Molecular acoustics. Part 5.—Internal rotation in some α,β-unsaturated aldehydes

Abstract

The barriers to internal rotation of the aldehyde group in several α,β-unsaturated aldehydes have been determined using the ultrasonic relaxation method. An attempt has been made to relate these barriers to some parameter associated with the double-bond character of the C—C bond adjacent to the carbonyl group. The parameter chosen in this study was the (J¹³C—H) coupling constant of the carbon atom in the carbonyl group. In three β-2-furyl substituted α,β-unsaturated carbonyl compounds the conformational equilibrium being perturbed by the sound wave is that involving internal rotation of the furan group. In addition, we have also measured the ultrasonic absorption and velocities in several related compounds, but no relaxation was observed. The possible reason for these observations are also considered.