Collect. Czech. Chem. Commun. 1985, 50, 208-214

Secondary steric effects in 2,4,6-trialkylated acetophenones and nitrobenzenes: Dipole moments and 13C NMR spectra

Juraj Koudelka, David Šaman and Otto Exner

Institute of Organic Chemistry and Biochemistry, Czechoslovak Academy of Sciences, 166 10 Prague 6


Dipole moments of 2,4,6-trialkylated acetophenones (II) and bromobenzenes (III) with variable size of the alkyl groups were measured in two solvents. The main intention was to shed more light on the previous results with similarly substituted nitrobenzenes (I). The comparison reveals a steady decrease of the dipole moment with the increasing size of the alkyl group in all three series. In addition, there is a stronger effect of steric inhibition of mesomerism in the series II and a comparably strong effect of the anisotropic polarizability in I; the two cannot be distinguished on the basis of dipole moments. Similarly an analysis of 13C NMR shifts reveals at least two effects. In one of them the series I behaves like III, in the other like II. Summarizing, our results are in agreement with the conception of negligible conjugation of the nitro group but they do not contribute significantly to its evidence which is based mainly on other physical quantities.