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. A hypothesis (from Greek) consists either of a suggested explanation for a phenomenon (an event that is observable or of a reasoned proposal suggesting a possible In Chemistry, transition state theory is a conception of Chemical reactions or other processes involving rearrangement of matter as proceeding through a continuous The transition state of a Chemical reaction is a particular configuration along the Reaction coordinate. A chemical reaction is a process that always results in the interconversion of Chemical substances The substance or substances initially involved in a chemical reaction are called

Contents 1 Interpreting the postulate 2 Applying Hammond's Postulate 3 Explaining seemingly contrary results 4 History of the Postulate 5 See also 6 External links 7 References

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. In Thermodynamics, the term thermodynamic free energy refers to the amount of work that can be extracted from a System, and is helpful in Engineering That is to say that the transition state of an endothermic reaction resembles the products, while that of an exothermic reaction resembles the reactants. In Thermodynamics, the word endothermic "within-heating" describes a process or reaction that absorbs Energy in the form of Heat. An exothermic reaction is a Chemical reaction that releases Heat. A reagent or reactant is a substance or compound consumed during a Chemical reaction.

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. Organic chemistry is a discipline within Chemistry which involves the scientific study of the structure properties composition reactions, and preparation

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. The reaction rate or rate of reaction for a Reactant or product in a particular reaction is intuitively defined as how fast a reaction takes 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”). In Chemistry, activation energy, also called midnight energy, is a term introduced in 1889 by the Swedish scientist Svante Arrhenius, that is defined In a Chemical process, chemical equilibrium is the state in which the chemical activities or Concentrations of the reactants and products have no net change In Thermodynamics, the Gibbs free energy ( IUPAC recommended name Gibbs energy or Gibbs function) is a Thermodynamic potential which 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. The rate-determining step (RDS is a Chemistry term for the slowest step in a Chemical reaction. However, one must be cautious when examining a multistep reaction or one with the possibility of rearrangements during an intermediate stage. A rearrangement reaction is a broad class of Organic reactions where the carbon skeleton of a Molecule is rearranged to give a Structural isomer of the original 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). Thermodynamic reaction control or kinetic reaction control in a Chemical reaction can decide the composition in a reaction product when competing reactions lead to Thermodynamic reaction control or kinetic reaction control in a Chemical reaction can decide the composition in a reaction product when competing reactions lead to 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. A carbocation (ˌkɑrboʊˈkætaɪɒn is an Ion with a positively-charged Carbon Atom. 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. The principle of Microscopic reversibility in Chemistry states that in a Reversible reaction the mechanism in one direction is exactly the reverse of the mechanism 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 is 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

External links

• Hammond Principle - Definition in the IUPAC Gold Book. In Chemistry, Bema Hapothle is an extended Acronym for Bell - Marcus - Hammond - Polanyi - Thornton - Leffler, referring

References

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