Hammond's Postulate

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:^[1]

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.

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Interpreting the postulate

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.

This interpretation ignores extremely exothermic and endothermic reactions which are relatively unusual and relates the transition state to the intermediates which are usually the most unstable.

Applying Hammond's Postulate

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.

Explaining seemingly contrary results

Hammond's postulate is especially important when looking at the rate-limiting step of a reaction. However, one must be cautious when examining a multistep reaction or one with the possibility of rearrangements during an intermediate stage. In some cases, the final products appear in skewed ratios in favor of a more unstable product (called the kinetic product) rather than the more stable product (the thermodynamic product). In this case one must examine the rate-limiting step and the intermediates. Often, the rate-limiting step is the initial formation of an unstable species such as a carbocation. Then, once the carbocation is formed, subsequent rearrangements can occur. In these kinds of reactions, especially when run in cooler temperatures, the reactants simply react before the rearrangements necessary to form a more stable intermediate have time to occur. At higher temperatures when microscopic reversal is easier, the more stable thermodynamic product is favored because these intermediates have time to rearrange. Whether run in high or low temperatures, the mixture of the kinetic and thermodynamic products will eventually reach the same ratio, one in favor of the more stable thermodynamic product, when given time to equilibrate due to microreversal.

History of the Postulate

The postulate in named after its creator, George S. Hammond. He first stated it in 1955 while he was a professor of Chemistry at Iowa State. Hammond first put his postulate in print in the Journal of the American Chemical Society. John E. Leffler of Florida State University proposed a similar idea a few years before Hammond ^[2], but Hammond's version became more popular. One alternate name giving credit to both scientists is the Hammond-Leffler postulate.

See also

• Bema Hapothle

References