Kinetics and mechanism of the denitrosation of nitrosamines in ethanol

Abstract

Rate constants have been determined for the denitrosation of N-methyl-N-nitrosoaniline (NMNA), N-nitrosodiphenylamine (NDA), and N-methyl-N-nitrosotoluene-p-sulphonamide (MTS) in ethanolic solutions of hydrogen chloride. For both NMNA and NDA the change is reversible in the absence of a trap for the nitrosyl chloride formed; the equilibrium position is dependent upon [HCl]. A kinetic analysis in the case of NMNA gives the rate constants for both the forward and reverse reactions. A more accurate determination of the forward rate constant was obtained for both NMNA and NDA by using an excess of added ascorbic acid to remove the free nitrosating species. Under these conditions the rate law, rate =k₂[Nitrosamine][HCl], prevails. The reaction of MTS was irreversible even in the absence of a nitrite trap and the same rate law was established. All three reactants show the absence of catalysis by added sodium bromide and potassium thiocyanate and gave kinetic deuterium solvent isotope effects k_EtOH/k_EtOD in the range 2.6–3.8. The results are in accord with a rate-determining proton transfer to the nitrosamine, contrasting with the behaviour (of NMNA and NDA) in water solvent. The rate of denitrosation of NMNA decreases as water is added to the solvent, and nucleophilic catalysis begins to be effective. The Fischer–Hepp rearrangement of NDA is observed when ascorbic acid is absent; the rate constant for rearrangement is much less than that for denitrosation at the same acidity.