Role of homogeneous steps in the heterogeneous catalytic hydrogenation of substituted 2-nitroazobenzenes

摘要

The homogeneous reactions of nitrohydrazo derivatives contribute substantially to the benzotriazole selectivity of the hydrogenation of substituted 2-hydroxynitroazobenzene. The rate of the homogeneous steps is governed by the solvent composition rather than by the substituent in the starting nitroazobenzene. An increase in pH in both aqueous and aqueous-alcohol solutions increases the rate constants of the homogeneous reactions of various substituted nitrohydrazo derivatives and considerably enhances the selectivity of the reaction with respect to the compounds containing a triazole ring. A kinetic model has been suggested to qualitatively describe the influence of the pH value of the medium on the rate of the cyclization of substituted 2'hydroxy-5'-hydrazobenzenes into the corresponding benzotriazole N-oxides.