91Ó°ÊÓ

An atom or group of atoms containing odd or unpaired electrons 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 the regeneration of reactive species takes place in the propagation step.

(a) Formation of reactive intermediates takes place first in the initiation step.

Initiation

In the first step of propagation, a radical reacts to generate another radical.

Propagation

In the second step, chloromethyl radical reacts with CI₂ to generate the product and chlorine radical.

Chloromethyl radical reacts withCI₂

(b) A halogen atom is substituted for a hydrogen atom in free-radical halogenation. A different compound is produced from the first substitution of a hydrogen atom. The hydrogen atoms left unreacted can compete for halogen atoms that are available. For example, in the chlorination of methane, different substitution products are formed such as CH₄,CH₃Cl,CH₂CI₂, CHCI₃, and CCI₄. Hence, free-radical halogenation usually gives a mixture of products.

(c)The amount ofCI₂ (chlorine) reacted with CH₄ (methane) should be in excess for favoring the production ofCCI₄ . This is to make certain that the reaction progresses so that all the C-H bonds are replaced with C-l bonds.

The amount ofCI₂ (chlorine) reacted with CH₄ (methane) should be less to favor the production of . This is to make certain that the reaction progresses so that all the chlorine atoms would more likely react with methane to formCH₃Cl and not with CH₃Cl to form CH₂CI₂ .