The synthesis of methyl chloride (chloromethane) from methane is a common reaction in organic chemistry. It is a free-radical substitution reaction that typically occurs when a mixture of methane (CH4​) and chlorine gas (Cl2​) is exposed to ultraviolet (UV) light or high temperatures. The reaction proceeds in three main steps:
- Initiation: The UV light provides the energy to break the chlorine molecule (Cl2​) into two highly reactive chlorine free radicals (Cl⋅). This is an example of homolytic fission. Cl2​→2Cl⋅
- Propagation: The chlorine radical reacts with a methane molecule, abstracting a hydrogen atom to form a methyl radical (CH3​⋅) and hydrogen chloride (HCl). The methyl radical then reacts with another chlorine molecule to form methyl chloride (CH3​Cl) and another chlorine radical. This new radical can continue the chain reaction. CH4​+Cl⋅→CH3​⋅+HCl CH3​⋅+Cl2​→CH3​Cl+Cl⋅
- Termination: The reaction stops when two free radicals combine to form a stable molecule. This can be two chlorine radicals, a chlorine radical and a methyl radical, or two methyl radicals. Cl⋅+Cl⋅→Cl2​ CH3​⋅+Cl⋅→CH3​Cl CH3​⋅+CH3​⋅→CH3​CH3​ (ethane)
A major challenge with this method is that the reaction doesn’t easily stop at methyl chloride. The newly formed methyl chloride can continue to react with chlorine radicals, leading to a mixture of products including dichloromethane (CH2​Cl2​), trichloromethane (CHCl3​), and tetrachloromethane (CCl4​). To maximize the yield of methyl chloride, a large excess of methane is typically used in the reaction.
