Azocoupling products. V1. Electronic spectroscopy study of two azocoupling products of 1-(4-hydroxy-6-methyl-pyrimidin-2-yl)-3-methyl-pyrazolin-5-one

A systematic study of the effect of solvent, sample concentration, time and acid or alkali addition, respectively of pH′, upon the electronic absorption spectra of two dyes 7 and 8, as well as the pKa′ determination of these dyes was carried out. In C6H6, CHCl3, DMSO, and acidic media (AcOH, acidified EtOH) these dyes that theoretically may be involved in azo–hydrazone tautomerism have been detected only as hydrazone tautomers 7c and 8c. In exchange, in neutral and alkalized EtOH, respectively in DMF, in concentration range 10−4–10−6 mol L−1 these tautomers are in equilibrium with monodeprotonated (9, 10) and twice deprotonated (13, 14) species. By increase of pH, or by dilution, the equilibrium shifts increasingly towards the deprotonated species. The monodeprotonated species, correspond also to a hydrazone structure 9, 10, whilst the doubly deprotonated species represent a common dianion that should be predominantly azo in character (13, 14).