Mesomeric dipole moments. Part 6. Steric inhibition of conjugation in 2,4,6-trimethylphenyl derivatives

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Abstract
The dipole moments of 2,4,6-trimethylphenyl derivatives (IIb—h) measured in benzene solution were compared to those of simple phenyl derivatives (Ib—h) in order to evaluate the steric inhibition of mesomerism of individual substituents. The best example of this phenomenon was found in the case of the BBr2 substituent [(If) and (IIf)], and it is demonstrable for B(OMe)2, B(SMe)2, COCH3, and CO2CH3; conjugation of these substituents with the benzene nucleus is also proved by results from other areas. In contrast, conjugation in aromatic nitro-compounds, often quoted as a classical example, is certainly very weak and can hardly be proven. The relatively small changes in the dipole moments of compounds (Ia) and (IIa) may originate for the most part in the strong inductive effect of the nitro-group combined with the anisotropic polarizability of the benzene nucleus. Also, analysis of other experimental data concerning nitro-compounds revealed a strong inductive effect and negligible conjugation with the benzene nucleus unless a donor substituent is present.

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