FLASH-PHOTOLYSIS OF 2,2,6-TRIMETHYL-4H-1,3-DIOXIN-4-ONE IN AQUEOUS-SOLUTION - HYDRATION OF ACETYLKETENE AND KETONIZATION OF ACETOACETIC ACID ENOL

Acetylketene was produced by flash photolysis of 2,2,6-trimethyl-4H-1, 3-dioxin-4-one in aqueous solution, and rates of hydration of the kete ne to acetoacetic acid enol and subsequent ketonization of the enol we re measured in this solvent across the acidity range [H+] = 1 - 10(-13 ) M. Acetylketene proved to be a remarkably reactive substance, underg oing uncatalyzed hydration with the rate constant k = 1.5 x 10(6) s(-1 ), some 10(4) times more rapidly than ketene itself; the acetylketene hydration reaction was also catalyzed weakly by hydroxide ion but not by hydrogen ion. Ketonization of acetoacetic acid enol was much slower with rates in the millisecond to second range. The reaction showed a complex rate profile that could be interpreted in terms of rate-determ ining carbon protonaton of the carboxylate-ionized form of the enol in the acid region and rate-determining carbon protonation of the doubly ionized carboxylate-enolate form in the basic region. Analysis of the data provided the acidity constant pQ(a) = 4.05 for the carboxylic ac id group of the enol and pQ(a)(E) = 13.18 for its enolic hydroxyl grou p. (These acidity constants are concentration quotients referring to a n ionic strength of 0.10 M). Combination of the present results with i nformation on the enolization of acetoacetic acid available from the l iterature gave K-E = 5.6 x 10(-3), pK(E) = 2.25, as an estimate of the keto-enol equilibrium constant.