The Infrared Absorption Spectra of Cyclic β-Ketoesters

Abstract

The infrared absorption spectra of a number of closely related cyclic /3-ketoesters have been observed. In the case of those compounds related to cyclohexanone-2-carboxylic ester in which enolization can occur absorption bands have been found at 1656-1660 cm.-1 and 1618-4624 cm."1 in addition to the absorption attributable to the ester carbonyl (1734-1745 cm."1) and the ketone carbonyl (1717-1720 cm."1). The additional bands are considered to be due, respectively, to carbonyl and to carbon-carbon unsaturation in the conjugated chelate enolic form. Analogous but weaker bands were found for compounds similarly related to cyclopcntanonc-2-carboxylic ester, indicating a lower degree of enolization for the five-membered ring analogs. Also, some of the ester and keto C-O vibrational frequencies differ for correspondingly substituted cyclopentanone and cyclohexanone homologs. The influence of ring strain on bond hybridization provides a basis to account for the differences.