User:MDP23/free radical sub

Initiation step:
 * Cl2 → 2Cl•

Propagation steps
 * Cl• + CH4 → •CH 3 + HCl
 * •CH3 + Cl2 → CH3Cl + Cl•

Termination steps
 * 2Cl• → Cl2
 * •CH3 + •CH3 → CH3CH3
 * •CH3 + Cl• → CH3Cl