The nature of conformational preference in a number of p-alkyl phenols and p-alkyl benzenes

The energy barriers to hydroxyl rotation and alkyl rotation in p-cresol, p-isopropylphenol, p-tert-butylphenol, isopropylbenzene and tert-butylbenzene have been calculated using the MP2/6-31g(d,p) model chemistry. The number of stable conformers predicted for isopropylbenzene, tert-butylbenzene, p-isopropylphenol and p-tert-butylphenol is consistent with experimental observations. The doubling of stable alkyl conformation, and hence spectral features, in the p-alkyl phenols is due to syn and anti orientations of the hydroxyl and alkyl substituents and can be related to an asymmetry in the electron density in the HOMO of phenol. Hyperconjugation is found to have an important influence on the torsional potential of the alkyl substituent.