page_title: Hammond's postulate
section_title: E1 reactions
hierarchical_section_title: Hammond's postulate / Structure of transition states / E1 reactions
caption_attribution_description: English: Unimolecular Elimination Reaction Mechanism
context_page_description: Hammond's postulate, is a hypothesis in physical organic chemistry which describes the geometric structure of the transition state in an organic chemical reaction. First proposed by George Hammond in 1955, the postulate states that:
  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.
context_section_description: An E1 reaction consists of a unimolecular elimination, where the rate determining step of the mechanism depends on the removal of a single molecular species.  This is a two-step mechanism.  The more stable the carbocation intermediate is, the faster the reaction will proceed, favoring the products.  Stabilization of the carbocation intermediate lowers the activation energy. The reactivity order is (CH3)3C- > (CH3)2CH- > CH3CH2- > CH3-.
  Furthermore, studies describe a typical kinetic resolution process that starts out with two enantiomers that are energetically equivalent and, in the end, forms two energy-inequivalent intermediates, referred to as diastereomers.  According to Hammond's postulate, the more stable diastereomer is formed faster.
caption: Unimolecular Elimination Reaction Mechanism
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