Competition between Radical and Nonradical Reactions of Halonitrobenzenes in Alkaline Alcoholic Solutions

Article abstract of DOI:10.1021/jo00298a044

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 S_NAr 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 S_NAr reaction.The radical processes proceed via a common intermediate, the radical anion ^*-, which can undergo unimolecular fragmentation to nitroaryl radical and X^- (path a, favored for X = 2-I, 2-Br), or reduction to the dianion ^2-, the direct precursor of the nitroso intermediate XC6H4NO (path c).In the presence of oxyen, an effective oxidant of the radical-anion intermediate, the S_NAr reaction prevails for the activated ortho and para substrates.Cation-anion interactions are also major consequence in determining the course of reaction.Ion pairing favors nitro reduction, whereas it slows both the S_NAr and the hydro dehalogenation reactions.