ABSTRACT
Free radicals are formed when a covalent bond breaks homolytically. Each fragment contains an unpaired election and, in the absence of significant stabilizing influences of a steric and/or electronic type, they exist only for a fraction of a second before reacting. The reactions of radicals may be divided into two classes. In the first, the product is formed from the combination of two radicals. In the second, the product results from the reaction of a radical with a molecule in a chain reaction; that is, the step in which the product is formed results in the formation of a new free radical, and this radical can bring about further reaction. The selectivity of a free radical towards carbon–hydrogen bonds is determined principally by two factors: bond dissociation energies and polar effects. The rate of abstraction increases as the carbon–hydrogen bond dissociation energy decreases.
