Calculation of Dipole Moments from Rates of Nitration of Substituted Benzenes and Its Significance for Organic Chemistry

Abstract

(1) From a set of assumptions which seem reasonable a priori, the authors calculate the charge distributions in the molecules of mono‐substituted benzenes, and the dipole moments of the latter from reaction rate data alone. These coincide very well with the observed moments. (2) From the dipole moments of aromatic and aliphatic compounds, the ortho‐, meta‐ and para‐percentages during nitration were calculated. These also accord satisfactorily with the experimental values. (3) It is concluded that these two facts give a quantitative basis to the usual explanation about the orienting power of substituents of benzene. (4) The nitration of benzene is an ionic reaction : The nitrating agent X·NO₂ dissociates into X^— and NO₂⁺ in the activated complex, and the latter ion interacts with the reacting carbon. The activation free energy is reduced by the amount of the electrostatic interaction, i.e., 4.80·10^—10ε_y/(rD). Here ε_y is the charge on the reacting carbon, r, the distance between NO₂⁺ and the reacting carbon in the activated complex, D, the dielectric constant of the medium between NO₂⁺ and the reacting carbon. (5) The authors find that r=1.6A and D=1 approximately, independent of the nature of the reacting media. The consequences of this theory are outlined. (6) Further this interpretation of reaction rates provides, for the first time, a method of calculating the atomic charges in the molecules as distinguished from the usual dielectric measurements which lead only to dipole moments.