Nitrosation by alkyl nitrites. Part 5. Kinetics and mechanism of reactions in acetonitrile

Abstract

Simple alcohols and thioglycolic acid react with alkyl nitrites and nitrous acid in acidic acetonitrile solution to give the O- and S-nitrosated products, in reactions which are kinetically zero order in the alcohol or thiol concentration. The results are consistent with rate-limiting NO⁺ formation. On addition of the parent alcohol (derived from the alkyl nitrite) reactions are slower and there is a change towards a first-order dependence upon the substrate concentration, indicating that under these conditions the reaction of NO⁺ with the substrate is partly rate limiting. The reactivity order is found to be HNO₂ > t-butyl nitrite > i-propyl nitrite > isopentyl nitrite. Similarly aromatic amines yield nitrosamine or diazonium ion products, but now the kinetics are consistent with rate-limiting attack of NO⁺ with the unprotonated amine. At higher acidities it is proposed that reaction occurs with the protonated form of the amine. The mechanistic implications of the kinetic results are discussed.