On the Nature of Solvent Effects in the Proton Resonance Spectra of Unsaturated Ring Compounds. I. Substituted Benzenes

Abstract

The relative chemical shift between the resonances of ortho and meta hydrogens in certain para‐disubstituted benzenes are markedly dependent on solvent and concentration. This behavior, which is observed also in other substituted benzenes, is characterized by a pronounced shift of some proton signals to low field in acetone solution and to high field in benzene solution. In compounds of the type [Complex chemical formula] where X is an electron‐withdrawing substituent, the resonance signals of protons meta to the X substituent are displaced selectively in donor solvents. When the CH₃ group is replaced by F, both ortho and meta hydrogens show pronounced shifts. The observations can be interpreted in terms of a specific molecular association involving hydrogen bonding. Although generally these bonds appear relatively weak, for some compounds the hydrogen‐bond strength is estimated to be comparable to those formed by chloroform in donor solvents. These effects offer a valuable practical aid in proton resonance measurements since in such compounds, within limits, it is possible to alter the relative chemical shifts by a suitable adjustment of solvent conditions.