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Intermediates of the transformation of azoxybenzene (AzOB) to hydroxyazobenzene isomers in concentrated sulfuric acid (the Wallach rearrangement) are found. It is considered that the charged nitrogen of the N+ → O− azo group induces electronic polarization in AzOB with the formation of a cation of the phenylaminyl type, and acts of the protonation of the second nitrogen of the azo group with the displacement of the hydroxy group in the form of a HO+ cation and appearance of protonated azobenzene AB+H follow the addition of a proton to oxygen with the formation of a N+ → OH group. It is found that AB+H participates in a reversible deprotonation ↔ protonation reaction (AB+H ↔ AB + H+) with one nitrogen atom of the azo group or another. HO+ cations in turn rapidly interact with sulfuric acid to generate hydroxyl sulfate cations HOSO2O+, which react with the neutral phenyl rings of the AB+H cations and the sulfuric acid. It is shown that the final products of rearrangement (hydroxy-substituted derivatives of AB) and side products that include azobenzene form as a result of the reaction system’s neutralization. An explanation of the kinetics of the Wallach rearrangement and the paths of the formation of side products is provided.

When the starting materials 4-acetyl-ONN- and -NNO-azoxybenzene(1α and 1β, respectively) were heated in 70/80% sulfuric acid, α/β isomerization and the Wallach rearrangement to 4-acetyl-4′-hydroxyazobenzene (2) were observed. However, neither the o-hydroxyazobenzene nor 4-acetyl azobenzene was detected in the reaction product. The isomerization of 1α was faster than that of 1β, demonstrating the higher themodynamic stability of the latter.

The major product in the reaction of 4-alkylazoxybenzenes with sulfuric acid is not 4-(p-alkylphenylazo)phenol, which is regarded as the usual product of Wallach rearrangement. The following azophenols were obtained as major products; 2-alkyl-5-(phenylazo)phenol (R : Me), 2-alkyl-4-(phenylazo)phenol (R : Et), and 4-(phenylazo)phenol (R : i-Pr and t-Bu).

Treatment of 4,4′-dihaloazoxybenzenes with sulfuric acid afforded 4,4′-dihaloazobenzenes (6) as the major product and 5-halo-2-(p-halophenylazo)phenols, the Wallach rearrangement products as minor products, besides 2-halo-5-(p-halophenylazo)phenols, 2-halo-4-(p-halophenylazo)phenols, and 4-(p-halophenylazo)phenols as abnormal rearrangement products. The reaction of fluoro compound gave the azophenols (3a or 5a) in the best yields. However, the ratio of 3a to 5a was found to vary with the concentration of sulfuric acid. Treatment of 4-haloazoxybenzenes with sulfuric acid gave 4-(p-halophenylazo)phenols and 4-haloazobenzenes (13) as major products together with 2-(p-halophenylazo)phenols and 5-halo-2-(phenylazo)phenols as minor products. Yields of reduction products 6 and 13 increased as the halosubstituent became heavier in the following sequence: F→Cl→Br→I.

The reactions of 4,4′-dialkylazoxybenzenes (1) with sulfuric acid afforded 2-alkyl-5-(p-alkylphenylazo)phenols (2) and 2-alkyl-4-(p-alkylphenylazo)phenols (3) as novel rearrangement products; in certain cases 4-(p-alkylphenylazo)phenols (4) were also obtained. The ratios of these azophenols were greatly affected by the alkyl group type. In addition, the reactions gave a very small amount of 5-alkyl-2-(p-alkylphenylazo)phenols (5), which were considered as normal Wallach rearrangement products.

Study of the acid-catalyzed Wallach rearrangement of azoxybenzene is extended into the 100% H2SO4 region. The rate of formation of 4-hydroxyazobenzene can be followed spectrally in a straight-forward manner until close to 99% H2SO4, but in higher acidities sulfonation of this product becomes kinetically important. The advent of second equilibrium protonations of 4-hydroxyazobenzene and of 4-hydroxyazobenzene-4′-slfonic acid further complicate the azoxybenzene rearrangement as followed spectrally. Above 100% H2SO4 a second sulfonation is also observed. A method is given for dissecting the rate data for the primary rearrangement process from the first of the sulfonation reactions.The rate of the azoxybenzene rearrangement is observed to increase continuously up to 99.99% H2SO4 (the upper limit of the present kinetic method). This suggests that the second proton transfer step to azoxybenzene is rate-determining and not an equilibrium process. These results permit a clarification of a previously proposed mechanism (1).

The rate of rearrangement of azoxybenzene to p-hydroxyazobenzene has been measured in 75.3–96.4% sulfuric acid at 25° and in 65.0–90.4% sulfuric acid at 75.5° by spectrophotometric methods. The pKa of azoxybenzene in aqueous sulfuric acid has also been determined. It is found that although azoxybesssnzene is almost completely protonated over the entire range of acid concentration studied, the rate increases by more than 1 000-fold. A two-proton process is therefore indicated and mechanisms are proposed involving a dication (II) as 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.

In the Wallach rearrangement of α-p-methylazoxybenzene with concentrated sulfuric acid as a catalyst, 2-hydroxy-4-methylazobenzene and 4-hydroxy-4'-methylazobenzene were formed as rearrangement products. The ο-isomer, 2-hydroxy-4'- methylazobenzene, has been recognized in the previous research.It is interesting that the yield of the p-isomer decreased and that of the ο-isomer increased as the concentration of sulfuric acid approached 100%.When α-p-methylazoxybenzene is irradiated by ultraviolet ray in various solvents, we obtained 2-hydroxy-4-methylazobenzene. In addition, it bas deen reported that 2-hydroxy-4'-methylazobenzene was formed by photo-chemical rearrangement.

Treatment of 4,4′-dihaloazoxybenzenes with sulfuric acid afforded 4,4′-dihaloazobenzenes (6) as the major product and 5-halo-2-(p-halophenylazo)phenols, the Wallach rearrangement products as minor products, besides 2-halo-5-(p-halophenylazo)phenols, 2-halo-4-(p-halophenylazo)phenols, and 4-(p-halophenylazo)phenols as abnormal rearrangement products. The reaction of fluoro compound gave the azophenols (3a or 5a) in the best yields. However, the ratio of 3a to 5a was found to vary with the concentration of sulfuric acid. Treatment of 4-haloazoxybenzenes with sulfuric acid gave 4-(p-halophenylazo)phenols and 4-haloazobenzenes (13) as major products together with 2-(p-halophenylazo)phenols and 5-halo-2-(phenylazo)phenols as minor products. Yields of reduction products 6 and 13 increased as the halosubstituent became heavier in the following sequence: F→Cl→Br→I.

The major product in the reaction of 4-alkylazoxybenzenes with sulfuric acid is not 4-(p-alkylphenylazo)phenol, which is regarded as the usual product of Wallach rearrangement. The following azophenols were obtained as major products; 2-alkyl-5-(phenylazo)phenol (R : Me), 2-alkyl-4-(phenylazo)phenol (R : Et), and 4-(phenylazo)phenol (R : i-Pr and t-Bu).

The treatment of azoxybenzenes bearing electron-withdrawing substituents at 4,4′-positions with sulfuric acid gave the corresponding 2-hydroxyazobenzenes, together with 4′-substituted 4-hydroxyazobenzenes. The treatment of 4,4′-dimethoxyazoxybenzene with p-toluenesulfonic acid and acetic anhydride afforded 4,4′-dimethoxy-3-acetoxyazobenzene and 4,4′-dimethoxy-2-tosyloxyazobenzene as the main rearrangement products.