Elimination reaction of free radicals

Radical disproportionation via radical elimination mechanism

Depolymerization of polystyrene via radical elimination mechanism

Radical elimination reaction in acyl-CoA dehydrogenase-catalyzed reaction.

Free radicals can undergo elimination reactions to form olefins, a reaction known as elimination reaction of free radicals.^[1] Such reactions are usually not major pathways for radical mediated reactions.

Radicals can undergo disproportionation reaction through radical elimination mechanism (Figure "Radical disproportionation via radical elimination mechanism"). A radical abstracts a hydrogen atom from another same radical to form two non-radical species: an alkane and an alkene.

Radicals can also undergo elimination reaction to generate a new radical as the leaving group. For example, when polystyrene decomposes upon heating at temperature above 300oC, styrene monomer is generated via radical elimination mechanism (Figure "Depolymerization of polystyrene via radical elimination mechanism").^[2] Here, the new radical is generated on the polymer chain, which can further undergo similar type of reaction to generate more styrene molecules. This process is known as the radical mediated depolymerization of polystyrene.

Radical elimination reactions are found in enzyme-catalyzed pathways. In the dehydrogenation reaction of acyl-CoA to form enoyl-CoA, FAD accepts two protons and two electrons to form FADH2 under the catalysis of acyl-CoA dehydrogenase.^[3] The mechanism involves formation of acyl-CoA β-radical that undergo elimination to form the enoyl-CoA product (Figure "Radical elimination reaction in acyl-CoA dehydrogenase-catalyzed reaction").

References

1. Ansylen, E. V.; Dougherty, D. A. Modern Physical Organic Chemistry. p. 586, ISBN 978-1-891389-31-3
2. Grassie, N.; Kerr, W. W. Trans. Faraday Soc., 1957, 53, 234-239
3. Thorpe, C.; Kim, J. J.; FASEB J., 1995, 9, 718-725