Flash photolysis of 2-diazocyclohexane-1, 3-dione in aqueous solution produced 2- ketocyclopentylideneketene, which hydrated to the enol of 2-oxocyclopentanecarboxylic acid, and the enol then isomerized to the keto form of the acid. This ketene proved to be a remarkably reactive substance, with an uncatalyzed hydration rate constant of k= 1.4× 106 s- 1 and a hydroxide-ion-catalyzed rate constant of 7.5× 107 M-1 s-1; no acid catalysis of ...