ACTIVATION HARDNESS - NEW INDEX FOR DESCRIBING THE ORIENTATION OF ELECTROPHILIC AROMATIC-SUBSTITUTION

The activation hardness concept is defined and used to predict the orientation of electrophilic aromatic substitution. With the transition state defined much in the manner of Wheland, it is shown that the activation energy is the negative of twice the change in hardness in going from reactant to transition state (called the activation hardness). The smaller the activation hardness is the faster the reaction is; the harder the transition state is the better. Calculations presented show that the activation hardness is an excellent index for predicting orientation effects. Several new principles of maximum hardness or softness are stated and discussed. © 1990, American Chemical Society. All rights reserved.