Kinetics and mechanisms of the reactions of ketenes with water and alcohols in dioxane solutions
Abstract
The spontaneous addition of water to diphenyl- and to dimethyl-ketene in dioxane–H2O and dioxane–D2O mixtures is third order in the stoicheiometric water concentration, [H2O]s, when [H2O]s≲ 1.5M. At higher water concentrations (up to 19M) the observed order falls, and the patterns of the results for the two ketenes are similar, but differ in detail. The isotope effect kH2O/kD2O= 1.7–2.3 for all the systems studied. The addition of ethanol to diphenylketene in dioxane–ethanol mixtures has a kinetic pattern very similar to that for addition of water to this ketene, except that the change to a lower order occurs when [EtOH]S≃ 5M. The variation of the activation parameters with medium composition is also very similar for all these spontaneous additions: at low water or ethanol concentrations Ea= 1 ± 1 kcal mol–1, and ΔS‡=–60 ± 8 cal K–1 mol–1; at higher concentrations Ea rises somewhat, and ΔS‡ becomes less negative. The results are considered in the light of previous work. It is concluded that water and alcohol add to ketenes by a common mechanism in non-hydroxylic solvents of low dielectric constant, and that this involves a cyclic, hydrogen-bonded transition state such as (1). Hydroxide ions (ca. 5 × 10–3M) have no effect on the rate of water addition in dioxane, but hydrogen ions have powerful effects: the addition to diphenylketene is inhibited, that to dimethylketene is catalysed. These observations are explained. The isotope effect kH3O+/kD3O+≃ 1.9 for the catalysis. We propose a catalytic mechanism with a cyclic transition state (3) involving a dominant slow proton transfer to the ketene β-carbon atom.