A bond can be broken either by heterolytic bond cleavage to formions or homolytic bond cleavage to yieldradicals. 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 orelectron 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 abstractsmostly 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 areformed from a starting radical compound.