Radiation chemical oxidation of aniline derivatives

Abstract

The reactions of ˙OH, O˙⁻ and N₃˙ with chloro- and hydroxy-anilines were studied by pulse radiolysis. The rates of the OH radical reaction are higher (k ∼ 5 × 10⁹ dm³ mol⁻¹ s⁻¹) than those found for the O˙⁻ and N₃˙ reactions (k ∼ 2 × 10⁹ dm³ mol⁻¹ s⁻¹). Neither the position of the substituent nor the introduction of an additional Cl to monochloroanilines has any significant effect on the rates of the ˙OH reaction. The intermediates formed in all the aniline derivatives studied herein have λ_max values around 310–320 and 350–380 nm. The OH radical reacts both by addition and direct H abstraction giving rise to OH adducts (350–380 nm) and anilino radicals (310–320 nm). The extent of these two reactions depends on the position of the substituent, the former being more predominant in the meta than in the ortho and para isomers. The initially-formed OH adducts subsequently undergo dehydration, leading to anilino radicals in the case of chloroanilines and phenoxyl radicals with hydroxyanilines. The OH attack at the carbon bonded to Cl in all three monochloroanilines is not significant (≤15%). Semi-empirical quantum calculations using the PM3 method were carried out to evaluate the possible sites of the OH radical attack. The charge distribution and the heats of formation data reveal that the ˙OH attack extends over more than one carbon center. The relative stabilities of the isomeric OH adducts formed from the attack at the unsubstituted carbons of chloro- and hydroxy-anilines are nearly the same, their respective heats of formation being approximately −70 and −230 kJ mol⁻¹.