Reactions of keten acetals. Part I. The hydrolysis of cyanoketen dimethyl acetal; general acid catalysis and kinetic hydrogen isotope effects

Abstract

The hydrolysis of cyanoketen dimethyl acetal to methyl cyanoacetate is found to be subject to general acid catalysis Catalytic coefficients are reported for nine aliphatic acids and the hydrogen ion, the reactivity of which is much smaller than expected from the parameters of the Brønsted catalysis law (G= 100, α= 0·62) established for the aliphatic acids.

At high concentrations of buffer solutions, especially for the weaker acids, the rate of the catalysed reaction contribution to the rate increases less rapidly than expected from the results at low buffer concentrations. It is suggested that dimerisation of the undissociated acid and association between carboxylic acid and carboxylate anion are responsible for the effect. For acetic acid, the results for both cyanoketen dimethyl acetal and 2-dichloromethylene-1,3-dioxolan (Part II) are satisfied by K_ass= 1·6 l. mole^–1, K_d= 0·4 l. mole^–1.

General acid catalysis has also been studied for three carboxylic acids in deuterium oxide as solvent. The value of the Brønsted exponent α remains virtually unaltered. The rate isotope effects for proton (deuteron) transfer from the carboxylic acids (k_H/k_D) are approximately 5·3.

The corresponding isotope effect for the hydrogen ion is approximately 3·0. In mixtures of H₂O and D₂O the velocity of the hydrogen ion-catalysed reaction falls off in accordance with the expression given by Gold (Trans. Faraday Soc., 1960, 56, 255). The best value of α required to fit the data to this equation is in the range 0·5–0·65, which straddles the value of the Brønsted exponent for carboxylic acids.

The catalytic coefficient of acetic acid decreases practically linearly with the deuterium content of the medium, and it is shown that this result points to the relative unimportance of a ‘transfer’ or ‘medium’ isotope effect superimposed on the exchange effect.