Bond cleavage

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Bond cleavage, or bond fission, is the splitting of chemical bonds. This can be generally referred to as Dissociation (chemistry) when a molecule is cleaved into two or more fragments. [1]


In general, there are two classifications for bond cleavage: homolytic and heterolytic, depending on the nature of the process. The triplet and singlet excitation energies of a sigma bond can be used to determine if a bond will follow the homolytic or heterolytic pathway. [2] A metalmetal sigma bond is an exception because the bond's excitation energy is extremely high, thus cannot be used for observation purposes. [2]

In some cases, bond cleavage requires catalysts. Due to the high bond-dissociation energy of CH bond, around 100 kcal/mol (420 kJ/mol), a large amount of energy is required to cleave the hydrogen atom from the carbon and bond a different atom to the carbon. [3]

Homolytic cleavage

Homolytic cleavage Homolysis (Chemistry) V.1.svg
Homolytic cleavage

In homolytic cleavage, or homolysis, the two electrons in a cleaved covalent bond are divided equally between the products. This process is also known as homolytic fission or radical fission. The bond-dissociation energy of a bond is the amount of energy required to cleave the bond homolytically. This enthalpy change is one measure of bond strength.

The triplet excitation energy of a sigma bond is the energy required for homolytic dissociation, but the actual excitation energy may be higher than the bond dissociation energy due to the repulsion between electrons in the triplet state. [2]

Heterolytic cleavage

Heterolytic cleavage Heterolysis (chemistry).svg
Heterolytic cleavage

In heterolytic cleavage, or heterolysis, the bond breaks in such a fashion that the originally-shared pair of electrons remain with one of the fragments. Thus, a fragment gains an electron, having both bonding electrons, while the other fragment loses an electron. [4] This process is also known as ionic fission.

The singlet excitation energy of a sigma bond is the energy required for heterolytic dissociation, but the actual singlet excitation energy may be lower than the bond dissociation energy of heterolysis as a result of the Coulombic attraction between the two ion fragments. [2] The singlet excitation energy of a siliconsilicon sigma bond is lower than the carboncarbon sigma bond, even though their bond strengths are 80kJ/mol and 70kJ/mol respectively, because silicon has higher electron affinity and lower ionization potential than carbon. [2]

Heterolysis occurs naturally in reactions that involve electron donor ligands and transition metals which have empty orbitals. [4]


Epoxide opening Epoxide opening.png
Epoxide opening

In a ring-opening, the cleaved molecule remains as a single unit. [5] The bond breaks, but the two fragments remain attached by other parts of the structure. For example, an epoxide ring can be opened by heterolytic cleavage of one of the polar carbon–oxygen bonds to give a single acyclic structure. [5]


In biochemistry, the process of breaking down large molecules by splitting their internal bonds is catabolism. Enzymes which catalyse bond cleavage are known as lyases, unless they operate by hydrolysis or oxidoreduction, in which case they are known as hydrolases and oxidoreductases respectively.

In proteomics, cleaving agents are used in proteome analysis where proteins are cleaved into smaller peptide fragments. [6] Examples of cleaving agents used are cyanogen bromide, pepsin, and trypsin. [6]

Related Research Articles

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In organic chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The general formula is R−:C−R' or R=C: where the R represents substituents or hydrogen atoms.


Silene, or silalkenes, are unsaturated hydrosilicons, which means that they consist only of hydrogen and silicon atoms and all bond, with the exception of one double bond, are either single or double bonds. By definition cycles are excluded, so that the silenes comprise homologous series of inorganic compounds with the general formula Si
2n - 2k + 2
, k > 0, where k is defined as the number of double bonds. There are no commercial sources.

In chemistry, orbital hybridisation is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the pairing of electrons to form chemical bonds in valence bond theory. For example, in a carbon atom which forms four single bonds the valence-shell s orbital combines with three valence-shell p orbitals to form four equivalent sp3 mixtures in a tetrahedral arrangement around the carbon to bond to four different atoms. Hybrid orbitals are useful in the explanation of molecular geometry and atomic bonding properties and are symmetrically disposed in space. Usually hybrid orbitals are formed by mixing atomic orbitals of comparable energies.

The bond-dissociation energy is one measure of the strength of a chemical bond A−B. It can be defined as the standard enthalpy change when A−B is cleaved by homolysis to give fragments A and B, which are usually radical species. The enthalpy change is temperature-dependent, and the bond-dissociation energy is often defined to be the enthalpy change of the homolysis at 0 K, although the enthalpy change at 298 K is also a frequently encountered parameter.

In chemistry, bond energy (BE), also called the mean bond enthalpy or average bond enthalpy is the measure of bond strength in a chemical bond. IUPAC defines bond energy as the average value of the gas-phase bond-dissociation energy for all bonds of the same type within the same chemical species. The larger the average bond energy, per electron-pair bond, of a molecule, the more stable and lower-energy the molecule.

In chemistry, homolysis or homolytic fission is the dissociation of a molecular bond by a process where each of the fragments retains one of the originally bonded electrons. During homolytic fission of a neutral molecule with an even number of electrons, two free radicals will be generated. That is, the two electrons involved in the original bond are distributed between the two fragment species. Bond cleavage is also possible by a process called heterolysis.

