Postulate of Hammond

The postulate of Hammond , sometimes called postulate of Hammond-Leffler, is a Hypothèse, derived from the theory of the state of transition. In the organic reactions this theory stipulates that:

When two states, for example a state of transition and an unstable intermediary, follow one another in a reactional process and have almost same energy, then their interconversion requires only one weak reorganization of the molecular structures.

Interpretation of the postulate

In an effective way, this postulate indicates that the structure of a state of transition is similar to those of the species closest in terms to free energy (or, to speak in term about Surface about potential energy, of the closest species topologically). I.e. the state of transition from a endergonic Réaction is close to the products whereas that of a exergonic Réaction is similar to the Réactant s.
Another current interpretation that one can find in the books of Organic chemistry is the following one:

One considers that the states of transition from reactions implying of the unstable intermediaries can be relatively quite approximate by the intermediaries themselves.

This interpretation puts side however the reactions extremely Endothermique S or Exothermique S which is relatively not very current and connects the state of transition to the intermediaries aui are usually most unstable.

Application of the postulate

The postulate of Hammond is very useful for the comprehension of the relation between the Taux of reaction and the stability of the products of réaction.
When the rate of reaction only depends on the energy of activation (sometimes indicated in thermochemistry by the notation ΔG^‡ delta G doubles scraping-knife ), the final rates of products in chemical balance depend only on the variation of free energy standard ΔG ( delta G ). The rate of finished products to balance is then directly related to the stability of these produits.
The postulate of Hammond binds the rate of a reactional process with the structural factors of the states which form part of it, by saying that the molecular reorganizations must be small at the time of these stages which imply two very close stages in energy. This allows structural comparisons between starting materials, the products and them intermediate states possible which makes it possible to include/understand why the most stable product thermodynamically is not always that which is favoured during the reactional process.

Explanation of apparently contradictory results

The postulate of Hammond is particularly important when one studies the limiting stage of a reaction. However, one must proceed with prudence when it is question of a reaction in several stages or a reaction in which a rearrangement at the time of an intermediate stage occurs. In certain cases, the finished products are in proportions significantly important in favor of the least stable product (Produit kinetic) and thus with the detriment of the most stable product (Produit thermodynamic). In this case, one must study the stage limiting as well as the intermediaries. The limiting stage often corresponds to the initial training of unstable species like the Carbocation S. Ensuite, once the species (or species) unstable is formed, of the important rearrangements can occur. In this kind of reactions, particularly when they are carried out at low temperatures, the reagents react before the rearrangements necessary to form a more stable Isomère can occur. At higher temperatures, when the microscopic Réversibilité is easier, the most stable product thermodynamically is favoured because it time left to carry out the rearrangements. According to whether one is with high or low temperatures, the mixture of products kinetic and thermodynamic will reach possibly the same proportion, in favor of the thermodynamically stable product, if one gives time to the balancing of microreversibility.

Note historical

The postulate was named according to the name of its creator, George S. Hammond. It stated it the first in 1955 when he was chemistry teacher of the Université of State of Iowa. Hammond initially published it in Journal off the American Chemical Society . John E. Leffler, of the Université of State of Florida proposed a similar idea a few years before Hammond, but the version of Hammond was most known. However, this postulate is also known under the name of postulate of Hammond-Leffler.

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