Bond-dissociation energy

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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,[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 and related parameters[edit]

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.[2] Nevertheless, the term bond-dissociation energy and the symbol D0 have been used for the reaction enthalpy at 298 K as well.[3][page needed]

Bond energy[edit]

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.[4][page needed][volume & issue needed]

For example, dissociation of HOH 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 OH bonds in water is said to be 458.9 kJ/mol, the average of these values.[5]

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.

Following dissociation, if new bonds of greater bond-dissociation energy are formed, these products are at lower enthalpy, there is a net loss of energy, and thus the process overall is exothermic. In particular, the conversion of the weak double bonds in O2 to the stronger bonds in CO2 and H2O makes combustion exothermic.[6]


Homolytic versus heterolytic dissociation[edit]

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
(kcal/mol) (kJ/mol) (eV/Bond)
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.[2]

Bond Bond Bond-dissociation energy at 298 K Comment
(kcal/mol) (kJ/mol) (eV/Bond)
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

See also[edit]

References[edit]

  1. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "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. ^ 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. 
  4. ^ Morrison, Robert Thornton; Boyd, Robert Neilson (1983). Organic Chemistry. Boston: Allyn & Bacon. ISBN 0-205-05838-8. 
  5. ^ 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. 
  6. ^ 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.