Effect of steric hindrance and π electrons on alcohol self-association
A general discussion on the effect of steric hindrance and π electrons on the self-association of linear, branched, cyclic, aromatic and phenolic alcohols in inert solvents is presented. A relation between the self-association ability of an alcohol and its molecular geometry is proposed using as a measure of the steric hindrance the ratio (q) of the polar surface area of the alcohol to the polar surface area of a linear alcohol. Surface areas were calculated using a molecular-mechanics program. The ratio q correlates with the free energy for tetramer formation and is a good measure of the local steric hindrance on the hydroxyl caused by groups in its immediate neighbourhood. The effect of π electrons on self-association was studied through quantum-mechanical calculations (MNDO), providing an explanation for the experimental finding that the enthalpy for hydrogen-bond formation for phenolic alcohols is systematically smaller than for any other type of alcohol. The MNDO results indicate that for any alcohol whose hydroxyl group is directly attached to the aromatic ring the electron density in the oxygen atom is small and hence phenolic alcohols form weak hydrogen bonds. Values of the enthalpy and entropy for hydrogen-bond formation are correlated and almost compensate for the alcohols.