Buffer catalysis in the hydrazinolysis of benzylpenicillin

The Brønsted plot for general base catalysis of the reaction of hydrazine with benzylpenicillin in water is non-linear with limiting slopes of <0.2 and >0.8 for strong and weak bases, respectively. It is concluded that there is a change in the rate-limiting step of the reaction with changing basicity of the catalyst and it is suggested that the mechanism proceeds by a step-wise mechanism of proton transfer. For strongly basic catalysts the rate-limiting step is thought to be the diffusion-controlled encounter of the pre-formed tetrahedral intermediate and the catalysing base, whereas for weakly basic catalysts it is the diffusion apart of the deprotonated tetrahedral intermediate and the conjugate acid of the catalyst or the breakdown of the deprotonated tetrahedral intermediate in the presence of the conjugate acid of the catalyst. The -effect for hydrazine is manifested by an increase in the equilibrium constant for formation of the tetrahedral intermediate of 350-fold, which is reflected by an increase of 15-fold in the value of k1 and a decrease of 22-fold in the value of k–1.