View source for Chemistry:Homolysis

{{short description|Breaking a molecular bond such that both fragments retain an electron}}

In [[Chemistry:Chemistry|chemistry]], '''homolysis''' ({{ety|el|''ὅμοιος'' (homoios)|equal||''λύσις'' (lusis)|loosening}}) or '''homolytic fission''' is the dissociation of a molecular bond by a process where each of the fragments (an [[Physics:Atom|atom]] or [[Physics:Molecule|molecule]]) retains one of the originally bonded [[Physics:Electron|electron]]s. During homolytic fission of a neutral molecule with an even number of electrons, two [[Chemistry:Free-radical halogenation|free radical]]s will be generated.<ref>{{GoldBookRef|title=homolysis (homolytic)|file=H02851}} </ref> 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 [[Chemistry:Heterolysis|heterolysis]]. 

[[File:Homolysis (Chemistry) V.1.svg|350px]]

The energy involved in this process is called bond dissociation energy (BDE).<ref>St. John, P.C., Guan, Y., Kim, Y. et al. Prediction of organic homolytic bond dissociation enthalpies at near chemical accuracy with sub-second computational cost. Nat Commun 11, 2328 (2020). https://doi.org/10.1038/s41467-020-16201-z</ref> BDE is defined as the "[[Physics:Enthalpy|enthalpy]] (per [[Mole (unit)|mole]]) required to break a given bond of some specific [[Chemistry:Molecular entity|molecular entity]] by homolysis," symbolized as '''''D'''''.<ref>IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). Online version (2019-) created by S. J. Chalk. {{ISBN|0-9678550-9-8}}. https://doi.org/10.1351/goldbook.</ref> BDE is dependent on the [[Physics:Bond energy|strength]] of the bond, which is determined by factors relating to the stability of the resulting [[Biology:Radical|radical species]].

Because of the relatively high energy required to break bonds in this manner, homolysis occurs primarily under certain circumstances:
* Light (i.e. [[Physics:Ultraviolet|ultraviolet]] radiation)
[[File:Homolysis2.png|thumb|The O-O σ bond in dibenzoyl peroxide is cleaved homolytically, distributing a radical to each benzoyloxy.]]
* Heat
**Certain intramolecular bonds, such as the O–O bond of a [[Chemistry:Peroxide|peroxide]], are weak enough to spontaneously homolytically dissociate with a small amount of heat.
**High temperatures in the absence of oxygen ([[Chemistry:Pyrolysis|pyrolysis]]) can induce homolytic elimination of [[Chemistry:Compounds of carbon|carbon compounds]].<ref>I. Pastorova, "Cellulose Char Structure: a Combined Analytical Py-GC-MS, FTIR, and NMR Study", Carbohydrate Research, 262 (1994) 27-47.</ref>
**Most bonds homolyse at temperatures above 200°C. <ref name="foo">Clayden, Jonathan, Greeves, Nick, Warren, Stuart. (2012). Organic Chemistry (Second ed.). Oxford: OUP. {{ISBN|978-0-19-927029-3}}</ref>

Additionally, in some cases pressure can induce the formation of radicals.<ref>Kristina Lekin, Hoa Phan, Stephen M. Winter, Joanne W. L. Wong, Alicea A. Leitch, Dominique Laniel, Wenjun Yong, Richard A. Secco, John S. Tse, Serge Desgreniers, Paul A. Dube, Michael Shatruk, and Richard T. Oakley "Heat, Pressure and Light-Induced Interconversion of Bisdithiazolyl Radicals and Dimers", Journal of the American Chemical Society 2014 136 (22), 8050-8062 DOI: 10.1021/ja502753t.</ref> These conditions excite electrons to the next highest molecular orbital, thus creating a singly occupied molecular orbital (SOMO).

[[Chemistry:Adenosylcobalamin|Adenosylcobalamin]] is the [[Biology:Cofactor (biochemistry)|cofactor]] which creates the [[Chemistry:Deoxyadenosyl radical|deoxyadenosyl radical]] by homolytic cleavage of a cobalt-carbon bond in reactions catalysed by [[Biology:Methylmalonyl-CoA mutase|methylmalonyl-CoA mutase]], [[Biology:Isobutyryl-CoA mutase|isobutyryl-CoA mutase]] and related enzymes. This triggers rearrangement reactions in the carbon framework of the substrates on which the enzymes act.<ref>{{cite journal |doi=10.1074/jbc.M115.676890 |title=Structural Basis for Substrate Specificity in Adenosylcobalamin-dependent Isobutyryl-CoA Mutase and Related Acyl-CoA Mutases |year=2015 |last1=Jost |first1=Marco |last2=Born |first2=David A. |last3=Cracan |first3=Valentin |last4=Banerjee |first4=Ruma |last5=Drennan |first5=Catherine L. |journal=Journal of Biological Chemistry |volume=290 |issue=45 |pages=26882–26898 |pmid=26318610 |pmc=4646380 |doi-access=free }}</ref>

== 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 ([[Physics:Enthalpy|enthalpy]]). If the radical species is better able to stabilize the free radical, the energy of the SOMO will be lowered, as will the bond dissociation energy. Bond dissociation energy is determined by multiple factors:<ref name="foo" />
[[File:Homolysis electronegativity.jpeg|thumb|The bond dissociation energy depends on the electronegativity of the species bonded.]]
*[[Chemistry:Electronegativity|Electronegativity]]
**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. <ref name="foo" />
*[[Physics:Polarizability|Polarizability]] 
**The larger the electron cloud, the better an atom can stabilize the radical (i.e. Iodine is very polarizable and a radical stabilizer). <ref name="foo" />
[[File:Hybridation sp3-anim 100.gif|thumb|An sp3 hybridized atom is the most stable configuration for a radical because of the low s-character.]]
*Orbital hybridization
**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. sp<sup>3</sup> hybridization is most stabilizing).<ref>Mendenhall, G. (1978). Long-lived free radicals. Science Progress (1933- ), 65(257), 1-18. Retrieved December 5, 2020, from http://www.jstor.org/stable/43420441</ref>
[[File:Bromine-dioxide-radical-resonance-2D.png|thumb|This bromine dioxide radical is stabilized by the resonance of the molecule. Structure from J. Chem. Phys. (1997) 107, 8292-8302.<ref>Holger, S. P. Müller et al., J. Chem. Phys. 107, 8292 (1997); https://doi.org/10.1063/1.475030</ref>]]
*[[Physics:Resonance|Resonance]]
**Radicals can be stabilized by the donation of negative charge from resonance, or in other words, electron delocalization. 
*[[Chemistry:Hyperconjugation|Hyperconjugation]]
**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 (as opposed to the before accepted paradigm, which suggests that carbon radicals are stabilized via alkyl groups). <ref>J. Org. Chem. 2006, 71, 3, 1209–1219
Publication Date:January 4, 2006
https://doi.org/10.1021/jo052363t</ref>
[[File:Radical stability substitution.jpeg|thumb|The most substituted carbon radical is the most stable.]]
*The captodative effect 
**Radicals can be stabilized by a synergistic effect of both [[Chemistry:Electron-withdrawing group|electron-withdrawing group]] 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. <ref name="foo" />
  
 

== See also ==
* [[Chemistry:Alpha cleavage|Alpha cleavage]]

== References ==
{{reflist}}

{{Chemical bonds}}

[[Category:Chemical reactions]]

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