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chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...
, homolysis () or homolytic fission is the dissociation of a molecular bond by a process where each of the fragments (an
atom Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...
or
molecule A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...
) retains one of the originally bonded
electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
s. During homolytic fission of a neutral molecule with an even number of electrons, two
radical Radical (from Latin: ', root) may refer to: Politics and ideology Politics *Classical radicalism, the Radical Movement that began in late 18th century Britain and spread to continental Europe and Latin America in the 19th century *Radical politics ...
s 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. The energy involved in this process is called
bond dissociation energy The bond-dissociation energy (BDE, ''D''0, or ''DH°'') is one measure of the strength of a chemical bond . It can be defined as the standard enthalpy change when is cleaved by homolysis to give fragments A and B, which are usually radical ...
(BDE). BDE is defined as the "
enthalpy Enthalpy () is the sum of a thermodynamic system's internal energy and the product of its pressure and volume. It is a state function in thermodynamics used in many measurements in chemical, biological, and physical systems at a constant extern ...
(per
mole Mole (or Molé) may refer to: Animals * Mole (animal) or "true mole" * Golden mole, southern African mammals * Marsupial mole Marsupial moles, the Notoryctidae family, are two species of highly specialized marsupial mammals that are found i ...
) required to break a given bond of some specific
molecular entity In chemistry and physics, a molecular entity, or chemical entity, is "any constitutionally or isotopically distinct atom, molecule, ion, ion pair, radical, radical ion, complex, conformer, etc., identifiable as a separately distinguishable en ...
by homolysis," symbolized as ''D''. BDE is dependent on the
strength Strength may refer to: Personal trait *Physical strength, as in people or animals *Character strengths like those listed in the Values in Action Inventory *The exercise of willpower Physics * Mechanical strength, the ability to withstand ...
of the bond, which is determined by factors relating to the stability of the resulting radical species. Because of the relatively high energy required to break bonds in this manner, homolysis occurs primarily under certain circumstances: * Light (i.e.
ultraviolet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...
radiation) * Heat **Certain intramolecular bonds, such as the O–O bond of a
peroxide In chemistry, peroxides are a group of Chemical compound, compounds with the structure , where the R's represent a radical (a portion of a complete molecule; not necessarily a free radical) and O's are single oxygen atoms. Oxygen atoms are joined ...
, are sufficiently weak to spontaneously homolytically dissociate near room temperature. **Most bonds homolyse at temperatures above 200°C.Clayden, Jonathan, Greeves, Nick, Warren, Stuart. (2012). Organic Chemistry (Second ed.). Oxford: OUP.
Adenosylcobalamin Adenosylcobalamin (AdoCbl), also known as coenzyme B12, cobamamide, and dibencozide, is one of the biologically active forms of vitamin B12. Adenosylcobalamin participates as a cofactor in radical-mediated 1,2-carbon skeleton rearrangements. T ...
is the cofactor which creates the deoxyadenosyl radical by homolytic cleavage of a cobalt-carbon bond in reactions catalysed by
methylmalonyl-CoA mutase Methylmalonyl-CoA mutase (, MCM), mitochondrial, also known as methylmalonyl-CoA isomerase, is a protein that in humans is encoded by the ''MUT'' gene. This vitamin B12-dependent enzyme catalyzes the isomerization of methylmalonyl-CoA to succin ...
,
isobutyryl-CoA mutase In enzymology, an isobutyryl-CoA mutase () is an enzyme that catalyzes the chemical reaction :2-methylpropanoyl-CoA \rightleftharpoons butanoyl-CoA Hence, this enzyme has one substrate, 2-methylpropanoyl-CoA, and one product, butanoyl-CoA. Th ...
and related enzymes. This triggers rearrangement reactions in the carbon framework of the substrates on which the enzymes act.


Factors that drive homolysis

Homolytic cleavage is driven by the ability of a molecule to absorb energy from light or heat, and the bond dissociation energy (
enthalpy Enthalpy () is the sum of a thermodynamic system's internal energy and the product of its pressure and volume. It is a state function in thermodynamics used in many measurements in chemical, biological, and physical systems at a constant extern ...
). If the radical species is better able to stabilize the radical, the energy of the SOMO will be lowered, as will the bond dissociation energy. Bond dissociation energy is determined by multiple factors: *
Electronegativity Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the ...
**Less electronegative atoms are better stabilizers of radicals, meaning that a bond between two electronegative atoms will have a higher BDE than a similar molecule with two less electronegative atoms. *
Polarizability Polarizability usually refers to the tendency of matter, when subjected to an electric field, to acquire an electric dipole moment in proportion to that applied field. It is a property of particles with an electric charge. When subject to an elect ...
**The larger the electron cloud, the better an atom can stabilize the radical (i.e. Iodine is very polarizable and a radical stabilizer). *
Orbital hybridization In chemistry, orbital hybridisation (or hybridization) is the concept of mixing atomic orbitals to form new ''hybrid orbitals'' (with different energies, shapes, etc., than the component atomic orbitals) suitable for the pairing of electrons to f ...
**The s-character of an orbital relates to how close electrons are to the nucleus. In the case of a radical, s-character more specifically relates to how close the single electron is to the nucleus. Radicals decrease in stability as they are closer to the nucleus, because the electron affinity of the orbital increases. As a general rule, hybridizations minimizing s-character increase the stability of radicals, and decreases the bond dissociation energy (i.e. sp3 hybridization is most stabilizing). *
Resonance Resonance is a phenomenon that occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency (or resonance frequency) of the system, defined as a frequency that generates a maximu ...
**Radicals can be stabilized by the donation of negative charge from resonance, or in other words,
electron delocalization In chemistry, delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or a covalent bond.IUPAC Gold Boo''delocalization''/ref> The term delocalization is general and can have slightly diff ...
. *
Hyperconjugation In organic chemistry, hyperconjugation (σ-conjugation or no-bond resonance) refers to the delocalization of electrons with the participation of bonds of primarily σ-character. Usually, hyperconjugation involves the interaction of the electron ...
**Carbon radicals are stabilized by hyperconjugation, meaning that more substituted carbons are more stable, and hence have lower BDEs. **In 2005, Gronert proposed an alternative hypothesis involving the relief of substituent group
steric strain Van der Waals strain is strain resulting from Van der Waals repulsion when two substituents in a molecule approach each other with a distance less than the sum of their Van der Waals radii. Van der Waals strain is also called Van der Waals repul ...
(as opposed to the before accepted paradigm, which suggests that carbon radicals are stabilized via alkyl groups). *The captodative effect **Radicals can be stabilized by a synergistic effect of both
electron-withdrawing group An electron-withdrawing group (EWG) is a Functional group, group or atom that has the ability to draw electron density toward itself and away from other adjacent atoms. This electron density transfer is often achieved by resonance or inductive effe ...
and electron-donating group substituents. **Electron-withdrawing groups often contain empty π* orbitals that are low in energy and overlap with the SOMO, creating two new orbitals: one that is lower in energy and stabilizing to the radical, and an empty higher energy orbital. Similarly, electron-donating orbitals combine with the radical SOMO, allowing a lone pair to lower in energy and the radical to enter the new higher energy orbital. This interaction is net stabilizing.


See also

* Alpha cleavage


References

{{Chemical bonds Chemical reactions