The chemistry of nitroso-compounds. Part 11. Nitrosation of amines by the two-phase interaction of amines in solution with gaseous oxides of nitrogen

Abstract

The formation of secondary N-nitrosoamines when MeCN solutions of amines are brought into contact with gaseous NO, N₂O₃, and N₂O₄ at 25 °C is reported. With NO, N-nitrosoamine formation from piperidine, morpholine, and diphenylamine occurs very slowly (t½ca. 8 days). Reaction rates are largely independent of the amine, suggesting that oxidation of NO by adventitious oxygen is the slow step. Very much faster reactions are observed, however, with N₂O₃ and N₂O₄. With a ca. 10-fold excess of N₂O₃ or N₂O₄, quantitative yields of both N-nitrosopiperidine and N-nitrosodiphenylamine are found in less than 3 min. With a 27-fold excess of piperidine and N₂O₄ only, ca. 8%N-nitropiperidine and 92%N-nitrosopiperidine are obtained. Rapid reactions also ensue when solutions of either primary aromatic or secondary amines dissolved in 0.1M-aqueous NaOH are brought into contact with gaseous N₂O₃ and N₂O₄. With a ca. 2–400-fold excess of these nitrogen oxides, 12–65% of the amine is converted either to N-nitrosoamine or diazonium ion in less than 3 min. Competitive hydrolysis of the nitrogen oxide by the solvent is not HO^–-catalysed and the amine : H₂O reactivities are in ca. 1 000 : 1. The extent of N-nitrosation varies insignificantly over a wide range of basicities (pK_a, 11.12 to –1.0), but no reaction occurs with either 2,4-dinitroaniline or N-butyl-acetamide. With N₂O₄, smaller amounts of N-nitroamines form concurrently and increase with decreasing amine basicity. The results are discussed in relation to amine nitrosation by N₂O₃ and N₂O₄ in aqueous acidic solutions. It is suggested that the lower selectivity for the dissolved gaseous reagents may relate to the presence of more reactive N₂O₃ and N₂O₄ isomers. The results also show that carcinogenic N-nitrosoamines form under a much wider range of experimental conditions than previously known, some of which are relevant to atmospheric pollution.