Alkanes do not react with most chemicals because the C-C bond and the C-H bond is not that polar. Alkanes will react though with things that are not polar (diatomic atoms for example), if you give them enough energy
Halogens do react with alkanes to form haloalkanes. Halogens react with alkanes in photochemical reactions, photochemical reactions start by UV light
A hydrogen atom is substituted (replaced) by a halogen. This is called free-radical substitution reaction. (Free radicals are particles with unpaired electron, you get them when a bond is split equally, they are also highly reactive)
Chlorine and Methane form Chloromethane. The mechanism has 3 stages: Initiation step, propagation step and termination step.
Free Radical Substitution
Initiation reaction -
- Sunlight provides enough energy to break the Cl-Cl bond, this is called photodissociation.
- The bond splits equally and each atom gets to keep one electron. The atom is now a highly reactive free radical (Cl-)
Propagation reaction(s) -
- Cl- attacks a methane molecule forming CH3 and HCl
- The CH3 can then attack a Cl2 molecule, forming a CH3Cl and a chlorine free radical
- The new chlorine free radical can attack another methane atom and so on until all the chlorine and methane is used up
Termination Reaction -
- If 2 free radical atoms join together, this forms a stable molecule. So this what happens between chlorine and CH3
Destroying the O-Zone
Chlorine free radicals are formed when the C-Cl bond (found in CFC's) is broken down by the UV light
These free radicals are catalysts. They react with the O - Zone (O3) to form an intermediate ClO free radical and an oxygen molecule
This ClO free radical can then attack another O - Zone atom, and form an oxygen atom and a chlorine free radical.
So the overall reaction is 2O3 ---> 3O2