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 ketene to acetoacetic acid enol and
subsequent ketonization of the enol were measured
in this solvent across the acidity range [H+] =
1−10-13 M. Acetylketene proved to be a
remarkably reactive substance,
undergoing uncatalyzed hydration with the rate constant k
= 1.5 × 106 s-1, some
104 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-determining 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 acid group of the enol and
pQ
a
E = 13.18 for its enolic
hydroxyl group. (These acidity constants are
concentration quotients referring to an ionic strength of 0.10 M).
Combination of the present results with information
on the enolization of acetoacetic acid available from the literature
gave K
E = 5.6 ×
10-3, pK
E = 2.25, as
an estimate
of the keto−enol equilibrium constant.
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