Hindered Rotation in Ethyl Alcohol

Abstract

Previous evaluations of the barriers hindering the internal rotations in ethyl alcohol are shown to be unsatisfactory because they fail to produce agreement between molecular and calorimetric entropies over a wide enough temperature range. The principal source of difficulty is found in an inconsistency between the modified Berthelot equation of state and the calorimetric data and vapor pressures used in calculating the third law entropies. Corrections for gas imperfection should be much higher at the temperatures at which the barrier evaluations have been made, but these corrections cannot be determined accurately with the available data. The larger entropies permit the assignment of lower and more reasonable potential barriers. For the hydroxyl group rotation, to keep the heat capacity, and therefore the change of entropy with the temperature, within the indicated limits, the simple expression V = V₀(1—cos3θ)/2 is more suitable than more detailed potential energy functions. With a reasonable assignment of the vibrational frequencies, the sum of the barriers restricting the hydroxyl and methyl group rotations is approximately 6000 cal.