A brief history of free radicals 
It is now 100 years since Moses Gomberg discovered the first organic free radical. During this time, research has revealed that free radicals are present in the atmosphere, in our bodies and in some very important chemical reactions. Indeed, free radicals have an impact on all of our lives. Examples range from the body's ageing process, to the large-scale preparation of plastics used in the household. So, what are free radicals?

What are free-radicals? 
A free radical is an atom or compound which contains an unpaired electron.  All free radicals contain an odd number of electrons and nowadays the term "radical" is often used in place of "free radical". In 1900, Moses Gomberg (1866-1947) reported his results on the reaction of triphenylmethyl halides with metals. The reactions produced a yellow syrup (in the absence of air) and he attributed this to the formation of the triphenylmethyl radical, Ph ₃ C ^. . He noted that "the radical so formed is apparently stable, for it can be kept both in solution and in the dry crystalline state for weeks". The observation of a stable radical was a remarkable achievement, as most radicals exist only momentarily. So, why was Gomberg able to observe this particular radical? Well, this can be explained by the presence of three bulky benzene rings (see below). These protect the carbon atom bearing the radical, which slows down radical reactions. Radicals of this type, which do not combine at even relatively high concentrations, are now called persistent radicals.

 

Today, thousands of industrial and biological processes are very much dependent on reactions involving free radicals. Research into their reactivity and future uses are extensively being investigated by both industrialists and academics.

How are radicals produced and destroyed?
Initiation
Some molecules contain relatively weak covalent bonds, which under the correct energetic conditions, can break to give two molecules or atoms, each containing an unpaired electron. This type of bond breaking is known as homolysis or homolytic bond cleavage.

These reactions, which form radicals from non-radical reactants are known as initiation reactions. The energy source can be  provided by heating (thermolysis) or electromagnetic radiation (visible-light, UV or X-rays). Alternatively, radicals can be generated by redox reactions involving transition metals.

Propagation
Once the radical has been formed it can undergo abstraction (e.g. cyclisation ) or addition (e.g. polymers ) reactions, to form more stable radical or non-radical products. These propagation reactions generally work because they break weak bonds in the reactants and make strong bonds in the products.

Termination
In competition with propagation reactions are termination reactions. Whereas propagation reactions have one radical reactant and one radical product, termination reactions lead to the destruction of radicals (i.e. no new radicals are produced from these reactions). An important termination reaction involves the combination (or coupling) of radicals to form dimers.

Chain reactions 
Most radical reactions involve a chain mechansim in which an initial radical undergoes a series of propagation reactions. These reactions ultimately lead to regeneration of the initial radical and the formation of a chain.

Chlorination of methane

© Department of Chemistry, University of York.