Barrier height Hammond postulate

These simple considerations yield several corollaries, sometimes known together as the Bell-Evans-Polanyi (BEP) principle [14]. First, there is an approximately linear relation between the barrier height and the reaction energy this is the basis of the Bronsted relation (and other LFERs). Second, the proportionality constant a in Eq. (19.2) tends to be smaller for exothermic reactions (but larger for endothermic reactions). Third, the position of the crossing point between the curves lies closer to the reactants for more exothermic reactions this is the basis of the Hammond postulate, that the TS for a more exothermic reaction more closely resembles the reactants (and that for a more endothermic reaction more closely resembles the products). 

The Hammond postulate does not predict the height of the barrier compared to the reactant and product, only its position along the reaction coordinate. For example, we are not forced to draw the continuum as shown in Figure 7.9 in order to obey the Hammond postulate. In fact. Figure 7.10 shows a different overlay of reaction coordinate diagrams, each of which also conforms to the Hammond postulate, and you may occasionally encounter a system that exhibits this kind of behavior. However, most reactions that are of similar type but vary in their thermodynamics will have reaction coordinates that resemble those drawn in Figure 7.9. 

Donahue, N. M. (2001). Revisiting the Hammond postulate the role of reactant and product ionic states in regulating barrier heights, locations, and transition state frequencies. J. 

We have found that second-order resonance structures can be used to predict the regiochemistry of an EAS reaction. A more rigorous prediction comes from the heights of the activation barriers. According to the Hammond Postulate, the relative energies of the intermediates can be used to approximate the relative heights of these activation barriers. Simply put... The pathway with the most favorable carbocation intermediate will likely dominate the product mixture. 

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