EFFECT OF HYDROXIDE ION ON THE CIS-TRANS THERMAL-ISOMERIZATION OF AZOBENZENE DERIVATIVES

Azobenzenes can be photochemically transformed from the trans (E) form , which is thermodynamically more stable, to the cis (Z) form. In the dark, the latter reverts to the former. We report here a kinetic study of the thermal isomerization ion of methyl orange (1), 4-(dimethylami no)-4'-methoxyazobenzene (2), and naphthalene-1-azo-(4'-(dimethylamino )benzene) (3) in water or in water/cosolvent solution measured at diff erent hydroxide ion concentrations. It was found that the reaction is strongly dependent on the hydroxide ion concentration. The observed ra te constant for 2 changes from 1.43 x 10(-2) s(-1) at NaOH(1.0 x 10(-3 ) M) to 7.7 x 10(-4) s(-1) at NaOH (0.10 M). These results can be inte rpreted in terms of the isomerization rate constant of the protonated substrates which is at least 10(8) times faster than that of the neutr al compound.