Freeradical Chemistry

Interest in free radicals began with the work of Gomberg (1900), who demonstrated the existence of the triphenylmethyl radical (Ph3C). The triphenylmethyl radical is stable enough to exist in solution at room temperature, often in equilibrium with a dimeric form. At room temperature, this concentration of Ph3C in benzene is 2%. Although triphenylmethyl-type radicals are stabilized by resonance, it is steric hindrance to dimerization and not resonance that is the cause of their stability.

Free radicals are formed from molecules by breaking a bond such that each fragment keeps one electron (free radicals can also be formed by collision of the nonradical species by a reaction between a radical and a molecule, which must then result in a radical, since the total number of electron is odd), by cleavage of a radical to give another radical, and by oxidation or reduction reactions.

One technique widely used in free-radical research, electron spin resonance (ESR) or electron paramagnetic resonance, detects radicals by measuring the energy changes that occur when unpaired electrons change their direction of spin. Only free radicals can give an ESR spectrum, hence the technique is sensitive for detecting and determining the concentration of free radicals. This technique provides information concerning the electron distribution and hence the structure of the free radicals—the structure being deduced from the splitting pattern of the ESR spectrum.

Whereas ESR depends on reorientation of unpaired electrons in a magnetic field, NMR (nuclear magnetic resonance) depends on reorientation of magnetic nuclei in the presence of a magnetic field. In the mid 1960s, it was discovered that if an NMR spectrum is taken during the course of a reaction involving free-radical species, certain signals can be enhanced (positively or negatively) or reduced. The occurrence of this chemically induced dynamic nuclear polarization (CIDNP) is indicative that, in such reaction, a portion of the product that was formed was via a radical intermediate. However, this may not be an absolute proof, since the absence of CIDNP does not

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prove that a free-radical intermediate is necessarily absent. CIDNP results when protons in a reacting molecule become dynamically coupled to an unpaired electron while traversing the path from reactants to products.

So a free radical is any chemical species (capable of independent existence) possessing one or more unpaired electrons, an unpaired electron being one that is alone in an orbital. The simplest free radical is an atom of the element hydrogen. The hydrogen atom contains one proton and a single unpaired electron, which qualifies it as a free radical. The radical dot (which denotes the radical species) is inserted to indicate that one or more unpaired electrons is present. Electrons are more stable when paired together in orbitals: the two electrons in a pair have different directions of spin. Hence radicals are generally less stable than nonradicals, although their reactivity may vary. The reader is referred to the literature (Bensasson et al., 1993; Butler et al., 1984; Cadogan, 1993; Hay and Waters, 1937; and Moad and Solomon, 1995) for accounts of free-radical chemistry.

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