March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 7th Edition (2013)

Chapter 6. Mechanisms and Methods of Determining Them

6.G. The Hammond Postulate

Transition states are not detectable and have zero lifetimes, so it is impossible to observe them directly. Information about their geometries must be obtained from inference and modeling. In some cases, the inferences can be very strong. For example, in the S_N2 reaction (Sec. 10.A.i) between CH₃I and I⁻ (a reaction in which the product is identical to the starting compound), the transition state should be perfectly symmetrical. In most cases, however, it is not possible to reach such easy conclusions, and conclusions are greatly aided by the Hammond postulate,²⁴ which states that for any single reaction step, the geometry of the transition state for that step resembles the side to which it is closer in free energy. Thus, for an exothermic reaction like that shown in Fig. 6.1, the transition state resembles the reactants more than the products, although not much more because there is a substantial ΔG^‡ on both sides.

The postulate is most useful in dealing with reactions with intermediates. In the reaction illustrated in Fig. 6.2a, the first transition state lies much closer in energy to the intermediate than to the reactants, and it is possible to predict that the geometry of the transition state resembles that of the intermediate more than it does that of the reactants. Likewise, the second transition state also has a free energy much closer to that of the intermediate than to the products, so that both transition states resemble the intermediate more than they do the products or reactants. This is generally the case in reactions that involve very reactive intermediates. More is usually known about the structure of intermediates than of transition states, so a knowledge of intermediates is used to draw conclusions about the transition states (e.g., see Sec. 10.G.i and 15.B.i).