There is a non-linear dependence of the rate of aminolysis of benzylpenicillin and 6-β-aminopenicillanic acid upon hydroxide ion concentration which is interpreted in terms of formation of a tetrahedral addition intermediate. At high concentrations of hydroxide ion the rate-limiting step is formation of the tetrahedral intermediate but at low concentrations it is the diffusion-controlled encounter of the intermediate and hydroxide ion. Rate constants for the formation of the intermediate and its breakdown to reactants are reported for a variety of amines. The dependence of these rate constants upon the pKa of the conjugate acid of the amine yield Brønsted β values of ca. 0.3 and -0.6 for the formation and breakdown of the intermediate, respectively. There is thus quite a large dependence of the rate of expulsion of the amine from the intermediate upon the basicity of the amine despite the rate constants for this step being ca. 10 9-1010 s-1. Possible stereoelectronic control in the breakdown of the tetrahedral intermediate is discussed. There is no evidence for intramolecular general base catalysis in the formation of the tetrahedral intermediate from 6-β-aminopenicillanic acid.
|Number of pages||6|
|Journal||Journal of the Chemical Society, Perkin Transactions 2|
|Publication status||Published - 1 Jan 1979|
|Event||Annual Research Conference of the Royal Institution of Chartered Surveyors - London, United Kingdom|
Duration: 7 Sep 2006 → 8 Sep 2006