Kinetics and mechanism of the reaction of nitrous acid with 2,4-dinitrophenylhydrazine

Abstract

Arylhydrazines react with excess nitrous acid to form mixtures of the diazonium ion and aryl azide. By use of the weakly basic 2,4-dinitrophenylhydrazine (DNP), pK_a= 1.55, information about the mechanism of the diazotisation reaction has been obtained. Three successive stages can be observed. The initial reaction consists of parallel nitrosations by N₂O₃ and NO⁺ of the free base DNP at the terminal nitrogen. This is followed by a stage with the rate independent of [HNO₂], almost certainly a tautomerisation, probably to form ArNNNHOH and ArNHNNOH. The former is converted to ArN₃ while the latter undergoes a further electrophilic nitrosation by NO⁺ of a conjugate base species to yield a di-N,N′-nitrosoarylhydrazine, a precursor to the diazonium ion.