The Wallach Rearrangement: Part Ii. Kinetics and Mechanism of the Acid-Catalyzed Rearrangement of Azoxybenzene

Abstract

The rate of rearrangement of azoxybenzene to p-hydroxyazobenzene has been measured in 75.3-96.4% sulfuric acid a t 25' and in 65.0-90.4% sulfuric acid a t 75.5' by spectrophotometric methods. The pK, of azoxybenzene in aqueous sulfuric acid has also been determined. I t is found thathlthough azoxybenzene is almost completely protonated over the entire range of acid concentration studied, the rate increases by more than 1000-fold. A two-proton process is therefore indicated and mechanisms are proposed involving a dication (11) a s the key intermediate. The rate data do not allow differentiation between two proposed mechanisms, one involving two equilibrium protonations, and the other a single equilibrium protonation followed by rate-determining proton transfer. Past mechanisms of the Wallach rearrangement are discussed.

Orientation in the Wallach Rearrangement for the Naphthyl Azoxy Series

“…may be obtained when the reactions are run a t high substrate and acid concentrations, high temperatures, and prolonged reaction times (1 2, 13). S u b s e q~~e n t work which has been carried out under controlled kinetic conditions, using low substrate concentrations ( 6 M), has shown the formation of a single primary product, the hydroxyazobenzene (8 ' M). Even so, the addition of organic cosolvents, 0.5% v/v ethanol for the phenyl-naphthyl azoxy compounds and 5.0% v/v dioxane in the case of the even less soluble naphthyl-naphthyl derivatives, was necessary to assist in solubilization.…”

“…Kinetic studies of the Wallach rearrangement have provided evidence concerning t h e reaction mechanism in establishing the form of the acid catalysis (2,3,8). T h e sin~ultaneous measurement of the rate of rearrangement a n d the extent of monoprotonation of the substrate has revealed a continued dependence of rate on acidity beyond the monoprotonation stage (8).…”

“…T h e sin~ultaneous measurement of the rate of rearrangement a n d the extent of monoprotonation of the substrate has revealed a continued dependence of rate on acidity beyond the monoprotonation stage (8). The reaction mechanism proposed to account for these data is given in Scheme 1 and involves a dicationic intermediate (7).…”

“…have included evaluation of the effect of substituents in one or both phenyls on the course of rearrangement (2, 3), the employment of isotopic tracers including 14C (4), 15N (9, and " 0 (6,7), and kinetic investigations, performed on azoxybenzene and erne substituted derivatives (2,3,8). As a result of these studies a number of mechanisms have been proposed (9), involving some rather unusual species as reaction intermediates.…”

Basicities in the naphthyl azoxy series

“…Previous studies on the protonation behavior of other aryl azoxy cpmpounds have been confined to azoxybenzene (3)(4)(5) and a number of mono-and disubstituted azoxybenzenes (4,5). The measurements of refs.…”