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Issue 7, 2002
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Halogenation of enol tautomers of 2-cyanoacetamide and malonamic acid

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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 KE, 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 ke 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.

Graphical abstract: Halogenation of enol tautomers of 2-cyanoacetamide and malonamic acid

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Publication details

The article was received on 12 Mar 2002, accepted on 13 May 2002 and first published on 29 May 2002

Article type: Paper
DOI: 10.1039/B202542F
Citation: J. Chem. Soc., Perkin Trans. 2, 2002,0, 1316-1319
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    Halogenation of enol tautomers of 2-cyanoacetamide and malonamic acid

    A. Eberlin and D. L. H. Williams, J. Chem. Soc., Perkin Trans. 2, 2002, 0, 1316
    DOI: 10.1039/B202542F

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