One major aspect of determining leaving group ability is the ability to stabilize developing negative charge. Compare properties of some common leaving groups. (Group charges are based on a PM3 calculation in the gas phase. Estimated heats of solution are the calculated differences between the ions in a vacuum and in aqueous solution, calculated at the PM3 level with the Cramer-Truhlar dielectric correction.)
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Acetate (OAc) |
Group charge: -0.80 | Heat of solution (aq.): -75.45 kcal/mol (est.) |
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p-Nitrobenzoate (OPNB) |
Group charge: -0.74 | Heat of solution (aq.): -66.79 kcal/mol (est.) |
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Methanesulfonate
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Group charge: -0.60 | Heat of solution (aq.): -84.42 kcal/mol (est.) |
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p-Toluenesulfonate
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Group charge: -0.53 | Heat of solution (aq.): -81.78 kcal/mol (est.) |
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p-Bromobenzenesulfonate
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Group charge: -0.51 | Heat of solution (aq.): -79.42 (est.) |
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p-Nitrobenzenesulfonate
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Group charge: -0.49 | Heat of solution (aq.): -75.43 kcal/mol (est.) |
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Trifluoromethanesulfonate
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Group charge: -0.37 | Heat of Solution (aq.): -70.22 (est.) |
Comparison of rates of ionization
Reaction: Solvolysis of 1-phenylethyl ester. Rates are sec-1 at 76°C in 80% ethanol/water.
Ref: Noyce, D. S.; Virgilio, J. A. J. Org. Chem., 1972, 37, 2643.
| OAc | 1.4 x 10-6 |
| OPNB | 5.5 x 10-6 |
| OMs | 3.0 x 104 |
| OTs | 3.7 x 104 |
| ONs | 4.4 x 105 |
| OTf | 1.4 x 108 |
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Last updated: 09/25/03