Diazotisation Mechanism of Heteroaromatic Amines. Diazotisation of 2-Aminothiazole as an Equilibrium

Article abstract of DOI:10.1002/hlca.19890720421

The rate of diazotisation of 2-aminothiazole was measured in 65 to 75 percent (w/w) aqueous H2SO4.The kinetic solvent isotope effect of the diazotisation in 72 percent D2SO4/D2O was determined (k_H/k_D = 5.8+/-0.2).These data are consistent with a mechanism in which the 2-aminothiazole, protonated at the ring N-atom, but not at the NH2 group, is attacked by the NO⁺ ion.The reaction does not go to completion, but to an equilibrium.The equilibrium concentration of the diazonium ion was determined in 30, 50, 70, and 90 percent (w/w) H2SO4 at three initial concentrations of reagents (0.001, 0.01, and 0.1 M).The final concentrations of the reagents and the diazonium ion are consistent with a reversible process.This is the first diazotisation for which quantitative evidence for equilibration has been found.In the very large range of acidities used (H₀ = -1.73 to -9.01), it was not possible to calculate meaningful equilibrium constants that are independent of the acidity.