Hammond Postulate

HAMMOND POSTULATE

The relationship between energy of the transition state and the structure of the activated complex is summarized by the Hammond postulate, which states that the structure of the activated complex for any reaction step is most similar to the species (reactant or product) to which it is most similar in energy. As seen previously in Figure 5.8, exergic reactions (where the reactants are higher in energy than the products) have early transition states and activated complexes that resemble the reactants. Moreover, endergic reactions (in which the products are higher in energy than the reactants) have late transition states and activated complexes that resemble the products, as in Figure 5.10.

A most useful application of the Hammond postulate involves reactions which proceed by the formation of unstable intermediates, such as the carbocations, carbanions, carbenes, free radicals, and so on. 

The rate-determining step of such reactions is necessarily endothermic, and the Hammond postulate serves as a use-ful tool for identifying structural characteristics of the activated complex leading to that intermediate. The logical next step is to ask how structural features in reactants change the structure and thus energy of the activated complex.

The Hammond postulate states that in endergic reactions, features which sta-bilize and thus lower the energy of a product lower the energy of the transition state leading to that product. This is shown in Figure 5.12. If product 2 (P₂) is lower in energy than product 1 (P₁), then transition state 2 (±₂) will be lower than transition state 1 (±₁). It will also be earlier. As a consequence, P₂ will have a lower activation barrier and be formed faster than P₁. A simplified restatement of the Hammond postulate is that more stable products are formed faster. It must be remembered that this analysis is for endothermic reactions and assumes that the reactants have the same or similar energies.

The ionization of alkyl tosylates to give carbocations is an endothermic reac-tion. Knowing that 3^◦ carbocations are more stable than 2^◦ carbocations, we would conclude that the activation barrier for ionization of the 3^◦ tosylate is lower than that of the 2^◦ tosylate, and thus the 3^◦ tosylate should ionize faster (Figure 5.13).

We would also predict that the transition state for ionization of the 3^◦ tosylate would be earlier, so there should be less C–O bond breaking and less charge development than in the activated complex for ionization of the 2^◦ tosylate.

The Hammond postulate provides a key relationship between the rate of reaction and the activated complex of that reaction (Figure 5.14). In practice, structural changes are made in the reactant(s) and the influence of those changes on the rate of reaction is measured. If the reaction is faster, then the change in the reactant has led to a lower product energy and hence a lower activation energy and an earlier transition state (one which has more reactant character).

 If the reac-tion is slower, then the change in the reactant has led to a higher product energy and hence a higher activation energy and a later transition state (one with more product character). The results of rate studies can thus be translated into structural changes (bonding, charge distribution, geometry) in the activated complex, which further translates into the mechanistic information about the reaction.

The Hammond postulate is best applied to reactions with unstable intermedi-ates, such as the carbocations in the above example. In such cases the transition state is late and the activated complex more resembles the intermediate. Thus changes in the energy of the intermediate have the greatest effect on the energy of the transition state and thus the rate of the reaction.

However, the Hammond postulate also holds for exergic reactions where the transition state is early and the activated complex more resembles the reactant. For such reactions changes in the energy of the reactants have the greatest effect on the energy of the transition state and thus the rate of the reaction.