Deuterium solvent isotope effects in methanol solution. Part II. The general acid-catalysed reaction of cyanoketen dimethyl acetal with solvent

Abstract

The kinetics of the acid-catalysed reaction of cyanoketen dimethyl acetal in methanol solution, involving addition of solvent to the olefinic double bond of the substrate, have been studied by spectrophotometry at 24·8°C. Catalysis by dilute strong mineral acid (hydrochloric acid) and by dichloroacetic acid were observed. The dependence of the rate constant on concentration of hydrochloric acid is linear in the presence of, but not strictly linear in the absence of, added electrolyte in swamping concentration, which suggests the operation of a significant salt effect. For reactions in methanolic dichloroacetic acid–sodium dichloroacetate buffers, the results at high buffer concentrations point to a significant amount of association between acid and anion. This conclusion is compatible with molecular weight determinations by vapour-pressure osmometry. The kinetic results are analogous to those previously obtained for the acid-catalysed hydrolysis of cyanoketen dimethyl acetal in aqueous solution. A common mechanism with a rate-limiting proton attachment to the olefinic double bond is indicated. Small amounts of residual water in the solvent do not greatly complicate the kinetic analysis.

The hydrochloric acid-catalysed reaction has also been studied in mixtures of MeOH and MeOD. The theory of the solvent isotope effect appropriate to this type of solvent system and reaction is developed and applied, again pointing to a close similarity to the solvent isotope effect of the reaction in aqueous solution. It is shown that the study of the solvent isotope effect in alcoholic solutions offers a convenient alternative procedure to the use of buffers for the investigation of general acid catalysis.