The chemistry of nitroso-compounds. Part 14. Nitrosation of amines in aqueous solution by dissolved gaseous NOCl

Abstract

Quantitative results are reported for the interaction of several primary arylamines and secondary alkylamines in either 0.1M-NaOH or phosphate buffer (pH 6.85) at 25 °C with gaseous NOCl. These reactions, which are complete in a few seconds, yield substantial amounts of diazonium ion or N-nitrosamine products in addition to NO₂ ^– from competing hydrolysis. With an excess of amine, however, the conversion of NOCl into N-nitrosation products is quantitative. The dependence of product yields on initial reactant concentrations suggest that N-nitrosamine and diazonium ion formation follows Rate =k₂ ^Am[Amine][NOCl]. The reactivity of amines more basic than N-methyl-4-nitroaniline (pK_A 1.49) towards NOCl is largely independent of their basicity and most react ca. 600–2 000 times faster than H₂O. The reactivity of less basic amines is proportional to their basicity, and no reaction is observed for 2,4-dinitroaniline (pK_A–4.53). For the competitive hydrolysis reaction, HO^– reacts only 94 times faster than H₂O itself. The results are interpreted as evidence that NOCl reacts with the more basic amines (pK_A >1.49) on encounter, with substrate reactivity being kinetically important only for less basic compounds. This conclusion is supported by a comparison of independent rate measurements for the interaction of aniline and H₂O with NOCl in dilute acid.