Radicals - Introduction
Radical Chain Reactions
As a result of an unpaired electron, free radicals are highly reactive and instable molecules. Therefore, radicals are usually impossible to isolate. Nevertheless, they appear in small concentrations at intermediate stages of radical reactions. Radical reactions are often chain reactions. In a typical radical chain reaction, for instance, the reaction between a radical A* and a starting product B yields a new radical B*. This radical B* is then converted into the product C. During the conversion of B*, in addition, a new radical, namely A* is formed. As a result, the starting product B has been converted into the product C, while the initially consumed radical A* has been recovered. Accordingly, the radical A* is again available to convert B into B*. The intermediate radical A* is consumed and recovered over and over again in this radical chain reaction. Therefore, the chain reaction is a cyclic process that is fed with the starting product B and yields the product C.
Take a look at the radical chlorination of methane, for example. This is a radical substitution reaction, which yields chloromethane, as well as dichloromethane, trichloromethane (chloroform), and tetrachloromethane (carbon tetrachloride). The formation of a product mixture is representative of the high reactivity of chlorine in radical chlorinations.
At the initial stage of the chain reaction, a small number of chlorine radicals must definitely be generated. In this so-called initiation reaction, the chlorine molecule is homolytically cleaved into two chlorine radicals by the application of light (or radiation) or heat.
Subsequently, in the first step of the chain propagation, a chlorine radical abstracts a hydrogen atom from methane, thus yielding hydrogen chloride and a methyl radical. In the second step of the chain propagation, the methyl radical reacts with a chlorine molecule yielding chloromethane and a new chlorine radical. Thus, the chlorine radical that was consumed in the first chain propagation step has been recovered in the second propagation step. As a result, the cyclic process can then start over again, beginning with the first chain propagation step.
The chain reaction can be terminated by undesirable side reactions, in which the radicals are consumed before being able to form new methyl or chlorine radicals. This so-called chain termination is either caused by impingements with the wall or by the recombination of two radicals. However, the recombination of two chlorine radicals, which forms a chlorine molecule, is unlikely, since the high energy of the chlorine radicals cannot be distributed sufficiently among the two atoms of the chlorine molecule. This results in the re-cleavage into chlorine radicals. In contrast, the recombination of a chlorine radical with a methyl radical or of two methyl radicals that yields chloromethane (but without recovering a chlorine radical) or ethane, respectively, can actually occur without immediate re-cleavage, as the high energy of the radicals can be distributed among more atoms of the product molecule.
Radical chain reactions play an important part in organic chemistry both as radical substitution and radical addition reactions. Radical addition chain reactions, for instance, may be found in radical polymerization.