Hammond's postulate

Hammond's postulate, also known as the Hammond–Leffler postulate, is a concept in physical organic chemistry that deals with the shape of the transition state during an organic reaction. It was first proposed by George Hammond in 1955. The idea is that if two states, such as a transition state and an unstable intermediate, appear one after another in a reaction and have nearly the same energy, then converting between them requires only a small change in molecular structure. This means the geometry of a state can be predicted by looking at how its energy compares to the species next to it on the reaction path. For an exothermic reaction, the transition state is closer in energy to the reactants, so it will look more like the reactants. For an endothermic reaction, the transition state is closer in energy to the products, so it will resemble the products. This comparison is useful because most transition states cannot be studied directly in experiments.

The postulate also helps explain the Bell–Evans–Polanyi principle, which notes that a reaction's rate and activation energy are influenced by its enthalpy. Hammond's postulate accounts for this by showing that a change in reaction enthalpy alters the transition state's structure, which in turn changes its energy, the activation energy, and the reaction rate. It has also been used to predict the shape of reaction coordinate diagrams. For instance, in electrophilic aromatic substitution, which has a distinct intermediate and two less defined states, applying Hammond's postulate to the effects of aromatic substituents led to the conclusion that the rate-determining step forms a transition state that resembles the intermediate complex.

In the 1940s and 1950s, chemists struggled to explain why small changes in reactants caused big differences in reaction rates and product mixtures. In 1955, George Hammond, then a young professor at Iowa State University, proposed that transition-state theory could qualitatively explain these structure-reactivity relationships. John E. Leffler of Florida State University had suggested a similar idea in 1953, but Hammond's version gained more attention because its qualitative nature was easier to understand and apply than Leffler's complex math. The postulate is sometimes called the Hammond–Leffler postulate to credit both scientists.

In practice, the postulate says the transition state's structure is most like the species closest to it in free energy. On a potential energy diagram, for an exothermic reaction (case a), the transition state's energy is nearer the reactant's, so its structure also resembles the reactant. If the transition state's energy is close to neither the reactant nor the product (case b), neither is a good model, and more information is needed. For an endothermic reaction (case c), the transition state should look more like the intermediate or product. This allows chemists to discuss transition states in terms of reactants, intermediates, or products. When the transition state resembles the reactants, it is called "early"; when it resembles the intermediate or product, it is called "late."

An example of an early transition state is chlorination. Chlorination is exothermic, so the products are lower in energy than the reactants. The transition state appears right after the reaction starts. An example of a late transition state is bromination. Bromination is endothermic, so the reactants are lower in energy than the products. The transition state appears right before the reaction is complete. Another common interpretation found in textbooks is that, for reactions involving unstable intermediates, the transition state can be closely approximated by the intermediates themselves, ignoring extremely exothermic or endothermic reactions, which are less common.