AZO HYDRAZO TAUTOMERIZATION AND CIS-TRANS ISOMERIZATION OF 2-CARBOXY-2'-HYDROXY-4'-METHYL AZOBENZENE

2-Carboxy-2'-hydroxy-4'-methyl azobenzene in its trans configuration a bsorbs at 380 nm in isopropanol. In aqueous medium, it remains as the ate (lambda(max)=355 nm) and hydrate (lambda(max)=480 nm) monoanions i n 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 isopropano l-water (1:1) mixture. On photoexcitation, the trans isomer changes to the cia isomer which absorbs in the region 420-430 nm. The back conve rsion of the cis to trans isomer occurs via an intermediate hydrate ta utomeric monoanion. The rate-determining step of the conversion proces s depends on the amount of base present or the alkalinity of the syste m. The conversion of the cis isomer to the hydrate monoanion and the e quilibrium relaxation of the hydrate monoanion to the trans isomer are competing reactions. In the absence of base, the cis to hydrate conve rsion is slower than the equilibrium relaxation of the hydrate to the trans form, while in the presence of base, it is faster.