SOME STEREOCHEMICAL STUDIES ON α,β-UNSATURATED KETONES USING RAMAN SPECTROPHOTOMETRY

Abstract

The Raman spectra of 12 α,β-unsaturated open-chain and cyclic ketones have been measured in the region of the C = O and C = C stretching vibrations, and the band positions and intensities have been compared with the corresponding bands in the infrared spectra. If due allowance is made for solvent effects, the infrared and Raman bands are observed at the same positions, but the relative intensities of the C = O and C = C bands are different; in particular, the Raman C = O stretching bands of the ketones in the s-trans configuration are from 5 to 10 times more intense than the Raman bands for the s-cis isomers. The cyclic s-trans ketones with a hydrogen atom on the α-carbon atom exhibit a solvent-sensitive doubling of the carbonyl maximum similar to that noted previously for certain α,β-unsaturated lactones, unsaturated acid anhydrides, and Δ²-cyclopentenone derivatives. These doublet carbonyl maxima are observed also in the infrared spectra. The C = C frequency is not similarly perturbed and the effect is distinct from the doubling of both the C = O and the C = C bands exhibited by some open-chain α,β-unsaturated ketones that exist in the liquid phase as equilibrium mixtures of s-cis and s-trans isomers.