Effects of Methyl and Hydroxyl Substitution on Barriers to Internal Rotation about Single Bonds

Abstract

The electrostatic model for threefold barriers to internal rotation developed in earlier papers is updated. The barrier in methyl arsine is predicted to be 1.57 kcal/mole. It is also shown that the barrier recently measured for nitrosomethane is in fair agreement with the model result. The effect upon the barrier of methyl substitution in ethanelike molecules is shown to correlate well with the electronegativity of the central atom where substitution occurs. The very different barrier increases caused by methyl substitution in propene and methyl ketene are reproduced and shown to be simply related to structure. An explanation is proposed for the experimentally observed lowering of the barrier in going from acetaldehyde to acetone. The very low barrier in acetic acid is shown to be consistent with delocalization of hydroxyl pi electrons. A low barrier for ethanol is not supported by the model.