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.

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.

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.

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.

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.

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).

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.

  Back to Home Page
  Back to Main Staff Page

Last Updated: 6th January 2006. These pages are maintained by Izzi Montgomery.
Any comments or suggestions are welcome

© Department of Chemistry, University of York.