Kinetics of reactions of 1,2,4-triazole-3-diazonium ions with phenol and with hydroxyl ion

Abstract

5-Methyl- and 5-phenyl-l,2,4-triazole-3-diazonium ions (IIIa, IIIb) react with undissociated phenol in diluted hydrochloric acid. At pH > 1 the reactions with phenolate ions become kinetically significant, their bimolecular rate constants approaching those of the diffusion-controlled reactions. At the same time, the diazonium ions are dissociated into the dipolar ions IIIb and IVb. At pH > 4 (for the phenyl derivative) and pH > 5 (for the methyl derivative), the reaction of the dipolar ions with phenolate ion becomes the main reaction path. The rate constant of the reaction of the dipolar ion IIIb with hydroxyl ion (3 . 103 l mol-1 s-1) is comparable with that of the analogous reaction of benzenediazonium ion, but the reverse reaction of the dipolar ion IIIb is slower by about 5 orders of magnitude, and the pKA value of the diazo hydroxide formed is higher by about 4 units than that of benzenediazo hydroxide. The high stability of the heterocyclic diazo hydroxide and its low acidity are explained by formation of a strong intramolecular hydrogen bond. In sodium hydroxide solutions, the (Z)-diazotate IIId initially formed is transformed into the (E)-isomer (the corresponding half-life being about 10 h) which, in contrast to the (Z)-isorner, does not practically react with aromatic hydroxy compounds in basic medium. The transformation of the (E)-isomer to the diazonium ion is general acid-catalyzed reaction. Transformation of nitrosamine into diazo hydroxide is suggested to be the rate-limiting step of this reaction in solutions of pH < 5.