Molecular polarisation studies in relation to mesomerism in aromatic compounds

The dipole moments of a number of substituted anilines, in both benzene and dioxan, have been evaluated from measurements of the dielectric constants, densities and refractive indices of dilute solutions. In selecting the compounds for study attention was paid to the desirability of comparing, on the one hand, the effects of o-p and m-directing substituents and, on the other hand, the effects of the positions of the groups on the deviations from the moments to be expected from the simple vector addition theory. Hence, the dipol9 moments of m- and p-nitro- and bromo-, 2:4- and 3:5-dinitro- and dibromo- and 2:4:6-trinitroaniline, p-nitro- and 2:4-dinitromethyl-aniline and 2-methyl-4-nitroaniline have been measured in both solvents.The results have been interpreted in terms of the mesomeric and inductive effects of the substituents. In the absence of substituents in positions ortho to the amino-group the difference between the apparent moments of the amines in dioxan and benzene solutions increased progressively with increase in the interaction moment. The presence of methyl groups, bromine atoms or nitro-groups in both positions ortho to the amino-group decreased the dioxan increment although there appeared to be no appreciable interference when there was one nitro-group in such a position and only slight inhibition of the mesomeric effect when there was one bromine atom in the ortho-posit ion. The presence of one nitro-group in the ortho-position to a methylamino-group caused a decrease in the dioxan increment. All these results are attributable to steric effects in the molecules.