Effect of Hydroxide Ion on the Cis-Trans Thermal Isomerization of Azobenzene Derivatives

Article abstract of DOI:10.1021/jo00115a010

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 of methyl orange (1), 4-(dimethylamino)-4'-methoxyazobenzene (2), and naphthalene-1-azo-(4'-(dimethylamino)benzene) (3) in water or in water/cosolvent solution measured at different hydroxide ion concentrations.It was found that the reaction is strongly dependent on the hydroxide ion concentration.The observed rate constant for 2 changes from 1.43E-2 s^-1 at NaOH (1.0E-3 M) to 7.7E-4 s^-1 at NaOH (0.10 M).These results can be interpreted in terms of the isomerization rate constant of the protonated substrates which is at least 1E8 times faster than that of the neutral compound.