Azoxybenzene formation from nitrosobenzene and phenylhydroxylamine. A unified view of the catalysis and mechanisms of the reactions

Abstract

Azoxybenzene formation from nitrosobenzene and N-phenylhydroxylamine has been studied in the pH range 1–11 with different buffers in 10% methanol at 25 °C and ionic strength 0.5 (KCl). The reaction exhibits general-acid (α= 0.29), general-base (β= 0.15), and specific-base catalysis by hydroxide ion. The plot of log k₂(k₂= second-order rate constant)vs. pH, the specific-base catalysis by hydroxide ion, and the substituent effects demonstrate that in the pH range studied only one step, i.e. the dehydration of a N,N′-dihydroxy intermediate, rapidly formed from the reagents, is rate-determining. The mechanism suggested in this work is also supported by the general acid- and base-catalysis and by the effect of the solvent polarity on the rate of the reaction. Other mechanisms and intermediates are considered, and shown to be inadequate in explaining all the experimental results.