Restricted Rotation in Aromatic Ketones. I. Substituent Effects on the Barrier to Rotation about the Benzene-to-Carbonyl Bond

Several aryl isopropyl ketones and aryl benzyl ketones were synthesized and their NMR spectra were recorded at various temperatures. Their temperature dependence was accounted for in terms of the restricted rotation about the benzene-to-carbonyl bond, whose free energies of activation at the coalescence temperatures varied from 9 to 15 kcal/mol according to the steric effect of ortho substituents, the buttressing effect of meta substituents, and the steric effect of the side-chain groups.