Bond-dissociation energy
Bond dissociation energy (BDE or D0) is one measure of the strength of a chemical bond. It can be defined as the standard enthalpy change when a bond is cleaved by homolysis,[1] with reactants and products of the homolysis reaction at 0 K (absolute zero). For instance, the bond-dissociation energy for one of the C–H bonds in ethane (C2H6) is defined by the process:
- CH3CH2–H → CH3CH2• + H•
Definitions and related parameters
The bond-dissociation energy is sometimes called the bond-dissociation enthalpy (or bond enthalpy), but these terms may not be strictly equivalent. Bond-dissociation enthalpy usually refers to the above reaction enthalpy at 298 K (standard conditions) rather than at 0 K, and differs from D0 by about 1.5 kcal/mol (6 kJ/mol) in the case of a bond to hydrogen in a large organic molecule.[2] Nevertheless, the term bond-dissociation energy and the symbol D0 have been used for the reaction enthalpy at 298 K as well.[3]
Bond energy
Except for diatomic molecules, the bond-dissociation energy differs from the bond energy. While the bond-dissociation energy is the energy of a single chemical bond, bond energy is the average of all the bond-dissociation energies of the bonds in a molecule.[4]
For example, dissociation of HO–H bond of a water molecule (H2O) requires 493.4 kJ/mol. The dissociation of the remaining hydroxyl radical requires 424.4 kJ/mol. The bond energy of the covalent O–H bonds in water is said to be 458.9 kJ/mol, the average of these values.[5]
In the same way for removing successive hydrogen atoms from methane the bond-dissociation energies are 104 kcal/mol (435 kJ/mol) for D(CH3–H), 106 kcal/mol (444 kJ/mol) for D(CH2–H), 106 kcal/mol (444 kJ/mol) for D(CH–H) and finally 81 kcal/mol (339 kJ/mol) for D(C–H). The bond energy is, thus, 99 kcal/mol or 414 kJ/mol (the average of the bond-dissociation energies). None of the individual bond-dissociation energies equals the bond energy of 99 kcal/mol.
Homolytic versus heterolytic dissociation
Bonds can be broken symmetrically or asymmetrically. The former is called homolysis and is the basis of the usual BDEs. Asymmetric scission of a bond is called heterolysis. For molecular hydrogen, the alternatives are:
- H2 → 2 H• ΔH = 104 kcal/mol (see table below)
- H2 → H+ + H− ΔH = 66 kcal/mol (in water)
| Bond | Bond | Bond-dissociation energy at 298 K | Comment | ||
|---|---|---|---|---|---|
| (kcal/mol) | (kJ/mol) | (eV) | |||
| C–C | C–C bond | 83–85 | 347–356 | 3.60–3.69 | Strong, but weaker than C–H bonds |
| Cl–Cl | Chlorine | 58 | 242 | 2.51 | Indicated by the yellowish colour of this gas |
| Br–Br | Bromine | 46 | 192 | 1.99 | Indicated by the brownish colour of Br2 Source of the Br• radical |
| I–I | Iodine | 36 | 151 | 1.57 | Indicated by the purplish colour of I2 Source of the I• radical |
| H–H | Hydrogen | 104 | 436 | 4.52 | Strong, nonpolarizable bond Cleaved only by metals and by strong oxidants |
| O–H | Hydroxyl | 110 | 460 | 4.77 | Slightly stronger than C–H bonds |
| O=O | Oxygen | 119 | 498 | 5.15 | Stronger than single bonds Weaker than many other double bonds |
| N≡N | Nitrogen | 226 | 945 | 9.79 | One of the strongest bonds Large activation energy in production of ammonia |
The data tabulated above shows how bond strengths vary over the periodic table. There is great interest, especially in organic chemistry, concerning relative strengths of bonds within a given group of compounds.[2]
| Bond | Bond | Bond-dissociation energy at 298 K | Comment | |
|---|---|---|---|---|
| (kcal/mol) | (kJ/mol) | |||
| H3C–H | Methyl C–H bond | 105 | 439 | One of the strongest aliphatic C–H bonds |
| C2H5–H | Ethyl C–H bond | 101 | 423 | Slightly weaker than H3C–H |
| (CH3)3C–H | Tertiary C–H bond | 96.5 | 404 | Tertiary radicals are stabilized |
| CH2CH–H | Vinyl C–H bond | 111 | 464 | Vinyl radicals are rare |
| HC2–H | acetylenic C–H bond | 133 | 556 | Acetylenic radicals are very rare |
| C6H5–H | Phenyl C–H bond | 113 | 473 | Comparable to vinyl radical, rare |
| CH2CHCH2–H | Allylic C–H bond | 89 | 372 | Such bonds show enhanced reactivity |
| C6H5CH2–H | Benzylic C–H bond | 90 | 377 | Akin to allylic C–H bonds Such bonds show enhanced reactivity |
| H3C–CH3 | Alkane C–C bond | 83–85 | 347–356 | Much weaker than a C–H bond |
| H2C=CH2 | Alkene C=C bond | 146–151 | 611–632 | About 2× stronger than a C–C single bond |
| HC≡CH | Alkyne C≡C triple bond | 200 | 837 | About 2.5× stronger than a C–C single bond |
See also
References
- ↑ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Bond-dissociation energy".
- 1 2 Blanksby, S. J.; Ellison, G. B. (2003). "Bond Dissociation Energies of Organic Molecules". Acc. Chem. Res. 36 (4): 255–263. doi:10.1021/ar020230d. PMID 12693923.
- ↑ Darwent, B. deB. (January 1970). Bond Dissociation Energies in Simple Molecules (PDF). NSRDS-NBS 31. Washington, DC: U.S. National Bureau of Standards. LCCN 70602101.
- ↑ Morrison, Robert Thornton; Boyd, Robert Neilson (1983). Organic Chemistry. Boston: Allyn & Bacon. ISBN 0-205-05838-8.
- ↑ Lehninger, Albert L.; Nelson, David L.; Cox, Michael M. (2005). Lehninger Principles of Biochemistry (4th ed.). W. H. Freeman. p. 48. ISBN 978-0-7167-4339-2. Retrieved May 20, 2016.
- ↑ Schmidt-Rohr, K. (2015). "Why Combustions Are Always Exothermic, Yielding About 418 kJ per Mole of O2". J. Chem. Educ. 92: 2094–2099. doi:10.1021/acs.jchemed.5b00333.