The 4,4,4-trifluoroacetoacetic acid keto-enol system in aqueous solution. Generation of the enol by hydration of trifluoroacetylketene

Trifluoroacetylketene was generated in aqueous solution by flash photolysis of 2,2-dimethyl-6-trifluoromethyl-4H-1,3-dioxin-4-one and its spiro analog ue, 4-trifluoromethyl-1,5-dioxaspiro[5.5]undec-3-en-2-one, and rates of hyd ration of the ketene to 4,4,4-trifluoroacetoacetic acid enol as well as sub sequent ketonization of the enol were measured in this solvent across the a cidity range [H+] = 10(-1) - 10(-12) M. Analysis of the rate profile produc ed by these data provides the acidity constants pQ(a,E) = 1.85 for the carb oxylic acid group of the enol and pQ(a)(E) = 9.95 for its enolic hydroxyl g roup, which make these groups 2 and 3 orders of magnitude more acidic, resp ectively, than the corresponding groups in the parent unfluorinated acetoac etic acid enol. Rates of enolization of the keto group of 4,4,4-trifluoroac etoacetic acid were also measured, by bromine scavenging, and these, togeth er with a value of the equilibrium constant for hydration of the keto group to its gem-diol derivative based upon a free energy relationship, K-h = 29 00, provide an estimate of the keto-enol equilibrium constant for this syst em. pK(E) = 0.28. This is greater, by 2 orders of magnitude, than the keto- enol equilibrium constant for the unfluorinated acetoacetic acid.