Conformation and steric effects in mono- and dimethoxybenzoic acids

The conformation of mono- and dimethoxybenzoic acids in solution was determined from their IR spectra and C-13 and H-1 NMR spectra. The main feature is a strong intramolecular hydrogen bond in all 2-methoxy derivatives that persists even in polar aprotic solvents but not in methanol. The methyl groups are mostly coplanar with the ring plane, the two planar conformations are almost equally abundant, and their conformation is not responsible for some enhanced values of the enthalpy of formation. The hydrogen bond (and possibly dimerization) is the reason for differences in the relative acidities (delta pK = pK - pK(H)) measured either in dimethyl sulfoxide or in methanol and also for the differences between dpK values of methoxybenzoic acids and corresponding methylbenzoic acids. Nevertheless, it was possible to separate with good approximation the polar and steric effects on the acidity. The steric effects in methanol are then parallel for methyl- and methoxybenzoic acids despite variable conformation of the latter. Steric effects in dimethyl sulfoxide are smaller and less regular. The acidity enhancement by ortho substituents is better described as electrostatic induction in the anion rather than a steric hindrance.