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, 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•
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. Nevertheless, the term bond-dissociation energy and the symbol D0 have been used for the reaction enthalpy at 298 K as well.[page needed]
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.[page needed][volume & issue needed]
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.
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.
For computing the Bond Dissociation Energy for splitting up a bond, what is needed is the energy E of an electron to remove it from a positively charged plate of 1 Volt Potential Energy to free space without any field, namely E = 0.160218 attojoules, and the fact that a mole of anything (free atoms, molecules, etc.,) is the Avogadro's number NA of them. The two quantities, when multiplied together, yield 96.485 kJ/mole as the energy density corresponding to 1 ev per bond, in the case of one broken bond per molecule.
For example, the energy needed to convert a mole of ethane to a mole of ethyl radicals, 423.0 kJ/mol, is, for each bond in that mole, 423.0/96.485 = 4.38 ev/bond ≈ 4.40 ev/bond. Likewise, 460 kJ/mol becomes 460/96.485 = 4.77 ev/bond.
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|
|C–C||Carbon||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||Hydroxide||110||460||4.77||Slightly stronger than C–H bonds|
|OH–H||Hydroxide-Hydron||64||268||2.78||Far weaker than C–H bonds|
|C–O||Monoxide||257||1077||11.16||Far stronger than C–H bonds|
|O–CO||Dioxide||127||532||5.51||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.
|Bond||Bond||Bond-dissociation energy at 298 K||Comment|
|H3C–H||Methyl C–H bond||105||439||4.550||One of the strongest aliphatic C–H bonds|
|C2H5–H||Ethyl C–H bond||101||423||4.384||Slightly weaker than H3C–H|
|(CH3)3C–H||Tertiary C–H bond||96.5||404||4.187||Tertiary radicals are stabilized|
|CH2CH–H||Vinyl C–H bond||111||464||4.809||Vinyl radicals are rare|
|HC2–H||acetylenic C–H bond||133||556||5.763||Acetylenic radicals are very rare|
|C6H5–H||Phenyl C–H bond||113||473||4.902||Comparable to vinyl radical, rare|
|CH2CHCH2–H||Allylic C–H bond||89||372||3.856||Such bonds show enhanced reactivity|
|C6H5CH2–H||Benzylic C–H bond||90||377||3.907||Akin to allylic C–H bonds
Such bonds show enhanced reactivity
|H3C–CH3||Alkane C–C bond||83–85||347–356||3.596-3.690||Much weaker than a C–H bond|
|H2C=CH2||Alkene C=C bond||146–151||611–632||6.333-6.550||About 2× stronger than a C–C single bond|
|HC≡CH||Alkyne C≡C triple bond||200||837||8.675||About 2.5× stronger than a C–C single bond|
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Bond-dissociation energy".
- 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.