Strong inhibition of cis-trans isomerization of azo compounds by hydroxide ion

Abstract

The thermel 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 2.17'710 -2 s -1 et NaOH 6'710 -3 M to 1.0'710 -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 ofthe protonated cis compounds than the neutral ones, and values for the pK a and rate constants for both species are calculated