Kinetic studies of the azo coupling reaction between 1-(4-hydroxy-6-methylpyrimidin-2-yl)-3-methylpyrazolin-5-one and substituted benzenediazonium salts

Abstract

Kinetic studies have been carried out on the azo coupling reactions between 1-(4-hydroxy-6-methylpyrimidin-2-yl)-3-methylpyrazolin-5-one and seven substituted benzenediazonium salts. The effects of acidity and temperature upon the rate have been investigated and activation parameters determined, and the involvement of deprotonated substrate in the rate determining step demonstrated. Acid dissociation equilibrium of the substrate is characterized by pK_a^HA= 5.07 ± 0.16 at 293 K and µ= 0.2. Close activation enthalpies, between 48 and 59 kJ mol^–1 and negative entropies between –22 and –53 J mol^–1 K^–1 have been found, depending on the electronic effects of substituents on the benzene nucleus of the diazonium salts. A Hammett relationship log₁₀k₂= 1.0 ± 0.1 +(3.3 ± 0.3)σ holds for coupling reactions. The greater the electron-withdrawing effect, the faster is the coupling rate. 1-(2-Pyrimidinyl)pyrazolin-5-one behaves similarly to 1-arylpyrazolin-5-one and phenols in reactions with arenediazonium cations.