On the Mechanism of the Acid/Base-Catalyzed Thermal Cis-Trans Isomerization of Methyl Orange.

The thermal cis-trans isomerization of methyl orange (i.e., sodium 4-[4'-(dimethylamino)phenylazo]benzenesulfonate) in acidic aqueous solutions has been investigated by means of laser-flash photolysis techniques. The thermal cis-trans isomerization is found to be catalyzed by general acids and general bases. Catalysis is attributed to acid/base-assisted tautomerization of cis-ammonium ions (formed by rapid protonation of cis-methyl orange generated upon photochemically induced trans-cis isomerization) into cis-azonium ions. The latter can easily isomerize via rotation around the -N=N- bond as a result of the concomitant decrease in the double bond character. Furthermore, the acidity of cis-ammonium ions is estimated to be significantly lower than that reported for the trans isomers (pK(a) values are 5.0 and 2.70-2.87, respectively). This result is attributed to a decrease in resonance interactions of the two aryl rings in the cis isomer compared with the trans form due to the nonplanar conformation of the former.