title: The 2-oxocyclobutanecarboxylic acid keto-enol system in aqueous solution: a remarkable acid-strengthening effect of the cyclobutane ring.
abstract: Flash photolysis of 2-diazocyclopentane-1,3-dione in aqueous solution produced 2-oxocyclobutylideneketene, which underwent hydration to the enol of 2-oxocyclobutanecarboxylic acid; the enol then isomerized to the keto form of this acid. Rates of the ketene and enol reactions were measured in acid, base, and buffer solutions across the acidity range [H+] = 10(-1)-10(-13) M, and analysis of these data, together with rates of enolization of the keto form of 2-oxocyclobutanecarboxylic acid determined by bromine scavenging, gave keto-enol equilibrium constants as well as acidity constants of the keto and enol forms. The keto-enol equilibrum constants proved to be 2 orders of magnitude less than those reported previously for the next higher homolog, 2-oxocyclopentanecarboxylic acid, reflecting the difficulty of inserting a carbon-carbon double bond into a small, strained carbocyclic ring. The acidity constant of the enol group of 2-oxocyclobutanecarboxylate ion, on the other hand, is greater, by 4 orders of magnitude, than that of the corresponding enol in the cyclopentyl system. This remarkable increase in acidity with diminishing ring size is consistent with the enhanced s character of the orbitals used to make the exocyclic bonds of the smaller cyclobutane ring.
authors.affiliation: Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada.
authors.forename: J A
authors.initials: JA
authors.lastname: Chang
authors.forename: Y
authors.initials: Y
authors.lastname: Chiang
authors.forename: J R
authors.initials: JR
authors.lastname: Keeffe
authors.forename: A J
authors.initials: AJ
authors.lastname: Kresge
authors.forename: V A
authors.initials: VA
authors.lastname: Nikolaev
authors.forename: V V
authors.initials: VV
authors.lastname: Popik