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The Curtin–Hammett principle is a principle in chemical kinetics proposed by David Yarrow Curtin and Louis Plack Hammett. It states that, for a reaction that has a pair of reactive intermediates or reactants that interconvert rapidly (as is usually the case for conformational isomers), each going irreversibly to a different product, the product ratio will depend both on the difference in energy between the two conformers and the free energy of the transition state going to each product. As a result, the product distribution will not necessarily reflect the equilibrium distribution of the two intermediates.〔Carey, Francis A.; Sundberg, Richard J.; (1984). ''Advanced Organic Chemistry Part A Structure and Mechanisms (2nd ed.).'' New York N.Y.: Plenum Press. ISBN 0-306-41198-9〕 The Curtin–Hammett principle has been invoked to explain selectivity in a variety of stereo- and regioselective reactions. ==Definition== The Curtin–Hammett principle applies to systems in which different products are formed from two substrates in equilibrium with one another. The rapidly interconverting reactants can be enantiomers, diastereomers, or constitutional isomers. Product formation must be irreversible, and the different products must be unable to interconvert.〔 For example, given species A and B that equilibrate rapidly while A turns irreversibly into C, and B turns irreversibly into D: : ''K'' is the equilibrium constant between A and B, and ''k''1 and ''k''2 are the rate constants for the formation of C and D, respectively. When the rate of interconversion between A and B is much faster than either k1 or k2, then the Curtin–Hammett principle tells us that the C:D product ratio is not equal to the A:B reactant ratio, but is instead determined by the relative energy of the transition states. If reactants A and B were at identical energies, the reaction would depend only on the energy of the transition states leading to each respective product. However, in a real-world scenario, the two reactants are likely at somewhat different energy levels, although the barrier to their interconversion must be low for the Curtin–Hammett scenario to apply. In this case, the product distribution depends both on the relative quantity of A and B and on the relative barriers to products C and D. The reaction coordinate free energy profile can be represented by the following scheme: The ratio of products depends on the value labeled Δ''G''‡ in the figure: C will be the major product, because the energy of TS1 is lower than the energy of TS2. The commonly made assertion that the product distribution does not in any way reflect the relative free energies of substrates A and B is incorrect.〔 As shown in the derivation below, the product ratio can be expressed as a function of ''K'', ''k''1, and ''k''2. 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「Curtin–Hammett principle」の詳細全文を読む スポンサード リンク
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