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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. [ABSTRACT FROM AUTHOR]

Abstract: The Wallach rearrangement was used as a method for preparing p-hydroxyazobenzocrown ethers starting from different azoxybenzocrowns as substrates. Synthesis of a series of p-hydroxyazobenzocrowns under modified conditions and characterization of the obtained products are presented. o-Hydroxyazobenzocrowns were identified among the products of the photochemical rearrangement of azoxybenzocrowns. Novel biscrowns were synthesized from p-hydroxyazobenzocrown ethers. The synthesized host molecules, differing in the size of the macrocycles and in the substituents in the aromatic rings, have the same dioxymethylene linkers. They were used as potential sodium or potassium ionophores in classic and miniature (screen-printed) ion-selective electrodes. The properties (tautomerism, acid–base equilibrium) of newly prepared o-hydroxyazobenzocrown ethers and their p-substituted analogs were studied using spectroscopic methods and compared. [Copyright &y& Elsevier]

Kinetic studies of the Wallach rearrangements of four azoxypyridines, four azoxypyridine N-oxides, and one azoxypyridine N-methiodide have been carried out in the 100% H2SO4 acidity region. For all of the β-isomers in the study the reactions proceeded at a spectrally measurable rate, and the log observed rate constants were found to be linear functions of the log H3SO4+ concentration, as previously found for azoxybenzene itself, suggesting that the reaction mechanism for these substrates is the same as that previously deduced for axozybenzene, i.e., a general-acid-catalysis A-SE2 process. For the α-azoxypyridines no reaction could be observed at all. The two α-azoxypyridine N-oxides in the study did react, albeit very slowly, but for these two compounds the log observed rate constants were not linear functions of the log H3SO4+ concentration, but were instead found to be linear in the H0 acidity function, which is known for the 100% H2SO4 acidity region. It follows that the reaction mechanism for these α-isomers is a different one, presumably an A1 process. This mechanism was proposed back in 1963 for azoxybenzene, but has never actually been observed for any substrate before the work reported in this study. Thus, the Wallach rearrangement story can be said to have come full circle. [ABSTRACT FROM AUTHOR]

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Extension of our studies of the generic Wallach rearrangement (of azoxybenzene to 4-hydroxyazobenzene) to the heteroaromatic series (azoxypyridines and axoxypyridine N-oxides) has revealed some dramatic reactivity differences, particularly for the α and β compounds. We have studied the 3-isomers and the 4-isomers in each series, each with α and β forms, eight compounds in all, in the 100 wt% sulfuric acid region of acidity. In those cases in which a product could be observed, the α and β isomers both give the same one, the corresponding 4′-hydroxyazo compounds. All the compounds react much more slowly than does azoxybenzene itself, presumably because of the extra positive charge present in the substrates, but the β isomers have half-lives of seconds and the α isomers half-lives of hundreds of hours in the 100 wt% H2SO4 acidity region. The α compounds have measurable pKBH+ values, but the β compounds do not, exhibiting only a medium effect in the acidity region in which the α compounds protonate. This means that for the β compounds, the protonated intermediates must be much less stable and the postulated reaction intermediates must be much more stable than for the α compounds. To clarify this, we have obtained Mulliken charge distributions for the various species concerned, calculating the charge carried by each half of the molecule, larger charge separations being taken to indicate lesser stability. As far as we can establish, this is the first time that this technique has been used to indicate the stabilities of carbocationic species. [ABSTRACT FROM AUTHOR]

Treatment of azoxybenzene and its derivatives with acids is known to result in the Wallach rearrangement, which leads to 2- or4-hydroxyazobenzenes. Starting in the 1960s, experimental findings have lead to the proposal of several mechanisms for this rearrangement. In this work, molecular orbital theory employing the semiempirical AM1 method is used to locate and discuss the energetics of the intermediates and the transition states for this rearrangement. Based on the results of AM1 calculations in vacuum and in solution, the most plausible mechanistic pathways are proposed and discussed. [ABSTRACT FROM AUTHOR]