Solvent and substituent nature effect on the 2-nitroazobenzenes hydrogenation selectivity on skeletal nickel

The elucidation of the reasons and character of the influence of nature substituent and solvent composition on the kinetic features and selectivity of liquid-phase catalytic hydrogenation of compounds containing several reactive capable groups is a theoretical and practically significant task. In the presented work the process of 2-nitroazobenzene derivatives hydrogenation with vari-ous substituents was investigated in order to obtain the corresponding benzotriazoles on a skeletal nickel catalyst in a neutral azeotropic aqueous solution of 2-propanol, and in the presence of acetic acid or sodium hydroxide. According to the data obtained, the solvent composition has a decisive influence on the selectivity of this process. The transformation of azo-and nitro groups, included in the initial compound, proceeds in parallel stages, and the introduction of acidic or basic additives leads to a change in the rate ratio of these stages. Appropriately, selectivity of 2-nitroazobenzene derivatives hydrogenation changes with respect to reaction products containing a triazole ring, in particular, with respect to N-oxides of substituted benzotriazoles, and also with respect to nitrohy-drazo compounds. The greatest influence of substituent nature on the hydrogenation selectivity in a solvent with any composition is manifested for the group whose transformation rate in a given medium is higher. In the presence of acid the substituent variation is more strongly reflected in the hydrogenation rate of azo group, and in the basic medium - of nitro group. It was found that the change in the transformation rate of both reactive capable groups corresponds to the Hammett constant alteration. The electron acceptor substituent has a deactivating effect on the transfor-mation rate of nitro as well azo group. To increase the selectivity of substituted 2-nitroazobenzenes hydrogenation with respect to compounds containing a triazole ring, it is recommended to introduce a base into the aqueous solution of 2-propanol.