Carbanion nitrosation. Reaction of malononitrile with nitrous acid

Abstract

Malononitrile reacts readily with nitrous acid in aqueous acid buffer solutions, particularly in the presence of nucleophilic catalysts X [Br^–, SCN^–, and SC(NH₂)₂] to give the corresponding oxime product C(CN)₂NOH. In the absence of catalysts and also with Cl^– as catalyst, reaction is slow and competes with the spontaneous decomposition of nitrous acid. The rate equation at one acidity for the catalysed reaction is established as rate =k₁[CH₂(CN)₂][HNO₂][X], which suggests that reaction occurs by rate-limiting attack of the XNO species. In the pH range 2–4.5 the dependence of k₁ upon acidity varies from being approximately first order in [H⁺] at low acidities to being approximately zero order in [H⁺] at higher acidities. All the results are consistent with a mechanism involving a rate-limiting reaction between XNO and the carbanion ^–CH(CN)₂. Analysis of the kinetic results reveals that the second-order rate constants k₂ for this step are, respectively, 1.1 × 10¹⁰, 4.2 × 10⁹, and 5.0 × 10⁹ dm³ mol^–1 s^–1(at 25 °C) for the BrNO, ONSCN, and ONSC(NH₂)₂ reactions. Thus each of these nitrosating species appears to react at the diffusion-controlled limit. Comparison with literature values of other substrate nitrosations shows that this carbanion is the most reactive substrate studied in nitrosation. There are no other reported examples where (the less reactive) ONSCN and ONSC(NH₂)₂ species react upon encounter. Reaction also occurs in aqueous acetic acid solutions, again at convenient rates only in the presence of catalysts X, i.e. involving reaction via XNO.