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

Free-Radical Substition

• 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:

$Cl-Cl\to Cl\bullet +Cl\bullet$

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

$Cl\bullet +C{H}_4\to C{H}_3\bullet +HCl$

(b) Methyl radical produced then abstracts a Cl atom from a Cl₂ molecule, chloromethane and Cl· radical

$C{H}_3\bullet +C{l}_2\to C{H}_{3}Cl+Cl\bullet$

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:

$2Cl\bullet \to C{l}_2$

$C{H}_3\bullet +Cl\bullet \to C{H}_{3}Cl$

$2C{H}_3\bullet \to C{H}_{3}C{H}_3$ (trace amount)

Overall: $C{H}_4+C{l}_2\to C{H}_{3}Cl+HCl$

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