Nitrosation of ketones: clear evidence for the involvement of the enol tautomers

Abstract

Nitrosation of acetone and ethyl methyl ketone in the presence of a fairly high concentration of Cl^–, Br^–, or SCN^– is first-order in [ketone] and [H⁺] and zero-order in [HNO₂] and [Cl^–], [Br^–], and [SCN^–]. This demonstrates that reaction occurs by nitrosation of the enol form of the ketone, and that under these conditions, enolisation is rate-limiting. The measured rate constant for enolisation in the nitrosation experiments is in excellent agreement with that obtained earlier for halogenation and for hydrogen-exchange reactions. The same behaviour occurs with 1,3-dichloroacetone, except that here the enolisation is not acid-catalysed–again this agrees with the bromination work. With lower concentrations of added nucleophiles, for all four ketones, the rate equation includes terms first-order in [HNO₂] and also in [Cl^–] or [Br^–], showing that the reaction of the enol with the nitrosating species is now rate-limiting. There is a very close analogy with the kinetic behaviour found in the halogenation of ketones. Analysis of the kinetic data reveals the reactivity sequence NOCl > NOBr > NOSCN (well known in N-nitrosation), and the reactivity of the enols is as expected for electrophilic addition. With acetylacetone, the rate-limiting step is always the reaction of the enol, due in part to the lower reactivity of the enol. In the absence of added nucleophiles, both acetone and ethyl methyl ketone react with N₂O₃, whereas the less-reactive 1,3-dichloro-acetone and acetylacetone react preferentially with H₂NO₂⁺ or NO⁺—a similar trend occurs in N-nitrosation of amines.