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In physical organic chemistry, Hammond's postulate, also referred to as the Hammond–Leffler postulate, is a hypothesis concerning the transition state of organic chemical reactions, proposed by George S. Hammond. The postulate helps chemists by providing information about the structure of transition state, which generally cannot be directly characterized experimentally. The postulate, first published in a ''Journal of the American Chemical Society'' paper in 1955, states that:〔 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.〕 :''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.'' In other words, species with similar energies along the reaction will also have similar structures. The postulate allows us to accurately predict the shape of a reaction coordinate diagram, and has been used extensively, for example, to explain the effects of aromatic substituents in electrophilic aromatic substitution. ==History== The postulate is named after its creator, George S. Hammond. He first suggested that transition-state theory could be used qualitatively to explain the relationships between reactants, transition states, and products. The postulate was published in 1955 while Hammond was a professor of Chemistry at Iowa State University. In fact, John E. Leffler of Florida State University proposed a similar idea a few years earlier in a ''Science'' paper in 1953, two years before Hammond published his version of the postulate.〔Leffler, J. E. Parameters for the Description of Transition States. ''Science'' 1953, ''117'', 340–341.〕 However, Hammond's version has received a much wider attention from scientific communities due to its qualitative nature while Leffler's version utilizes multiple mathematical equations. Hammond's postulate is sometimes called the Hammond-Leffler postulate to give credit to both scientists.〔Yarnell, Amanda. Hammond Postulate: 1955 paper used transition-state theory to explain structure-reactivity relationships. ''Chemical & Engineering News'' May 19, 2003, ''81(20)'', 42 ()〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Hammond's postulate」の詳細全文を読む スポンサード リンク
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