20.5: Radical Formation: Overview
A bond can be broken either by heterolytic bond cleavage to form ions or homolytic bond cleavage to yield radicals. A fishhook arrow is used to represent the motion of a single electron in homolytic bond cleavage. There are two main sources from which radicals can be formed:
Radicals from spin-paired molecules:
Radicals can be obtained from spin-paired molecules either by homolysis or electron transfer. While two radicals are formed in the former, an electron is added in the latter, also known as reduction.
Radicals from other radicals:
There are three ways of radical formation from other radicals: substitution or abstraction, addition, and elimination. During substitution, a radical interacts with a compound or spin-paired molecule. It abstracts mostly a hydrogen or halogen atom to form another spin-paired molecule and a radical. In case of addition, a radical is added to a pi bond of an unsaturated compound to produce a carbon-centered radical. Elimination is the reversal of the addition process, where a new radical and an unsaturated compound are formed from a starting radical compound.