituted benzenes, bi- and polycyclic hydrocarbons, and heteroaromatic compounds. Intramolecular Friedel–Crafts acylations are of particular value in building up cyclic systems; dibasic acid anhydrides are much used in these reactions.