Kinetics and mechanism of the addition of alcohols to ketenes in diethyl ether solution in the presence of boron trifluoride

Abstract

The kinetics are reported of the boron trifluoride-catalysed additions of seven alcohols to dimethylketene in diethyl ether solution at 25 °C. All the reactions involve the rapid initial formation of a 1:1 alcohol–boron trifluoride adduct (formation constant K) which transfers a proton to the ketene in the slow step of the addition. For t-butyl alcohol, ethanol, methanol, phenylmethanol, and 2-chloroethanol the K values are ≳ 1 000, 320 ± 50, 300 ± 50, 80 ± 20, and 65 ± 15 l mol^–1, respectively. The more acidic an alcohol the faster is the reaction of its 1 : 1 boron trifluoride adduct with the ketene. For t-butyl alcohol the relatively slow reaction of its adduct is catalysed by a further molecule of the adduct and by fre boron trifluoride.

At 25 °C the spontaneous addition of ethanol to diphenylketene in ether (like that to dimethylketene) is first order in the ketene and third order in the ethanol concentration. In this reaction diphenylketene is ca. 10-fold more reactive than the dimethyl derivative in the concentration range studied. Whereas the addition of boron trifluoride to the solution catalyses the addition of ethanol (and the other alcohols) to dimethylketene to such an extent that the spontaneous addition makes a negligible contribution to the overall rate, with diphenylketene the alcohol–boron trifluoride adduct is relatively so unreactive that added boron trifluoride strongly inhibits the spontaneous addition. The ethanol–boron trifluoride adduct is > 10⁴-fold less reactive towards diphenylketene than towards dimethylketene. These results support our previous conclusions concerning additions of acidic species to ketenes.