Diamagnetic susceptibilities of some substituted anisoles and thioanisoles. Evidence for steric enhancement of resonance

Abstract

Evidence for steric enhancement of resonance in some benzene derivatives is obtained from their diamagnetic susceptibilities. Conjugative interaction of substituents in the benzene ring results in a decrease in diamagnetic susceptibility. For 2-methyl-4-nitroanisole and 2-halogeno-4-nitroanisoles the observed diamagnetic susceptibilities are significantly lower than the expected value, indicating that the 2-substituent in these compounds enhances the resonance interaction of the 1,4-substituents. Interestingly, the diamagnetic susceptibility data also show steric inhibition of resonance in 2,6-disubstituted-4-nitroanisoles. 2-Methyl-4-nitrothioanisole seems to exhibit a behaviour similar to that of its oxygen analogue, though the evidence for steric enhancement of resonance in this case is not beyond doubt. 2,6-Dimethyl-4-nitrothioanisole, however, convincingly shows the expected steric inhibition of resonance. The diamagnetic susceptibilities of 3-substituted-4-methoxyacetophenones also furnish further evidence for steric enhancement of resonance.