Illustrated Glossary of Organic Chemistry - Hammond postulate (Hammond-Leffler postulate; early transition state; late transition state)

Illustrated Glossary of Organic Chemistry

Hammond postulate (Hammond-Leffler postulate): Postulates that there is a relationship between transition state structure and the energy change involved in the mechanism step that contains this transition state structure. An exergonic mechanism step has an early transition state (a transition state that is early on the energy profile reaction coordinate, and resembles reactants more than products). An endergonic mechanism step has a late transition state (a transition state that is late on the energy profile reaction coordinate, and resembles products more than reactants).

		Many S_N2 reactions are exergonic (ΔG < 0), and therefore have early transition states. For example, the S_N2 reaction of HO^- with CH₃I is exergonic because a carbon-iodine bond is replaced by a stronger carbon-oxygen bond. The transition state resembles HO^- and CH₃I (the reactants) more than it resembles CH₃OH and I^- (the products). In this transition state the oxygen atom has almost a full negative charge whereas the iodide atom has only a tiny negative charge and the carbon-oxygen bond is very incomplete and the carbon-iodine bond is almost completely gone.

		Ionization of a carbon-leaving group bond is endergonic (ΔG > 0) because a carbon-leaving group bond is lost and no new bond is formed, and so this mechanism step has a late transition state. For example the ionization of the carbon-iodine bond of tert-butyl iodide resembles the tert-butyl carbocation and iodide ion (the products) more than it resembles tert-butyl iodide (the reactant). In this transition state the carbon atom has a nearly full +1 formal charge and the iodine atom a nearly full -1 formal charge, and the carbon-iodine bond is almost completely gone.