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 k_H2O/k_D2O= 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 E_a= 1 ± 1 kcal mol^–1, and ΔS^‡=–60 ± 8 cal K^–1 mol^–1; at higher concentrations E_a 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 k_H3O⁺/k_D3O⁺≃ 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.