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-dinitroph enylhydrazine (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 nitrosation s 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 cer tainly a tautomerisation, probably to form ArN=NNHOH and ArNHN=NOH. Th e former is converted to ArN3 while the latter undergoes a further ele ctrophilic nitrosation by NO+ of a conjugate base species to yield a d i-N,N'-nitrosoarylhydrazine, a precursor to the diazonium ion.