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In chemistry, bond dissociation energy, D0 or BDE, is one measure of the bond 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)

Contents

Definitions of BDE and related parameters

The bond dissociation energy is sometimes 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.[2]

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BDE vs 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.[3]

For example, an O-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]

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 BDE's is 99 kcal/mol.

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

Homolytic vs heterolytic dissociation

Bonds can be broken symmetrically or unsymmetrically. The former is called homolysis and is the basis of the usual BDE's. Unsymmetrical scission of a bond is called heterolysis. For molecular hydrogen, the alternatives are:

H2 → 2 H·
H2 → H+ + H-
Bond Bond Bond Dissociation Energy (kcal/mole)
C-H C-H bond 98
C-F C-F bond 117
C-Cl C-Cl bond 79
C-Br C-Br bond 69
C-C C-C bond 83
Cl-Cl chlorine 58
Br-Br bromine 46
I-I iodine 36
H-H hydrogen 104
O=O oxygen 119
NN nitrogen 226

See also

References

  1. ^ International Union of Pure and Applied Chemistry (1994). "Bond dissociation energy". Compendium of Chemical Terminology Internet edition.
  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. 
  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]

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