The 2-oxocyclobutanecarboxylic acid keto-enol system in aqueous solution: a remarkable acid-strengthening effect of the cyclobutane ring.
J Org Chem
; 71(12): 4460-7, 2006 Jun 09.
Article
en En
| MEDLINE
| ID: mdl-16749775
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.
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Colección:
01-internacional
Base de datos:
MEDLINE
Idioma:
En
Revista:
J Org Chem
Año:
2006
Tipo del documento:
Article
País de afiliación:
Canadá