COMPETITION BETWEEN RADICAL AND NONRADICAL REACTIONS OF HALONITROBENZENES IN ALKALINE ALCOHOLIC SOLUTIONS

The study of the reactivity of monohalonitrobenzenes in 2-propanol solutions of potassium 2-propoxide has led to the identification of three distinct reaction paths: (a) hydro dehalogenation to nitrobenzene, (b) alkoxy dehalogenation via the SNAr mechanism, and (c) nitro reduction to azoxy and anilino derivatives via nitroso intermediates. With the exception of 2- and 4-fluoronitrobenzene, radical processes c or a are faster than the SNAr reaction. The radical processes proceed via a common intermediate, the radical anion [XC6H4NO2], which can undergo unimolecular fragmentation to nitroaryl radical and X- (path a, favored for X = 2-I, 2-Br), or reduction to the dianion [XC6H4NO2]2-, the direct precursor of the nitroso intermediate XC6H4NO (path c). In the presence of oxygen, an effective oxidant of the radicalanion intermediate, the SNAr reaction prevails for the activated ortho and para substrates. Cationanion interactions are also of major consequence in determining the course of reaction. Ion pairing favors nitro reduction, whereas it slows both the SNAr and the hydro dehalogenation reactions. © 1990, American Chemical Society. All rights reserved.