Halogenation of enol tautomers of 2-cyanoacetamide and malonamic acid

Abstract

Reactions of 2-cyanoacetamide (CA) with both bromine and iodine at low halogen concentration in aqueous acid solution were first order in halogen and CA, consistent with an interpretation involving rate-limiting halogenation of the enol tautomer. There was no acid dependence in the range 0.1–0.5 mol dm⁻³. The observed rate constants for bromination and iodination were very similar, indicating that both probably occurred at the encounter limit. This enabled a value for K_E, the equilibrium constant for enolisation, to be determined as 6 × 10⁻¹⁰. There was strong evidence that at low acidities (pH ∼ 4) enolisation is rate-limiting, but we were unable to achieve a fully zero-order kinetic dependence upon halogen. However we did obtain a value for the acid-catalysed enolisation rate constant k_e of 2.7 × 10⁻³ dm³ mol⁻¹ s⁻¹ from measurements of rates of deuterium exchange with the solvent. The bromination of malonamic acid (ML) in dilute acid solution at low [Br₂] was zero-order in [Br₂] and we obtained a value of 1.3 × 10⁻² dm³ mol⁻¹ s⁻¹ for the acid catalysed enolisation rate constant. There is some evidence which suggests that enolisation of ML takes place via the amide group rather than the carboxylic acid group. The results are compared with earlier studies.