Diazotisation Mechanism of Heteroaromatic Amines. Diazotisation of 2-aminothiazole as an equilibrium

The rate of diazotization of 2-aminothiazole was measured in 65 to 75% (w/w) aqueous H₂SO₄. The kinetic solvent isotope effect of the diazotization in 72% 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% (w/w) H₂SO₄ at three initial concentration of reagents (0.001,0.01, and 0.1m). The final concentrations of the reagents and the diazonium ion are consistent with a reversible process. This is the first diazotization for which quantitative evidence for equilibration has been found. In the very large range of acidities used (H_o = ?1.73 to ?9.01), it was not possible to calculate meaningful equilibrium constants that are independent of the acidity.