Azo-hydrazo tautomerization and cis-trans isomerization of 2-carboxy-2′-hydroxy-4′-methyl azobenzene

Abstract 2-Carboxy-2′-hydroxy-4′-methyl azobenzene in its trans configuration absorbs at 380 nm in isopropanol. In aqueous medium, it remains as the azo ( λ max = 355 nm) and hydrazo ( λ max = 480 nm) monoanions in equilibrium. The equilibrium process requires an enthalpy change of 26.8 kJ mol −1 in aqueous medium and 15.3 kJ mol −1 in an isopropanol-water (1:1) mixture. On photoexcitation, the trans isomer changes to the cis isomer which absorbs in the region 420–430 nm. The back conversion of the cis to trans isomer occurs via an intermediate hydrazo tautomeric monoanion. The rate-determining step of the conversion process depends on the amount of base present or the alkalinity of the system. The conversion of the cis isomer to the hydrazo monoanion and the equilibrium relaxation of the hydrazo monoanion to the trans isomer are competing reactions. In the absence of base, the cis to hydrazo conversion is slower than the equilibrium relaxation of the hydrazo to the trans form, while in the presence of base, it is faster.