Homolysis (chemistry)
In
The energy involved in this process is called
Because of the relatively high energy required to break bonds in this manner, homolysis occurs primarily under certain circumstances:
- Light (i.e. ultraviolet radiation)
- Heat
Additionally, in some cases pressure can induce the formation of radicals.
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]
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). 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:[5]
- 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.[5]
- Polarizability
- The larger the electron cloud, the better an atom can stabilize the radical (i.e. Iodine is very polarizable and a radical stabilizer).[5]
- 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. sp3 hybridization is most stabilizing).[8]
- Resonance
- Radicals can be stabilized by the donation of negative charge from resonance, or in other words, electron delocalization.
- Radicals can be stabilized by the donation of negative charge from resonance, or in other words,
- 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).[10]
- The captodative effect
- Radicals can be stabilized by a synergistic effect of both electron-donating groupsubstituents.
- 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.[5]
- Radicals can be stabilized by a synergistic effect of both
See also
References
- ^ 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
- .
- ^ I. Pastorova, "Cellulose Char Structure: a Combined Analytical Py-GC-MS, FTIR, and NMR Study", Carbohydrate Research, 262 (1994) 27-47.
- ^ ISBN 978-0-19-927029-3
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- PMID 26318610.
- JSTOR 43420441. Retrieved December 5, 2020.
- doi:10.1063/1.475030.
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