Steric Effects in Aromatic Carbonyl Compounds: I. Basicities and Ultraviolet Spectra

The basicities of eight o-methyl and o-dimethyl carbonyl compounds have been determined by spectrophotometric methods in aqueous sulphuric acid. The basicities of a further 19 compounds of the same class have been recalculated from literature data using a recently determined H 0 scale of acidity. The effects of ortho substitution on the base strength have been found to increase with the steric requirements of the group attached to the carbonyl carbon.The ultraviolet intensities of 18 of the above compounds have either been freshly determined or carefully redetermined. The mean interplanar angle θ between carbonyl and phenyl groups has been obtained for both base and conjugate acid forms using the relationship ε/ε planar ≈ cos 2 θ.For the conjugate acids of the o-methyl derivatives a linear relation exists between ε/ε planar and the steric effect on basicity. This is discussed in terms of inhibition of resonance in the cations. No similar relation exists for the o-dimethyl derivatives. Steric effects on basicity in the latter are discussed in terms of steric inhibition of solvation of the cations.