Acylation. Part XXVIII. The kinetics and mechanism of the addition of hydrogen halides to dimethylketen in ether solution. Relative reactivities of substrates towards ketens and a comparison with mechanism of acylation by other acylating agents in non-hydroxylic solvents

Abstract

The formation of isobutyryl chloride from dimethylketen and hydrogen chloride in ether solution is very rapid, and, at –20°, is a reaction of the first order in keten. At hydrogen chloride concentrations >ca. 10^–2M it is also of the first order in hydrogen chloride but at lower concentrations the order in this component rises. The observed first-order rate constant for keten loss is given by k_obs(min.^–1)= 42·0[HCl]²/(3·3 × 10^–3+[HCl]). The results suggest a mechanism involving carbonyl addition and a subsequent catalysed prototropy, in which two molecules of hydrogen chloride are always involved, the catalytic role of the second molecule being kinetically relatively more important at low concentrations. Data for deuterium chloride (ca. 2-fold less reactive than hydrogen chloride) and hydrogen bromide (ca. 25-fold more reactive) are compatible with the scheme.

Hydrogen cyanide and sulphide react very slowly with dimethylketen in ether. The relative reactivities of a range of substrates towards ketens is established. Two general mechanisms of acylation by ketens are identified and compared with those postulated for acylation by more conventional reagents in non-hydroxylic solvents.