Ultrasonic relaxation in some alkyl esters

Abstract

Ultrasonic attenuation measurements have been made from 1–30 MHz over a variety of temperatures in methyl, ethyl and n-propyl formates, methyl and ethyl acetate, methyl and ethyl propionate and ethyl n-butyrate. The results are discussed in terms of equilibrium between cis and trans rotational isomers. The low energy state (the alkyl groups trans to each other) lies 8.8±1.4 kcal mole^–1 below the interconversion transition state in all the esters. However, the energy of the higher state (the alkyl groups cis to each other) rises with the increasing number of carbon atoms on both alkyl groups. These observations are not in accord with theories of high dipole energies (the cis state has a large dipole moment) in media of low dielectric constant, or of hydrogen-bonded stabilization of the lower (trans) state. However, they are in complete agreement with the theory that the states differ in energy because of steric repulsion between alkyl groups in the higher energy (cis) state.