Kinetic studies of nitrosations by alkyl nitrites
The reactions of tertiary butyl nitrite and isopropyl nitrite in aqueous acid solution have been studied. In the case of Isopropyl nitrite the rates of reaction with hydrazoic acid, sulphamic acid, thioglycolic acid and N-methylaniline are markedly reduced by the addition of propan-2-ol. This has been explained in terms of a rapid hydrolysis of the alkyl nitrite to yield an equilibrium concentration of nitrous acid which then effects nitrosation. Analysis of the results gives values for the equilibrium constant for isopropyl nitrite hydrolysis and for the rate constants for nitrous acid nitrosation of the substrate, which are in good agreement with the literature values obtained by direct measurement. For tertiary butyl nitrite the extent of hydrolysis is so large and rapid that the kinetics are identical to those obtained using nitrous acid. The reactions of isopropyl nitrite in isopropanol and tertiary butyl nitrite in tertiary butanol with thiourea and thioglycolic acid have been studied. In both cases the reactions are acid catalysed and the results are consistent with a direct reaction between the protonated alkyl nitrite and the substrate. The reactions with thioglycolic acid were subject to catalysis by chloride and bromide salts and thiourea. The results for chloride and bromide salt are best explained in terms of formation of hydrochloric or hydrobromic acid which can then act as a general acid catalyst. For thiourea the catalysis is due to the formation of an equilibrium concentration of s-nitrosothiourea. In acetonitrile the reactions of tertiary butyl nitrite, isopropyl nitrite, isoamyl nitrite and nitrous acid with alcohols and thioglycolic acid were found to be kinetically zero order with respect to the substrate concentration. The results have been interpreted in terms of rate limiting formation of the nitrosonium ion. With less reactive substrates, aniline, N-methylaniline, p-toluidine, acetylacetone, 1,1,1,trifluoroacetylacetone and 1,1,1, 5,5, 5, hexafluoroacetylacetone, the reactions are kinetically first order with respect to the substrate concentration and have been interpreted in terms of rate limiting reaction of the nitrosonium ion with the free (at low acidity) or protonated (at high acidity) form of the amine. For the ketones reaction occurs, in a rate limiting step with either the enols or enolate ions.