Use of the two-dimensional energy-reaction coordinate diagram to demonstrate the structure-reactivity relationship in organic reactions influenced bythe dominant polar or polar and resonance effects of substituents

An easy procedure is described to understand the substituent effect on rate s of organic reactions. The procedure uses the two-dimensional energy-react ion coordinate diagram to show whether the predominant substituent effect i s on the ground state or on the transition state of the reaction. Based upo n the usual definition of Hammett (sigma) and Taft (sigma*) substituent con stants, the signs of the reaction constants, rho and rho*, can be easily pr edicted by knowing qualitatively the relative polarity of the ground state and the transition state of the reaction. Conversely, if the relative polar ity of the reactant and the product states as well as the sign of rho or rh o* are known, the nature of the charge on the transition state in the rate- determining step can be qualitatively characterized.