Determination of Fractional Steric Inhibition of Resonance from σR Values

THE INFRARED FREQUENCIES AND INTENSITIES OF THE HYDROXYL BAND IN ORTHO-SUBSTITUTED PHENOLS

The infrared frequencies, intensities, and half-band widths of the fundamental stretching vibration of the hydroxyl group have been measured for a large number of ortho-alkyl phenols. It has been found that phenols containing a single ortho-alkyl substituent exist as cis and trans isomers in unequal amounts, the trans isomer being the more abundant. The difference in free energy between the two isomers in dilute solution in carbon tetrachloride at 24 °C has been measured for ortho-cresol, 2,3-dimethyl phenol, and several ortho-tert-alkyl phenols.

THE INFRARED FREQUENCIES AND INTENSITIES OF THE HYDROXYL GROUP IN 2,6-DI-tert-BUTYL-4-SUBSTITUTED PHENOLS

The infrared frequencies, intensities, and apparent half band widths of the O—H stretching band of a series of 2,6-di-tert-butyl-4-substituted phenols have been measured in carbon tetrachloride solution and compared with the corresponding 4-substituted phenols. The frequencies of the former are 36 cm−1 higher than the latter and the intensities are also slightly higher. Both the frequency and intensity follow a Hammett ρσ relation. It is shown that the O—H bond lies in the plane of the benzene ring, the increase in frequency and intensity being ascribed to a steric repulsion of the phenolic hydrogen by the adjacent tert-butyl group.