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An atom or group of atoms containing odd or unpaired electron is known as the free radical. The unpaired electron is represented by a single unpaired dot in the formula. Free radicals are electrically neutral. They are highly reactive species formed by homolytic fission of a covalent bond.

The formation of products with regeneration of reactive species takes place in the propagation step.

The highest energy state in a molecular collision that leads to reaction is the transition state. Transition state is most unstable. The transition state or activated complex gets converted into product molecules. During the formation of this complex, old bonds start breaking and the new bonds start making.

According to Hammond’s postulate, related species which are closer in energy are also closer in structure and the structure of the transition state resembles the structure of the closest stable species.

(a)

second propagation step in the chlorination of methane

transition state

From Hammond’s postulate, it is known that the transition state in the exothermic chlorination must resemble the reactants more than the products. Hence, the transition state is more reactant-like.

(b)

second propagation step in the bromination of methane

transition state