Hammond's postulate

George Hammond developed the postulate during his professorship at Iowa State University

Hammond's postulate (or alternatively the Hammond–Leffler postulate), is a hypothesis in physical organic chemistry which describes the geometric structure of the transition state in an organic chemical reaction.[1] First proposed by George Hammond in 1955, the postulate states that:[2]

If two states, as, 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.

Therefore, the geometric structure of a state can be predicted by comparing its energy to the species neighboring it along the reaction coordinate. For example, in an exothermic reaction the transition state is closer in energy to the reactants than to the products. Therefore, the transition state will be more geometrically similar to the reactants than to the products. In contrast, however, in an endothermic reaction the transition state is closer in energy to the products than to the reactants. So, according to Hammond’s postulate the structure of the transition state would resemble the products more than the reactants.[3] This type of comparison is especially useful because most transition states cannot be characterized experimentally.[4]

Hammond's postulate also helps to explain and rationalize the Bell–Evans–Polanyi principle. Namely, this principle describes the experimental observation that the rate of a reaction, and therefore its activation energy, is affected by the enthalpy of that reaction. Hammond's postulate explains this observation by describing how varying the enthalpy of a reaction would also change the structure of the transition state. In turn, this change in geometric structure would alter the energy of the transition state, and therefore the activation energy and reaction rate as well.[5]

The postulate has also been used to predict the shape of reaction coordinate diagrams. For example, electrophilic aromatic substitution involves a distinct intermediate and two less well defined states. By measuring the effects of aromatic substituents and applying Hammond's postulate it was concluded that the rate-determining step involves formation of a transition state that should resemble the intermediate complex.[6]

  1. ^ Fox MA, Whiteshell JK (2004). Organic Chemistry. Sudbury, Massachusetts: Jones and Bartlett Publisher s. pp. 355–357. ISBN 978-0-7637-2197-8.
  2. ^ Hammond, G. S. (1955). "A Correlation of Reaction Rates". J. Am. Chem. Soc. 77 (2): 334–338. doi:10.1021/ja01607a027.
    Solomons, T.W. Graham & Fryhle, Craig B. (2004). Organic Chemistry (8th ed.). John Wiley & Sons, Inc. ISBN 0-471-41799-8.
    Loudon, G. Marc. "Organic Chemistry" 4th ed. 2005.
  3. ^ Carey FA, Sundberg R (2007). Advanced Organic Chemistry Part A:Structure and Mechanisms. Norwell: Springer.
  4. ^ Anslyn EV, Dougherty DA (2006). Modern Physical Organic Chemistry. Sausalito, CA: University Science.
  5. ^ McMurry J (1992). Organic Chemistry. Pacific Grove, CA: Brooks/Cole. pp. 246–248.
  6. ^ Carey FA, Sundberg RJ (1990). Advanced Organic Chemistry.-Part A: Structure and Mechanism. New York, NY: Plenum.