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Kinetic studies have been carried out o n the azo coupling reactions between 1 -(4-hydroxy-6methylpyrimidin-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 dfJermining step demonstrated. Acid dissociation equilibrium of the substrate is characterized b y pK, = 5-07 + 0.16 at 293 K and 1. 1 = 0.2. Close activation enthalpies, between 48 and 59 kJ mol-' and negative entropies between -2 2 and -53 J mol-' K-I have been found, depending on the electronic effects of substituents on the benzene nucleus of the diazonium salts. A Hammett relationship log,, k2 = 1 .O +_ 0.1 + (3.3 +_ 0.3)a holds for coupling reactions. The greater the electron-withdrawing effect, the faster is the coupling rate. 1 -(2-Pyrimidinyl)pyrazolin-5-one behaves similarly t o 1 -arylpyrazolin-5one and phenols in reactions with arenediazonium cations.Paper 2/01967A

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