Reaction Mechanisms

Alkanes react with halogens in the presence of UV radiation or at a temperature of around 300c. Methane reacts with chlorine to produce chloromethane:

CH4 + Cl2 -----> CH3Cl + HCl

This is radical substitution:

Covalent bonds are broken by homolytic fission to form radicals

A hydrogen atom in the alkane is substituted by a halogen atom.

In the initiation stage, the Cl-Cl bond in a chlorine molecule is broken by homolytic fission, forming two chlorine radicals. UV radiation provides the energy for this bond fission.

Cl-Cl ------> Cl (radical) + Cl (radical)

After initiation, the reaction can continue without the need for further energy or more radicals.

The propagation stage has two steps:

In the first stage methane reacts with a chlorine radical. A single C-H bond is broken by homolytic fission, forming a methyl radical, CH3 (radical). Hydrogen chloride, HCl, is also formed.

CH4 + Cl (radical) -----> CH3 (radical) + HCl

In the second propagation step, the methyl radical reacts with a chlorine molecule. The organic product chloromethane, CH3Cl, is formed together with a further chlorine radical. The chlorine radical can be used again in the first propagation step.

CH3 (radical) + Cl2 ------> CH3Cl + Cl (radical)

Propagation reactions are rapid, and reactions continue until no reactants remain. Propagation is a chain reaction, and it continues until all the chlorine has been used or the termination stage has removed all the radicals.

In the termination stage, two radicals combine to form a molecule. There are a number of possible termination steps. This is because of the large numbers of radicals in the reaction mixture. In this example, the following molecules can be produced:

Cl (radical) + Cl (radical) -----> Cl2

CH3 (radical) + CH3 (radical) ------> C2H6

CH3 (radical) + Cl (radical) -----> CH3Cl

The termination stage removes radicals, stopping the reaction. Before this happens though, the propagation stage will have cycled through many thousands of times.

Chloromethane made in the propagation stage may react with further chlorine radicals until all of the hydrogen atoms have been replaced. This results in a mixture of chloromethane (CH3Cl), dichloromethane (CH2Cl2), trichloromethane (CHCl3) and tetrachloromethane (CCl4).