Substituent Constants for Aromatic Substitution

Abstract

A plot of the logarithms of the rate constants for the solvolysis of the meta and para substituted phenyldimethylcarbinyl chlorides against the Hammett σ-constants reveals an excellent correlation in the case of the meta derivatives but serious deviations for the para substituents. The deviations are in the direction to be anticipated for important resonance interactions between the para substituent and the electron deficient center of the incipient carbonium ion in the transition state. The meta derivatives are used to establish ρ for the reaction (−4.620). A new set of σ+-constants for the para substituents is then calculated from this ρ-value and the observed solvolysis rates. The available data on the rates of the nitration of monosubstituted aromatics, the halogenation of aromatics, the brominolysis of arylboronic acids and the protonolysis of aryltrimethylsilanes exhibit important deviations using the Hammett σ-values, but they give good agreement with the σ+ values. It is proposed that the σ+-values may provide a quantitative basis for the treatment of directive effects in aromatic substitution. © 1957, American Chemical Society. All rights reserved.