Bond-dissociation energy

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,^[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·

D₀ = ΔH = 101.1 kcal/mol = 423.0 kJ/mol = 4.40 eV (per bond)

Definitions of BDE and related parameters

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

1. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (1994) "Bond dissociation energy".
2. ^ ^{a b} Blanksby, S. J.; Ellison, G. B.; (2003). "Bond Dissociation Energies of Organic Molecules". Acc. Chem. Res. 36 (4): 255–263.  
3. ^ Morrison & Boyd Organic Chemistry 4th Ed. ISBN 0-205-05838-8
4. ^ Principles of biochemistry by Albert L. Lehninger, David Lee Nelson, Michael M. Cox; edition 4, page 48 [1]

References

• Bond strength
• Bond energy
• Binding energy
• Chemical bond

See also

[2] The data tabulated above shows how bond strengths vary over the periodic table. There is great interest, especially in

|69r>bromocarbons are often labile and are useful fire retardants

H₂ → 2 H· ΔG = 102 kcal/mol (see table below)

H₂ → H⁺ + H⁻ ΔG = 66 kcal/mol (in water)

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:

Homolytic versus heterolytic dissociation

Following dissociation if new bonds are formed at lower enthalpy there is a net loss of energy, and thus an overall exothermic process.

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.

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.^[4]

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.^[3]

BDE versus bond energy

[2]