Solvent-induced mechanistic changes in nitrosation reactions. Part 2. Effect of acetonitrile–water mixtures in the nitrosation of ureas

Abstract

The nitrosation of 1,3-dimethylurea in acetonitrile–water mixtures has been studied kinetically. The results obtained show that the addition of acetonitrile until the medium holds 70% acetonitrile by weight inhibits the reaction. The reaction is not catalysed by chloride ions in these circumstances, and the reaction mechanism is probably the same as in pure water. Addition of acetonitrile to a solution already containing more than 70% acetonitrile increases the reaction rate, and catalysis by halides becomes possible. The change in reaction mechanism this suggests was studied in detail in a medium containing 90% acetonitrile. The reaction rate increases non-linearly with increasing halide concentration and acidity, but seems to tend to the same limiting value in all cases, depending only on the nitrous acid and urea concentrations. Nitrosyl halides are therefore good nitrosating agents of ureas, though the catalytic efficiency of the different halides is the reverse of that in water, probably because of solvation-induced changes in their nucleophilicity. The tendency of the reaction rate towards a limiting value is evidence that the mechanism changes with the catalyst or acid concentration. In the limit, the reaction rate will only depend on nitrous acid and urea concentrations; this is consistent with a limiting step consisting of the rearrangement of the ‘nitrosourea’, the nitroso group transferring from the more nucleophilic O atom to the N atom. Thus, there is direct kinetic evidence that the nitrosation of amides occurs initially on oxygen.