Azocoupling products VI.1 The sensitivity to external factors of the UV–vis absorption spectra of the azocoupling product between 1-(4-hydroxy-6-methyl-pyrimidin-2-yl)-3-methylpyrazolin-5-one and 4-(N,N-dimethyl) aminobenzenediazonium salt

The pH-, water content-, sample concentration-, temperature- and time-dependences of the UV–vis spectra of the potentially tautomeric azocoupling product 1 in (aqueous) ethanol solutions have been systematically investigated. On the basis of pH-dependence were determined the pKa′ values corresponding to three interconnected acid–base equilibria 2b⇔1a⇔3b⇔4b2b⇔1a⇔3b⇔4b that originate from 1. The four species 1a, 2b, 3b, 4b involved in these equilibria are characterized on the basis of their different visible absorption bands. The hydrazone structure 1a of the initial species is supported by the 1H and 13C NMR data. The dependence of the UV–vis spectra of (aqueous) ethanol solution of 1 upon the water content, dye concentration, temperature or time seems to be due firstly to the acid–base equilibrium 1a⇔3b1a⇔3b. However, some irregularities in these dependences might be interpreted as being caused by eventual superimposition upon the acid–base equilibrium 1a⇔3b1a⇔3b of the azo–hydrazone (e.g. 1a⇔1b1a⇔1b) and/or aggregation equilibrium. By the time dependence has been observed practically a kind of oscillation of the position of the present equilibrium that is very difficult to explain.