Bond dissociation energy
Encyclopedia
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
, 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)
, which is an average calculated from the sum of the bond-dissociation energies of all bonds in a molecule
.
For example, an H
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.
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
.
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.
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
, bond-dissociation energy (BDE) or D0, is one measure of the bond strength
Bond strength
In chemistry, bond strength is measured between two atoms joined in a chemical bond. It is the degree to which each atom linked to another atom contributes to the valency of this other atom...
in a chemical bond
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...
. It is defined as the standard enthalpy change
Enthalpy
Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.Enthalpy is a...
when a bond is cleaved by homolysis
Homolysis
In general it means breakdown to equal pieces There are separate meanings for the word in chemistry and biology.-Homolysis in chemistry:...
, with reactants and products of the homolysis reaction at 0 K (absolute zero
Absolute zero
Absolute zero is the theoretical temperature at which entropy reaches its minimum value. The laws of thermodynamics state that absolute zero cannot be reached using only thermodynamic means....
). For instance, the bond-dissociation energy for one of the C-H bonds in ethane
Ethane
Ethane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane that is an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colorless, odorless gas....
(C2H6) is defined by the process:
CH3CH2-H → CH3CH2·
Ethyl group
In chemistry, an ethyl group is an alkyl substituent derived from ethane . It has the formula -C2H5 and is very often abbreviated -Et.Ethylation is the formation of a compound by introduction of the ethyl functional group, C2H5....
+ 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.BDE vs bond energy
Except in the case of diatomic molecules, the bond-dissociation energy is different from the bond energyBond energy
In chemistry, bond energy is the measure of bond strength in a chemical bond. It is the heat required to break one Mole of molecules into their individual atoms. For example, the carbon-hydrogen bond energy in methane E is the enthalpy change involved with breaking up one molecule of methane into...
, which is an average calculated from the sum of the bond-dissociation energies of all bonds in a molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
.
For example, an H
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
O
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
-H
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
bond of a water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
molecule (H
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
-O
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
-H
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
) has 493.4 kJ/mol of bond-dissociation energy, and 424.4 kJ/mol is needed to cleave the remaining O
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
-H
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
bond. The bond energy of the covalent O
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
-H
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
bonds in water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
is 458.9 kJ/mol, which is the average of the values. Hydrogen bond
Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the 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.
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
Exothermic reaction
An exothermic reaction is a chemical reaction that releases energy in the form of light or heat. It is the opposite of an endothermic reaction. Expressed in a chemical equation:-Overview:...
.
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 Organobromine compound Organobromine compounds are organic compounds that contain carbon bonded to bromine. The most pervasive is the naturally produced bromomethane. One prominent application is the use of polybrominated diphenyl ethers as fire-retardants. A variety of minor organobromine compounds are found in... |
69 | 285 | weaker than C-Cl bonds bromocarbons are often labile and are useful fire retardant Fire retardant A fire retardant is a substance other than water that reduces flammability of fuels or delays their combustion. This typically refers to chemical retardants but may also include substances that work by physical action, such as cooling the fuels; examples of these include fire-fighting foams and... s |
| C-C | C-C bond | 83-85 | 347-356 | strong, but weaker than C-H bonds, routinely broken by catalytic cracking |
| Cl-Cl | chlorine Chlorine Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine... |
58 | 243 | indicated by the yellowish colour of this gas |
| Br-Br | bromine Bromine Bromine ") is a chemical element with the symbol Br, an atomic number of 35, and an atomic mass of 79.904. It is in the halogen element group. The element was isolated independently by two chemists, Carl Jacob Löwig and Antoine Jerome Balard, in 1825–1826... |
46 | 192 | indicated by the brownish colour of Br2 source of the Br. radical |
| I-I | iodine Iodine Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor.... |
36 | 151 | indicated by the purplish colour of I2 source of the I. radical |
| H-H | hydrogen Hydrogen Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly... |
104 | 436 | strong, nonpolarizable bond cleaved only by metals and by strong oxidants |
| O-H | hydroxyl Hydroxyl A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in... |
110 | 460 | comparable to strength of O=O and C-H bonds |
| O=O | oxygen Oxygen Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition... |
119 | 494 | strong bond, but O-H bonds are of comparable strength |
| N≡N | nitrogen Nitrogen Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere... |
226 | 942 | one of the strongest bonds production of ammonia Ammonia Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or... consumes significant energy |
The data tabulated above shows how bond strengths vary over the periodic table. There is great interest, especially in organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
, concerning bond strengths of bonds within a given group of compounds.
| 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 Ethyl group In chemistry, an ethyl group is an alkyl substituent derived from ethane . It has the formula -C2H5 and is very often abbreviated -Et.Ethylation is the formation of a compound by introduction of the ethyl functional group, C2H5.... 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 Vinyl A vinyl compound is any organic compound that contains a vinyl group ,which are derivatives of ethene, CH2=CH2, with one hydrogen atom replaced with some other group... C-H bond |
111 | 464 | vinyl radicals are rare |
| HC2-H | acetylenic Acetylene Acetylene is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in pure form and thus is usually handled as a solution.As an alkyne, acetylene is unsaturated because... 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 Alkane Alkanes are chemical compounds that consist only of hydrogen and carbon atoms and are bonded exclusively by single bonds without any cycles... C-C bond |
83-85 | 347-356 | much weaker than a C-H bond |
| H2C=CH2 | Alkene Alkene In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond... C=C bond |
146-151 | 611-632 | about 2x stronger than a C-C single bond |
| HC≡CH | alkyne Alkyne Alkynes are hydrocarbons that have a triple bond between two carbon atoms, with the formula CnH2n-2. Alkynes are traditionally known as acetylenes, although the name acetylene also refers specifically to C2H2, known formally as ethyne using IUPAC nomenclature... C≡C triple bond |
200 | 837 | about 2.5x stronger than a C-C single bond |