Kinetics and Mechanisms of the Reactions of Ketenes with Water and Alcohols in Dioxane Solutions

Article abstract of DOI:10.1039/P29860001485

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, _s, when _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 _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*1E-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.

Full text of DOI:10.1039/P29860001485

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