A mechanistic study of the decomposition of phenylhydroxylamine to azoxybenzene and aniline and its catalysis by iron(II) and iron(III) ions stabilised by ethylenediaminetetra-acetic acid

Abstract

The catalysis by Fe^II and Fe^III of the slow decomposition 3PhNHOH → PhN₂OPh + PhNH₂ has been investigated kinetically under nitrogen by quantitative u.v. and e.s.r. spectroscopy and by potentiometric measurements at pH 6–8 in aqueous 30% t-butyl alcohol containing EDTA. The reactions of nitrosobenzene with (a) phenylhydroxylamine and (b) Fe^II have been examined similarly and a mechanism for all these reactions is put forward. Small percentages of nitrosobenzene and of phenylnitroxide radicals are formed during all these reactions and the rapidly attained equilibria (i) and (ii) appear to control their concentrations, the much slower formation of azoxybenzene being due to the dimerisation (iii). Aniline formation is due to reaction (iv) PhNO + 2Fe^II+ 2H⁺⇌ PhNHOH + 2Fe^III(i), PhNO + PhNHOH ⇌ 2 PhNHO·(ii), 2 PhNHO·→ PhN₂OPh + H₂O (iii) of which the first stage must be (V) and becomes dominant when high percentages of Fe^II are used. Reaction 2 Fe^II+ PhNHOH + 2H⁺→ PhNH₂+ 2Fe^III+ H₂O (iv), Fe^II+ PhNHOH → PhNH·+ Fe^III+(OH)^–(v)(iv) does not appear to be pH dependent. Azoxybenzene formation, initiated by reaction (vi), occurs more rapidly the higher the pH, this being due to the effect of base on the [PhNO]:[PhNHOH] ratio controlled by equilibrium (i). With low percentages of Fe^II or Fe^III catalysts, the reaction velocities approximately depend Fe^III+ PhNHOH → PhNHO·+ Fe^II+ H⁺(vi) on the total iron salt concentration. Under these conditions the transient PhNO concentrations change so slowly with time that both the decomposition of PhNHOH and the formation of azoxybenzene become pseudounimolecular processes. The noteworthy feature of this transition metal catalysed decomposition is the kinetic control of a sequence of one-electron transfer reactions by the formation of a molecular intermediate (PhNO).