Homolysis (chemistry)

For other uses, see Homolysis (disambiguation).

In chemistry, homolysis (from Greek ὅμοιος (homoios) 'equal' and λύσις (lusis) 'loosening') or homolytic fission is the dissociation of a molecular bond by a process where each of the fragments (an atom or molecule) 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.[1] 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 (BDE).[2] BDE is defined as the "enthalpy (per mole) required to break a given bond of some specific molecular entity by homolysis," symbolized as D.[3] BDE is dependent on the strength 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:

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 peroxide, are weak enough to spontaneously homolytically dissociate with a small amount of heat.
    • High temperatures in the absence of oxygen (pyrolysis) can induce homolytic elimination of carbon compounds.[4]
    • Most bonds homolyse at temperatures above 200°C.[5]

Additionally, in some cases pressure can induce the formation of radicals.[6] These conditions excite electrons to the next highest molecular orbital, thus creating a singly occupied molecular orbital (SOMO).

Adenosylcobalamin is the cofactor which creates the deoxyadenosyl radical by homolytic cleavage of a cobalt-carbon bond in reactions catalysed by methylmalonyl-CoA mutase, isobutyryl-CoA mutase and related enzymes. This triggers rearrangement reactions in the carbon framework of the substrates on which the enzymes act.[7]

  1. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "homolysis (homolytic)". doi:10.1351/goldbook.H02851
  2. ^ 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
  3. ^ 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.
  4. ^ I. Pastorova, "Cellulose Char Structure: a Combined Analytical Py-GC-MS, FTIR, and NMR Study", Carbohydrate Research, 262 (1994) 27-47.
  5. ^ Clayden, Jonathan, Greeves, Nick, Warren, Stuart. (2012). Organic Chemistry (Second ed.). Oxford: OUP. ISBN 978-0-19-927029-3
  6. ^ 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.
  7. ^ Jost, Marco; Born, David A.; Cracan, Valentin; Banerjee, Ruma; Drennan, Catherine L. (2015). "Structural Basis for Substrate Specificity in Adenosylcobalamin-dependent Isobutyryl-CoA Mutase and Related Acyl-CoA Mutases". Journal of Biological Chemistry. 290 (45): 26882–26898. doi:10.1074/jbc.M115.676890. PMC 4646380. PMID 26318610.