Radical Reactions

A free radical is a molecule which has an extra electron/open shell configuration. It does not satisfy the octet rule and is usually highly reactive.

## Types of Radical Reactions

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### Free-Radical Substition

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* Reaction : CH4 \+ Cl2 \--> CH3Cl + HCl
* The reaction involves a chain reaction consisting of 3 steps: initiation, propagation, termination

Step 1: Initiation

Formation of the Cl· free radical via homolytic fission of the Cl-Cl bond:

C l − C l -> C l ∙ + C l ∙ {\displaystyle Cl-Cl\to Cl\bullet +Cl\bullet }

Step 2: Propagation

Generation of more free radicals via the chain reaction:

(a) Highly reactive chlorine radicals collide with CH4 molecules and abstract one of its H atoms, forming HCl and a methyl radical

C l ∙ + C H 4 -> C H 3 ∙ + H C l {\displaystyle Cl\bullet +CH_{4}\to CH_{3}\bullet +HCl}
(b) Methyl radical produced then abstracts a Cl atom from a Cl2 molecule, chloromethane and Cl· radical

C H 3 ∙ + C l 2 -> C H 3 C l + C l ∙ {\displaystyle CH_{3}\bullet +Cl_{2}\to CH_{3}Cl+Cl\bullet }
Then (a), (b), (a), (b)...
The chlorine free radical produced can attack another CH4 molecule and hence the process continues - a chain reaction
Step 3: Termination

Termination of the chain reaction occurs upon the collision of two free radicals with the formation of a single covalent bond:

2 C l ∙ -> C l 2 {\displaystyle 2Cl\bullet \to Cl_{2}}
C H 3 ∙ + C l ∙ -> C H 3 C l {\displaystyle CH_{3}\bullet +Cl\bullet \to CH_{3}Cl}
2 C H 3 ∙ -> C H 3 C H 3 {\displaystyle 2CH_{3}\bullet \to CH_{3}CH_{3}} (trace amount)

Overall: C H 4 + C l 2 -> C H 3 C l + H C l {\displaystyle CH_{4}+Cl_{2}\to CH_{3}Cl+HCl}

* The reaction does not stop at this stage. Further substitution may arise, resulting in a mixture of multi-substituted products: