Chlorination of Alkanes (Methane) via Substitution Reaction

• The overall symbol equation for the chlorination of methane is CH4 + Cl2 -> CH3Cl + HCl.
• The overall word equation for the chlorination of methane is methane (CH4) + chlorine (Cl2) -> chloromethane (CH3Cl) + hydrochloric acid (HCl).
• The reaction of chlorine and methane will only occur at room temperature if it is exposed to UV radiation (sunlight).
• Stage One of the chlorination of methane is known as initiation. In this process, the UV radiation causes the chlorine molecule to be broken down into two chlorine radicals.
• The reason why the chlorine molecule is broken down into two chlorine radicals is because in the chlorine molecule, there is a covalent bond between the two chlorine atoms - a shared pair of electrons. Therefore, when broken, each chlorine atom takes one unpaired electron (a radical). This is the process of homolytic fission.
• Homolytic fission is the breaking of a covalent bond where each of the bonding electrons leaves with one species, forming a radical.
• A radical is a species that contains an unpaired electron.
• Stage Two of the chlorination of methane is known as propagation. Due to the high reactivity of chlorine radicals, they further collide with methane molecules, removing a hydrogen atom.
• The process of propagation can be split into two different reactions. Firstly, the chlorine radical reacts with methane to form hydrochloric acid and a methyl radical. Next, the highly reactive methyl radical further reacts with chlorine to form chloromethane and a chlorine radical.
• Stage Three of the chlorination of methane is known as termination. In this stage, some radicals may collide to form new molecules. This is because each atom has a free radical, so forms a shared pair of electrons in a covalent bond.
• Essentially, termination occurs when reactive species are being converted into unreactive species.
• One of the major disadvantages of radical substitution reactions in industry is that they produce a low yield of chloromethane. This is because there is a high likelihood of further reactions occurring with chloromethane, meaning that these unwanted products have to become separated.
• After the chlorination of methane, the chloromethane produced can react further with chlorine to form dichloromethane. This can further react with chlorine to form trichloromethane, and then again with chlorine to form tetrachlorine.