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Please explain Hammond-Leffler postulate

7 years ago

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Hammond's Postulate, also referred to as the Hammond-Leffler postulate, is a hypothesis, derived from transition state theory, concerning the transition state of organic chemical reactions, which states that:


If two states, for example, a transition state and an unstable intermediate, occur consecutively during a reaction process and have nearly the same energy content, their interconversion will involve only a small reorganization of the molecular structures.

Effectively, the postulate states that the structure of a transition state resembles that of the species nearest to it in free energy. That is to say that the transition state of an endothermic reaction resembles the products, while that of an exothermic reaction resembles the reactants.


One other useful interpretation of the postulate often found in textbooks of organic chemistry is the following:


Assume that the transition states for reactions involving unstable intermediates can be closely approximated by the intermediates themselves.

Hammond's postulate is useful for understanding the relationship between the rate of a reaction and the stability of the products. While the rate of a reaction depends just on the activation energy (often represented in organic chemistry as ΔG “delta G double dagger”), the final ratios of products in chemical equilibrium depends only on the standard free-energy change ΔG (“delta G”). The ratio of the final products at equilibrium corresponds directly with the stability of those products.


Hammond's postulate connects the rate of a reaction process with the structural features of those states that form part of it, by saying that the molecular reorganizations have to be small in those steps that involve two states that are very close in energy. This gave birth to the structural comparison between the starting materials, products, and the possible "stable intermediates" that lead to the understanding that the most stable product is not always the one that is favored in a reaction process.

7 years ago

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