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

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 =k2Am[Amine][NOCl]. The reactivity of amines more basic than N-methyl-4-nitroaniline (pKA 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 (pKA–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 (pKA >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.