Synthesis of Haloalkanes (Free Radical Substitution)
Halogens react with alkanes in photochemical reactions (reactions that involve light). The amount of light needed for the reaction to occur is given the value of a quanta (a packet) of light, and in reality it is usually UV light.
In free radical substitution reactions, a hydrogen atom is substituted by a halogen atom. A free radical is a particle with an unpaired electron. The symbol for a free radical is a dot next to the atom, this dot is showing that it has an unpaired electron, for example Cl∙
Free radicals occur when bonds are broken via homolytic fission. This means that when the bond is broken (remember we are talking about covalent bonds, which are shared pairs of electrons), one electron goes to one atom, and one goes to the other.
A free radical substitution reaction is split into three stages, Initiation, Propagation and Termination.
The Initiation stage is where free radicals are produced.
Cl2 –-U.V--> 2Cl∙
This is the initiation step, and produces two chlorine free radicals. The quanta of UV light is used to break the bond between the chlorine atoms and can sometimes be called ‘photodissociation’.
The propagation stage is known as the chain reaction stage. Here the free radicals produced via initiation are used, and a new free radical is produced (hence chain reaction).
Cl∙ + CH4 --> ∙CH3 + HCl
∙CH3 + Cl2 --> CH3Cl + Cl∙
These are just two examples of what could happen. You need to be able to construct your own propagations steps. In an exam question, they will often tell you which products to form.
The termination step is when two free radicals collide with each other.
Cl∙ + ∙CH3 --> CH3Cl
∙CH3 + ∙CH3 --> C2H6
Though free radical substitution is an easy reaction to complete, it is rarely used in industry because of its very poor yield, and numerous unwanted products.
CFC’s used to be used as an aerosol propellant (help shoot the product out of the canister), fridge coolant and for foaming plastics, but it was found that they are very damaging to the environment, mainly the ozone layer.
Ozone (O3) absorbs lots of UV radiation from the sun which can cause skin cancer. Ozone is formed by the reaction of oxygen with a quanta of UV light:
O2 + hν --> O∙ + O∙
The newly formed free radical then reacts with another molecule of oxygen which creates a molecule of ozone.
O2 + O∙ --> O3
If CFC’s are released into the atmosphere, then the chlorine free radicals that are created break down the ozone molecules:
Cl∙ + O3 --> O2 + ClO∙
ClO∙ + O3 --> 2O2 + Cl∙
If you look carefully at these two equations, you can see that the chlorine free radical is a reactant in the first equation, but a product in the second. This shows that the chlorine free radical is acting as a catalyst! It has been theorised that one chlorine free radical could be responsible for the breakdown of around thirty thousand ozone molecules.
The above two equations can be written overall as:
2O3 --> 3O2
This is formed from the cancelling of the chlorine free radicals and combining of the ozone molecules and oxygen molecules on both sides.