Conformations of α,β-unsaturated ketones: An IR spectroscopic study

Abstract

The conformational analyses of substituted methyl styryl ketones and phenyl styryl ketones have been carried out using IR spectroscopy. The split in the C=O bands of these compounds is attributed to the existence of two conformations, viz.s-cis ands-trans, in equilibrium. The methyl styryl ketones exist predominantly in thes-trans form whereas phenyl styryl ketones exist in thes-cis form. In all the ketones studied the proportion of thes-trans form increases with increase in the polarity of the solvent while that of thes-cis form decreases. This shows that thes-trans form is more polar than thes-cis. The field effects between the C=O and C=C groups are found to be electrostatic repulsions which play a dominant role in the determination of the relative stabilities of thes-cis ands-trans forms over the steric effects and the electrostatic attractions between the carbonyl oxygen and the β-carbon.