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.

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
 * $$ Cl\bullet + CH_4 \to CH_3\bullet + HCl $$
 * (b) Methyl radical produced then abstracts a Cl atom from a Cl2 molecule, chloromethane and Cl· radical
 * $$CH_3\bullet + Cl_2 \to CH_3Cl + Cl\bullet$$
 * 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:
 * $$2Cl\bullet \to Cl_2$$
 * $$CH_3\bullet + Cl\bullet \to CH_3Cl$$
 * $$2CH_3\bullet \to CH_3CH_3$$ (trace amount)
 * $$2CH_3\bullet \to CH_3CH_3$$ (trace amount)


 * Overall:   $$CH_4 + Cl_2 \to CH_3Cl + HCl$$


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