Nitro-benzene polarizability

It is obvious here to think of an interaction of the Debye type of the appreciable moment of the nitro or carbonyl group with the readily polarizable unsaturated C — C bond in the unsaturated hydrocarbons or with the likewise readily polarizable free electron pair in the amines, etc. Actually the stability increases with increasing polarizability, compare for example, the heat of formation of j-trinitrobenzene with, respectively, benzene, naphthalene and anthracene AH 0.6, 3.4 and 4.4 kcal. In the series mono-, di- and tri-nitro-benzene with acenaphthene AH also increases with increasing number of partial moments, namely AH — o, 1.35 and 2.45 kcal. Nevertheless this explanation is inadequate, nitromethane with a moment [x = 3.54 D gives very much less stable compounds than nitrobenzene with (X = 4,22 D, while nitrosobenzene with [x =3.2 D gives stronger colour effects than nitrobenzene. 

The association of nitrobenzene and nitronaphthalene in non-polar solvents, such as hexane and carbon tetrachloride, forms an exception. Here the association depends in the first place on interaction of the Keesom type with the very large moment (4.20 D). The stronger association of the last-mentioned compound points, however, in addition to an interaction due to complex resonance as observed between nitro compounds and aromatic hydrocarbons in general. It is plausible that when dissolved in benzene this association gives way to a solvation. This interaction between solute and solvent molecules is closely related to the association between like molecules. In benzene etc. no stoichiometric association is observed but, owing to the anisotropy of the polarizability, a more or less parallel... 

The second-order molecular polarizability, /, and the third-order nonlinear susceptibility, have been measured for many compounds (see Table 5.7). Note that the f value of nitroaniline, where the centrosymmetric benzene ring carries an electron withdrawing nitro-group and an electron-donating amino-group, is larger than that of monosubstituted benzenes. 

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