The amination of benzene and toluene to aniline and toluidines with hydroxylamine sulfate has been investigated in water-acetic acid and water-acetic acid-sulfuric acid media in the presence of transition metal compounds as catalysts. The process yields are strongly dependent on the temperature, added sulfuric acid and the composition of catalyst. For the Ε½ . amination of benzene, the soluble catalysts, NaVO and Fe III salts produce high yields of aniline without addition of 3 H SO , whereas Na MoO and FeSO exhibit substantial activity only in 5 M H SO . Amination is accompanied by a 2 4 2 4 4 2 4 disproportionation of hydroxylamine catalyzed by the redox active transition metal ions. The favorable effect of H SO on 2 4 the amination is due mostly to the greater stability of hydroxylamine in the strongly acidic medium. Mixed oxides containing Ε½ . Ε½ . V V and Mo VI are active amination catalysts when suspended in 5 M solution of H SO in acetic acid. Introduction of 2 4
metallic Pd into these oxide catalysts improves performance increasing the yield and selectivity of amination with respect to the aromatic substrate. Toluene exhibited a close to benzene reactivity in amination giving approximately equal yields of o-, m-, p-toluidines. Mechanistic considerations based on literature data and results of ab initio quantum mechanics calculations suggest that the aminating species is the protonated amino radical Γ NH q , which in the rate-determining step reacts with 3 benzene and toluene to yield the corresponding aminocyclohexadienyl and aminomethylcyclohexadienyl radical intermediates. These are then oxidatively aromatized to give, respectively, aniline and a non-regiospecific mixture of toluidines.