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

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 ArN=NNHOH and ArNHN=NOH. 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.