In chemistry, heterolysis or heterolytic fission is the process of cleaving/breaking a covalent bond where one previously bonded species takes both original bonding electrons from the other species. During heterolytic bond cleavage of a neutral molecule, a cation and an anion will be generated. Most commonly the more electronegative atom keeps the pair of electrons becoming anionic while the more electropositive atom becomes cationic.

Triplet oxygen Triplet state of the dioxygen molecule

Triplet oxygen, 3O2, refers to the S = 1 electronic ground state of molecular oxygen (dioxygen). It is the most stable and common allotrope of oxygen. Molecules of triplet oxygen contain two unpaired electrons, making triplet oxygen an unusual example of a stable and commonly encountered diradical: it is more stable as a triplet than a singlet. According to molecular orbital theory, the electron configuration of triplet oxygen has two electrons occupying two π molecular orbitals (MOs) of equal energy (that is, degenerate MOs). In accordance with Hund's rules, they remain unpaired and spin-parallel and account for the paramagnetism of molecular oxygen. These half-filled orbitals are antibonding in character, reducing the overall bond order of the molecule to 2 from a maximum value of 3 (e.g., dinitrogen), which occurs when these antibonding orbitals remain fully unoccupied. The molecular term symbol for triplet oxygen is 3Σ

Diatomic carbon Chemical compound

Diatomic carbon (systematically named dicarbon and 2,2λ2-ethene), is a green, gaseous inorganic chemical with the chemical formula C=C (also written [C2] or C2). It is kinetically unstable at ambient temperature and pressure, being removed through autopolymerisation. It occurs in carbon vapor, for example in electric arcs; in comets, stellar atmospheres, and the interstellar medium; and in blue hydrocarbon flames. Diatomic carbon is the second simplest form of carbon after atomic carbon, and is an intermediate participator in the genesis of fullerenes.

The di-pi-methane rearrangement is a photochemical reaction of a molecular entity that contains two π-systems separated by a saturated carbon atom, to form an ene- substituted cyclopropane. The rearrangement reaction formally amounts to a 1,2 shift of one ene group or the aryl group and bond formation between the lateral carbons of the non-migrating moiety.

Sextuple bond Covalent bond involving 12 bonding electrons

A sextuple bond is a type of covalent bond involving 12 bonding electrons and in which the bond order is 6. The only known molecules with true sextuple bonds are the diatomic dimolybdenum (Mo2) and ditungsten (W2), which exist in the gaseous phase and have boiling points of 4,639 °C (8,382 °F) and 5,930 °C (10,710 °F). There is strong evidence to believe that there is no element with atomic number below about 100 that can form a bond with a greater order than 6 between its atoms, but the question of possibility of such a bond between two atoms of different elements remains open. Bonds between heteronuclear systems with two atoms of different elements may not necessarily have the same limit.

An electronic effect influences the structure, reactivity, or properties of molecule but is neither a traditional bond nor a steric effect. In organic chemistry, the term stereoelectronic effect is also used to emphasize the relation between the electronic structure and the geometry (stereochemistry) of a molecule.

Mass spectral interpretation

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Radical (chemistry) Atom, molecule, or ion that has an unpaired valence electron; typically highly reactive

In chemistry, a free radical is an atom, molecule, or ion that has at least one unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemically reactive. Many radicals spontaneously dimerize. Most organic radicals have short lifetimes.

Fragmentation (mass spectrometry)

In mass spectrometry, fragmentation is the dissociation of energetically unstable molecular ions formed from passing the molecules in the ionization chamber of a mass spectrometer. The fragments of a molecule cause a unique pattern in the mass spectrum. These reactions are well documented over the decades and fragmentation pattern is useful to determine the molar weight and structural information of the unknown molecule. Fragmentation that occurs in tandem mass spectrometry experiments has been a recent focus of research, because this data helps facilitate the identification of molecules.

Methylene is an organic compound with the chemical formula CH
. It is a colourless gas that fluoresces in the mid-infrared range, and only persists in dilution, or as an adduct.

Collision-induced dissociation Mass spectrometry technique to induce fragmentation of selected ions in the gas phase

Collision-induced dissociation (CID), also known as collisionally activated dissociation (CAD), is a mass spectrometry technique to induce fragmentation of selected ions in the gas phase. The selected ions are usually accelerated by applying an electrical potential to increase the ion kinetic energy and then allowed to collide with neutral molecules. In the collision some of the kinetic energy is converted into internal energy which results in bond breakage and the fragmentation of the molecular ion into smaller fragments. These fragment ions can then be analyzed by tandem mass spectrometry.

Fluorenylidene Chemical compound

9-Fluorenylidene is an aryl carbene derived from the bridging methylene group of fluorene. Fluorenylidene has the unusual property that the triplet ground state is only 1.1 kcal/mol lower in energy than the singlet state. For this reason, fluorenylidene has been studied extensively in organic chemistry.

The Carter–Goddard–Malrieu–Trinquier model is a model in inorganic chemistry, used for the description and prediction of distortions in multiple bonding systems of main group elements.


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