Strong Inhibition of Cis-Trans Isomerization of Azo Compounds by Hydroxide Ion

Article abstract of DOI:10.1021/jo00060a026

The thermal cis-trans isomerization rate of methyl yellow (1), p-phenyl red (2), and o-methyl red (3) in aqueous solution was measured at different hydroxide ion concentrations, and it was found that there is a very strong inhibition as the pH increases.For 1 the rate changes from 2.17*10^-2 s^-1 at NaOH 6*10^-3 M to 1.0*10^-3 s^-1 at NaOH 0.1 M.The observed rate constant for 2 was too fast for the experimental technique used at a concentration of NaOH lower than 0.01 M.These results are interpreted in terms of a much faster rate of isomerization of the protonated cis compounds than the neutral ones, and values for the pK_a and rate constants for both species are calculated.