Hammond's Postulate
Encyclopedia
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 reaction
s, which states that:
. That is to say that the transition state of an endothermic reaction resembles the products, while that of an exothermic reaction
resembles the reactants. A transition state that resembles the reactants more than the products is said to be early, while a transition state that resembles the products more than the reactants is said to be late. Thus, the postulate predicts an early transition state for an exergonic reaction and a late transition state for an endergonic reaction. A dimensionless reaction coordinate quantifying the lateness of a transition state can be used to test the validity of the postulate for a particular reaction.
One other useful interpretation of the postulate often found in textbooks of organic chemistry
is the following:
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.
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.
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 at lower 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 at 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.
. 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, but Hammond's version became more popular. One alternate name giving credit to both scientists is the Hammond-Leffler postulate.
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
, derived from transition state theory
Transition state theory
Transition state theory explains the reaction rates of elementary chemical reactions. The theory assumes a special type of chemical equilibrium between reactants and activated transition state complexes....
, concerning the transition state
Transition state
The transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest energy along this reaction coordinate. At this point, assuming a perfectly irreversible reaction, colliding reactant molecules will always...
of organic chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
s, 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.
Interpreting the postulate
Effectively, the postulate states that the structure of a transition state resembles that of the species nearest to it in free energyThermodynamic free energy
The thermodynamic free energy is the amount of work that a thermodynamic system can perform. The concept is useful in the thermodynamics of chemical or thermal processes in engineering and science. The free energy is the internal energy of a system less the amount of energy that cannot be used to...
. That is to say that the transition state of an endothermic reaction resembles the products, while that of an exothermic reaction
Exothermic reaction
An exothermic reaction is a chemical reaction that releases energy in the form of light or heat. It is the opposite of an endothermic reaction. Expressed in a chemical equation:-Overview:...
resembles the reactants. A transition state that resembles the reactants more than the products is said to be early, while a transition state that resembles the products more than the reactants is said to be late. Thus, the postulate predicts an early transition state for an exergonic reaction and a late transition state for an endergonic reaction. A dimensionless reaction coordinate quantifying the lateness of a transition state can be used to test the validity of the postulate for a particular reaction.
One other useful interpretation of the postulate often found in textbooks of organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
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 the postulate
Hammond's postulate is useful for understanding the relationship between the rate of a reactionReaction rate
The reaction rate or speed of reaction for a reactant or product in a particular reaction is intuitively defined as how fast or slow a reaction takes place...
and the stability of the products.
While the rate of a reaction depends just on the activation energy
Activation energy
In chemistry, activation energy is a term introduced in 1889 by the Swedish scientist Svante Arrhenius that is defined as the energy that must be overcome in order for a chemical reaction to occur. Activation energy may also be defined as the minimum energy required to start a chemical reaction...
(often represented in organic chemistry as ΔG‡ “delta G double dagger”), the final ratios of products in chemical equilibrium
Chemical equilibrium
In a chemical reaction, chemical equilibrium is the state in which the concentrations of the reactants and products have not yet changed with time. It occurs only in reversible reactions, and not in irreversible reactions. Usually, this state results when the forward reaction proceeds at the same...
depends only on the standard free-energy change
Gibbs free energy
In thermodynamics, the Gibbs free energy is a thermodynamic potential that measures the "useful" or process-initiating work obtainable from a thermodynamic system at a constant temperature and pressure...
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 rearrangementsRearrangement reaction
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 molecule. Often a substituent moves from one atom to another atom in the same molecule...
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
Thermodynamic reaction control
Thermodynamic reaction control or kinetic reaction control in a chemical reaction can decide the composition in a reaction product mixture when competing pathways lead to different products and the reaction conditions influence the selectivity...
) rather than the more stable product (the thermodynamic product
Thermodynamic reaction control
Thermodynamic reaction control or kinetic reaction control in a chemical reaction can decide the composition in a reaction product mixture when competing pathways lead to different products and the reaction conditions influence the selectivity...
). 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
Carbocation
A carbocation is an ion with a positively-charged carbon atom. The charged carbon atom in a carbocation is a "sextet", i.e. it has only six electrons in its outer valence shell instead of the eight valence electrons that ensures maximum stability . Therefore carbocations are often reactive,...
. Then, once the carbocation is formed, subsequent rearrangements can occur. In these kinds of reactions, especially when run at lower 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
Microscopic reversibility
The principle of Microscopic reversibility in physics and chemistry is twofold:* First, it states that the microscopic detailed dynamics of particles and fields is time-reversible because the microscopic equations of motion are symmetric with respect to inversion in time ;* Second, it relates to...
is easier, the more stable thermodynamic product is favored because these intermediates have time to rearrange. Whether run at 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
The postulate is named after its creator, George S. HammondGeorge S. Hammond
George S. Hammond was a chemist at Iowa State University and the California Institute of Technology. Among his awards were the Norris Award in 1968 and the Priestley Medal in 1976 and the National Medal of Science in 1994...
. 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, but Hammond's version became more popular. One alternate name giving credit to both scientists is the Hammond-Leffler postulate.