Benzene and other aromatic rings obey Hückel's rule: the ring system must be cyclic, planar, fully conjugated, and possess \(4n+2\) pi electrons, where n is an integer (yielding 6, 10, 14, … pi electrons). The characteristic reaction of aromatic compounds is electrophilic aromatic substitution (EAS), in which an electrophile replaces a ring hydrogen while the aromatic pi system is regenerated—this contrasts with alkene electrophilic addition, which permanently breaks a pi bond. In IR spectroscopy, aromatic rings show C–H stretches just above 3000 cm\(^{-1}\), C=C ring stretches near 1450–1600 cm\(^{-1}\), and strong out-of-plane bending absorptions in the 675–870 cm\(^{-1}\) region whose pattern reveals the substitution pattern on the ring.
Substituents dramatically affect both ring reactivity and the regiochemistry of further substitution. Electron-donating groups such as -OH, -NH₂, -OR, and alkyl groups are ring-activating and ortho/para-directing because they donate electron density into the ring by resonance or induction, stabilizing the cationic intermediate at those positions. Electron-withdrawing groups such as -NO₂, -CN, -COOH, and carbonyls are ring-deactivating and meta-directing because they withdraw electron density and destabilize the ortho/para intermediates. Halogens are a special case: they are deactivating through inductive electron withdrawal but still ortho/para-directing because their lone pairs donate by resonance to the ortho and para positions.
Friedel–Crafts alkylation uses an alkyl halide with an \(AlCl_3\) catalyst to install an alkyl group on a benzene ring, but it is prone to carbocation rearrangements and polysubstitution. Friedel–Crafts acylation installs an acyl group via an acyl halide with \(AlCl_3\), and is preferred because the acylium ion is resonance-stabilized and the deactivated product ring prevents over-substitution. To access other functional groups from anilines, diazotization with nitrous acid (\(HNO_2\)) generates a diazonium salt (Ar-N₂⁺); Sandmeyer reactions using copper(I) chloride, bromide, or cyanide then convert the diazonium group into an aryl chloride, bromide, or nitrile, respectively.