Information AboutCurtin-hammett Principle |
| CATEGORIES ABOUT CURTIN-HAMMETT PRINCIPLE | |
| chemical kinetics | |
| physical organic chemistry | |
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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. The Curtin-Hammett principle tells us that the C:D product ratio will not reflect ''K'', but the relative energy of the transition states. The Reaction Coordinate free energy profile can be represented by the following scheme: The ratio of products will depend only on the value labeled ΔΔ''G''‡ in the figure: C will be the major product, because the energy of '''TS1''' is lower than the energy of '''TS2'''. It doesn't matter whether '''A''' is more stable than '''B''' or not, or by how much. This can be understood qualitatively by thinking what would happen if the free energy of '''A''' were increased, while keeping everything else constant. On one hand, Δ''G''1‡ would become smaller, which would make ''k''1 larger, therefore favoring the formation of C. But on the other hand, the amount of '''A''' in equilibrium would decrease, because the change in Δ''G'' would increase the value of ''K'', favoring '''B'''. These two effects cancel out, leading to the conclusion that the relative energies of '''A''' and '''B''' don't matter. This can also be proved algebraically. APPLICATION TO STEREOSELECTIVE REACTIONS The Curtin-Hammett principle is used to explain the selectivity ratios for molecule binds to a Chiral Catalyst , forming a pair of Diastereomer ic intermediates, depending on which face of the substrate was bound to the catalyst. These intermediates equilibrate rapidly (like A and '''B''' in the diagram above), and each one then leads to a different Enantiomer of the product through the Rate-determining Step . EXTERNAL LINKS |
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