Hammond's postulate

Lore & Background

During the 1940s and 1950s, chemists had trouble explaining why even slight changes in reactants caused significant differences in reaction rates and product distributions. Notably, John E. The postulate is sometimes called the Hammond–Leffler postulate to give credit to both scientists. The postulate has been used to predict the shape of reaction coordinate diagrams. For example, in electrophilic aromatic substitution, which involves a distinct intermediate and two less well defined states, measuring the effects of aromatic substituents and applying Hammond's postulate led to the conclusion that the rate-determining step involves formation of a transition state that should resemble the intermediate complex. The postulate also helps explain and rationalize the Bell–Evans–Polanyi principle, which describes how the rate of a reaction is affected by its enthalpy. Hammond's postulate explains this by describing how varying the enthalpy of a reaction changes the structure of the transition state, which in turn alters the activation energy and reaction rate.

Reader's Guide

Hammond's postulate is a foundational concept in physical organic chemistry that provides a qualitative framework for understanding transition state structures. Its significance lies in allowing chemists to infer the geometry of transition states—species that cannot be directly observed—by comparing their energy to reactants, intermediates, or products. For exothermic reactions, the transition state is closer in energy to the reactants and thus resembles them structurally (an 'early' transition state), while for endothermic reactions, it resembles the products (a 'late' transition state). This principle has been applied to various reaction mechanisms, including SN1, SN2, E1, and E2 reactions, helping to rationalize reactivity trends and rate-determining steps. The postulate also connects to the Bell–Evans–Polanyi principle, explaining how changes in reaction enthalpy affect activation energy.

Did You Know?

• The postulate states that if two states (e.g., a transition state and an unstable intermediate) have nearly the same energy, their interconversion involves only a small reorganization of molecular structures.
• In an exothermic reaction, the transition state is 'early' and resembles the reactants; in an endothermic reaction, it is 'late' and resembles the products.
• The postulate helps explain the Bell–Evans–Polanyi principle, which describes how reaction rate is affected by reaction enthalpy.