University of York Department of Chemistry

“A Tale of 2 Radicals”

Part (II)

The Influence of Free Radicals in
Industry and Biology

A brief history of free radicals
What are free-radicals?
How are radicals produced and destroyed?
Chain reactions
Classic free radical reactions in organic chemistry
Natural product synthesis
Polymers
Free radicals in nature
Antioxidants
Radicals working in harmony
Radicals in the environment
Recent free radical literature


Classic free radical reactions in organic chemistry top_of_page
HBr adds to alkenes in the presence of a peroxide via a RADICAL PATHWAY. The bromine atom adds to the less hindered carbon atom of the alkene to form a secondary (rather than a primary) radical. This radical then abstracts a hydrogen atom from HBr to continue the chain reaction.Anti Markovnikov Addition

Benzene can be reduced to 1,4-cyclohexadiene, using sodium in liquid ammonia. This produces an electron which adds to the benzene ring to generate a radical anion. This can be protonated to give a cyclohexadienyl radical which on further reduction and protonation produces the diene.

Birch Reduction

Other well known radical reactions include, the acyloin condensation, the Bouveault - Blanc reaction, the Norrish Type II reaction, the Sandmeyer reaction and the Barton decarboxylation.



Natural product synthesis top_of_page

One of the most important free radical reactions are cyclisations to form rings. This type of reaction has been used to prepare a variety of cyclic natural products. The most common method of cyclisation involves the reaction of tributyltin hydride (Bu3SnH) with unsaturated alkyl halides in the presence of AIBN. The tributyltin radical (Bu3Sn.) abstracts a halogen atom to form a carbon-centred radical, which can cyclise.

cyclisation
Synthesis of Kainic Acid

Kainic acid is extracted from Digenea simplex (a type of red algae/seaweed) found growing in warm temperate waters. This  amino acid has been shown to exhibit potent neurological activity, which has been attributed to its structural likeness to the neurotransmitter L-glutamic acid.

Kainic acid2cyclisation

Synthetic routes to analogues of kainic acid have been investigated within our group at York. A key step in the synthesis involves a RADICAL CYCLISATION reaction to make the 5-membered ring. This involves the formation of a tributyltin radical, Bu3Sn., which abstracts a chlorine atom from the precursor. The carbon radical can then add to the alkene double bond to form the ring. This chain reaction is completed by reaction of tributyltin hydride (Bu3SnH) with the cyclic carbon radical.

Cyclisation


Polymers top_of_page

Radical polymerisation methods are commonly used as a way to circumvent the limitations of other polymerisation processes involving ions. Radical polymerisation can be carried out in the presence of a variety of functional groups using a considerable range of solvents (from toluene to water). Radical processes are used to make around 75% of the worlds polymers (about 108 tonnes). An initial radical adds to the double bond of an alkene monomer and the resulting radical adds to another alkene monomer, and so on. This addition polymerisation is used to make a number of important polymers, including poly(vinyl chloride), polystyrene and polyethylene (polythene).
polymers

An example of this type of polymerisation is LIVING POLYMERISATION, in which the molecular weight of polymer can be increased by addition of extra or different monomer even after the apparent completion of the reaction (see figure). The living process is an equilibrium between dormant and active radical species. This activation/deactivation cycle keeps the level of radicals low, which leads to a controlled polymerisation.

living polymerisation

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Last Updated: 6th January 2006. These pages are maintained by Izzi Montgomery.
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