Ultrasonic Relaxation and Cis-Trans Isomerization in Methyl and Ethyl Formates

Abstract

The ultrasonic absorption of liquid methyl and ethyl formates has been measured over a frequency range of 0.6–10 Mc/sec from 60° to 140°C. The relationship with previous measurements is discussed. The absorption is consistent with the single-relaxation process of internal rotation about the carboxyl C–O bond and activation enthalpies of 7.8±0.5 and 8.0±0.4 kcal/mole for methyl and ethyl formates, respectively. The difference in energy between trans and cis isomers is estimated to be approximately 2–3 kcal/mole for both methyl and ethyl formates. Absorption measurements in dilute solutions of the formates in xylene and nitrobenzene indicate that the activation enthalpy is insensitive to molecular surroundings. The relation of the activation enthalpy to the barrier to internal rotation, bond order, and the theory of resonance is discussed.