Bond-dissociation energy

In chemistry, bond-dissociation energy (BDE) or D₀, 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)

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]

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 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]

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.

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 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)

Tabulated data

Bond	Bond	Bond-dissociation energy		Comment
		(kcal/mole)	(kJ/mole)
C-H	C-H bond	98	410	C-H bond strength affected by substituents (see next table)
C-F	C-F bond	117	490	one of the strongest single bonds to carbon
C-Cl	C-Cl bond	79	331	more labile than C-H and C-F bonds, chlorocarbons degrade photochemically
C-Br	C-Br bond	69	285	weaker than C-Cl bonds bromocarbons are often labile and are useful fire retardants
C-C	C-C bond	83-85	347-356	strong, but weaker than C-H bonds, routinely broken by catalytic cracking
Cl-Cl	chlorine	58	243	indicated by the yellowish colour of this gas
Br-Br	bromine	46	192	indicated by the brownish colour of Br2 source of the Br. radical
I-I	iodine	36	151	indicated by the purplish colour of I2 source of the I. radical
H-H	hydrogen	104	436	strong, nonpolarizable bond cleaved only by metals and by strong oxidants
O-H	hydroxyl	110	460	comparable to strength of O=O and C-H bonds
O=O	oxygen	119	494	strong bond, but O-H bonds are of comparable strength
N≡N	nitrogen	226	942	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.^[2]

Bond	Bond	Bond-dissociation energy		Comment
		(kcal/mole)	(kJ/mole)
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

See also

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

References

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. doi:10.1021/ar020230d. PMID 12693923. 
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]