In chemistry, bond-dissociation energy (BDE) or D0, is one measure of the strength in a chemical bond. It is 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 sometime also called the bond-dissociation enthalpy (or bond enthalpy), but these terms are not strictly equivalent, as they refer to the above reaction enthalpy at standard conditions, and differ from D0 by about 1.5 kcal/mol (6 kJ/mol) in the case of a bond to hydrogen in a large organic molecule.
BDE versus bond energy
Except in the case of diatomic molecules, the bond-dissociation energy is different from the bond energy, which is an average calculated from the sum of the bond-dissociation energies of all bonds in a molecule.
For example, an HO–H bond of a water molecule (H–O–H) has 493.4 kJ/mol of bond-dissociation energy, and 424.4 kJ/mol is needed to cleave the remaining O–H bond. The bond energy of the covalent O–H bonds in water is 458.9 kJ/mol, which is the average of the values. Hydrogen bond-dissociation energy in water is about 23 kJ/mol.
In the same way for removing successive hydrogen atoms from methane the bond-dissociating 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). Notice that none of the C-H BDEs is 99 kcal/mol.
Following dissociation if new bonds are formed at lower enthalpy there is a net loss of energy, and thus an overall exothermic process.
Homolytic versus heterolytic dissociation
Bonds can be broken symmetrically or asymmetrically. The former is called homolysis and is the basis of the usual BDEs. Asymmetrical scission of a bond is called heterolysis. For molecular hydrogen, the alternatives are:
- H2 → 2 H· ΔG = 102 kcal/mol (see table below)
- H2 → H+ + H− ΔG = 66 kcal/mol (in water)
|69r>bromocarbons are often labile and are useful fire retardants
|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||comparable to strength of O=O and C–H bonds|
|O=O||oxygen||119||497||5.15||strong bond, but O–H bonds are of comparable strength|
|N≡N||nitrogen||226||945||9.79||one of the strongest bonds
production of ammonia consumes significant energy
The data tabulated above shows how bond strengths vary over the periodic table. There is great interest, especially in organic chemistry, concerning bond strengths of bonds within a given group of compounds.
|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 2x stronger than a C–C single bond|
|HC≡CH||alkyne C≡C triple bond||200||837||about 2.5x stronger than a C–C single bond|
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (1994) "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.
- Morrison & Boyd Organic Chemistry 4th Ed. ISBN 0-205-05838-8
- Principles of biochemistry by Albert L. Lehninger, David Lee Nelson, Michael M. Cox; edition 4, page 48