Acylation. Part XXVII. The kinetics and mechanism of the spontaneous and acid-catalysed reactions of dimethylketen with water and alcohols

Abstract

We report a kinetic study of the spontaneous and the carboxylic acid-catalysed additions of water and of a series o alcohols to dimethylketen in ether at 25°. The products are isobutyric acid or an isobutyryl ester. The addition of water is an autocatalytic process. The rate equations for the carboxylic acid-catalysed reactions, which do not proceed via a carboxylic anhydride intermediate, are respectively –d[Keten]/dt=k_cat[H₂O][RCO₂H][Keten] and –d[Keten]/dt=k_cat[R¹OH][R²CO₂H][Keten]. The catalytic effect of an acid is inversely related to its conventional strength and trichloroacetic acid is a very poor catalyst.

The spontaneous additions lead to rate equations which approximate closely to –d[Keten]/dt=k_{water dimer}[(H₂O)₂][Keten] and –d[Keten]/dt=k_{alcohol trimer}[(ROH)₃][Keten] for the water and alcohol reactions respectively. Water reacts more rapidly than the alcohols studied, although the alcohols are the more extensively associated in ether. Measurements of this association are reported; its extent is rather little dependent on the nature of R. The value of k_{alcohol trimer} varies in a complex way with the nature of R; electron repulsion by, and the steric bulk of, R are two factors among others which affect the rate. The various results, including measurements of activation energy, considered in the light of our previous studies, constitute formidable evidence that both the spontaneous and the acid-catalysed addition of nucleophiles to ketens normally proceed via cyclic transition states.