In Chapter 2.2, we suggest that the experimental deprotonation energy (DPE) of cyclohexane is in doubt. G2MP2 predicts the DPE of cyclohexane is 414.5 kcal mol-1, a figure significantly higher than the experimental1 value of 404 kcal mol-1. Given that the deviation between the G2MP2 computed DPE and experiment is about 2 kcal mol-1, we suggest that cyclohexane should be re-examined.
In a recent JACS article,2 Kass calls into question the experimental bond dissociation energies (BDE) of the small cycloalkanes. With his experimental determination of the BDE of both the vinyl and allylic positions of cyclobutene, Kass can compare experimental and computed BDEs for a range of hydrocarbon environments, as listed in Table 1. The two composite methods G3 and W1 provide excellent BDE values for the small alkanes, one acyclic alkene, and the small cyclic alkenes. These composite methods appear to accurately predict BDEs of hydrocarbons.
However, the small cyclic alkanes are dramatic outliers. The well-accepted experimental BDEs of cyclopropane, cyclobutane, and cyclohexane are 3-5 kcal mol-1 lower than those predicted by the composite methods. Given the strong performance of the computational methods, and the difficulties associated with experimental determinations of BDEs, Kass suggests that the BDEs of these cycloalkanes are in error. Further experiments are deserved.
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Table 1. Computed and experimental BDEs (kcal mol-1) of some simple hydrocarbons. |
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| G3a | W1a | Expt. | |
| Methane | 104.2 | 104.3 | 105.0±0.1b 104.9±0.2c |
| Ethane | 101.2 | 101.2 | 100.5±0.3b 101.1±0.4c |
| (CH3)CH2 | 98.9 | 98.4 | 98.1±0.7b 97.8±0.5c |
| CH3CH2CH2CH3 | 98.8 | 98.8 | 98.3±0.5b 98.3±0.5c |
| Z-2-butene (allyl) | 86.0 | 87.0 | 85.6±1.5d |
| Cyclopropene (vinyl) | 109.6 | 109.8 | 106.7±3.7e |
| Cyclopropene (allyl) | 100.4 | 100.4 | 90.6±4.0f |
| Cyclobutene (vinyl) | 111.9 | 112.4 | 112.5±2.5a |
| Cyclobutene (allyl) | 90.6 | 91.7 | 91.2±2.3a |
| Cyclopentene (allyl) | 84.2 | 85.0 | 82.3±1.1g |
| Cyclohexene (allyl) | 83.9 | 85±1h | |
| Cyclopropane | 109.2 | 109.0 | 106.3±0.3b 106.3±0.3c |
| Cyclobutane | 100.5 | 99.9 | 96.8±1.0b 96.5±1.0c |
| Cyclopentane | 96.4 | 96.9 | 95.6±1.0b 96.4±0.6c |
| Cyclohexane | 100.0 | 99.5±1.2b 95.5±1.0c |
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| aRef. 2. bRef. 3. cRef. 4. dRef. 5. eRef. 6. fRef. 7. gRef. 8. hRef. 9 | |||
References
(1) NIST webbook, 2005, http://webbook.nist.gov/.
(2) Tian, Z.; Fattahi, A.; Lis, L.; Kass, S. R., "Cycloalkane and Cycloalkene C-H Bond Dissociation Energies," J. Am. Chem. Soc. 2006, 128, 17087-17092, DOI: 10.1021/ja065348u
(3) Yao, Y.-R. Handbook of Bond Dissociation Energies in Organic Compounds; CRC Press: Boca Raton, FL, 2003.
(4) CRC Handbook of Chemistry and Physics; 85th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2004.
(5) Tsang, W. In Energetics of Stable Molecules and Reactive Intermediates, NATO Science Series C; Minas da Piedade, M. E., Ed.; Kluwer Academic Publishers: Dordrecht, Netherlands, 1999; Vol. 535.
(6) Fattahi, A.; McCarthy, R. E.; Ahmad, M. R.; Kass, S. R., "Why Does Cyclopropene Have the Acidity of an Acetylene but the Bond Energy of Methane?," J. Am. Chem. Soc. 2003, 125, 11746-11750, DOI: 10.1021/ja035725s.
(7) DeFrees, D. J.; McIver, R. T., Jr.; Hehre, W. J., "Heats of Formation of Gaseous Free Radicals via Ion Cyclotron Double Resonance Spectroscopy," J. Am. Chem. Soc. 1980, 102, 3334-3338, DOI: 10.1021/ja00530a005.
(8) Furuyama, S.; Golden, D. M.; Benson, S. W., "Kinetic Study of the Gas-Phase Reaction c-C5H8+I2 c-C5H6+2HI. Heat of Formation and the Stabilization Energy of the Cyclopentenyl Radical," Int. J. Chem. Kinet. 1970, 2, 93-99.
(9) Alfassi, Z. B.; Feldman, L., "The Kinetics of Radiation-Induced Hydrogen Abstraction
by Trichloromethyl Radicals in the Liquid Phase: Cyclohexene," Int. J. Chem. Kinet. 1981, 13, 771-783.
