Detection of Radicals as Reaction Intermediates

Because of the low steady-state concentrations involved, this is not always easy. Evidence for free radical intermediates can be obtained from  

Kinetics. As outlined in Chapter 2, since radicals tend to be formed by first-order processes and destroyed in second-order reactions, steady-state radical concentrations are usually proportional to the square root of the precursor concentration [A]. This often leads to kinetic dependence on [A]0 5 or [A]15, so kinetic dependence of this form is strong evidence for a free radical mechanism. 

Initiation and inhibition. If a reaction is speeded up by the addition of compounds such as peroxides which are known to produce free radicals, this is evidence for a free radical mechanism. Inhibition (slowing down) of a reaction by compounds such as phenols is also evidence for free radical behaviour. 2,4,6-Tri- -butylphenol (20) acts as an inhibitor by hydrogen transfer (reaction 6.15) to one of the chaincarrying radicals involved, giving the phenoxyl radical 21, which is stabilized by delocalization and may be unable to carry on the chain reaction. 

By-products. The presence in the products of small quantities of compounds which would arise from combination of free radical intermediates can provide evidence for a free radical process. For example, the explosive reaction of methane with fluorine gives mainly hydrogen fluoride and a mixture of mono-, di-, tri- and tetrafluoromethanes, but small quantities of fluorinated ethanes, including C2F6, are also produced. These two-carbon products cannot be readily explained on the basis of possible molecular reactions (see reaction 6.16), but would arise naturally as combination products of the fluorinated methyl radicals produced in a radical chain reaction sequence (reaction 6.17). 

ESR spectra of radicals show some similarities with the NMF spectra of spin-paired molecules. The ESR g value corresponds to the NMR chemical shift. Hyperfine splitting in ESR caused by electron-nuclear spin interaction , have an NMR counterpart in the splitting of resonances by miteraclions between two nuclear spins. 

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