Alkanes are exceptionally boring and unreactive molecules chemically speaking but we can persuade them to react with halogens in the presence of high energy / high frequency UV light.
CH4(g) + Cl2(g) ⇾ CH3Cl(g) + HCl(g)
The radiation causes homolytic fission of the halogen-halogen bond, forming radical halogen atoms and starting a chain reaction which ends in a haloalkane (halogenoalkane) such as choromethane.
You need to be able to explain the terminology and draw out the mechanism for the radical substitution reaction.
A mechanism shows the movement of electrons in a reaction, breaking it down into simple steps.
This is the first of many that you will meet and is important for understanding the role of CFCs in the depletion of ozone in the stratosphere.
- Initiation
Homolytic fission of a covalent bond is when the bond breaks evenly and each fragment takes one electron from the bond. The resulting fragments each have an unpaired electron are are called radicals. Radicals are very reactive species.
- Propagation
There are two propagation steps. Each step produces a radical leading to a chain reaction.
The key to understanding this mechanism is to appreciate that of the two propagation steps, the first always makes the by-product and the haloalkane is made in the second propagation step. Both steps are exothermic as the bonds that are made in each stage are stronger than those broken.
- Termination
In the termination step, two radicals react to form a non-radical. Since the concentration of radicals in the reaction mixture is actually very low, chain termination is rare.
