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.
Sanchez
Ana M.
AM
Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1, and Instituto de Investigaciones en Físico-Química de Córdoba (INFIQC), Facultad de Ciencias Químicas, Departamento de Química Orgánica, Universidad Nacional de Córdoba, 5000, Córdoba, República Argentina.
Barra
Mónica
M
de Rossi
Rita H.
RH