1 point
9. Methane and chlorine gases
react with each other at 250°C to
400°C in presence of U.V. light to
form and hydrogenchloride,
Answers
a mixture of methane and chlorine is exposed to a flame, it explodes - producing carbon and hydrogen chloride. This is not a very useful reaction! The reaction we are going to explore is a more gentle one between methane and chlorine in the presence of ultraviolet light - typically sunlight. This is a good example of a photochemical reaction - a reaction brought about by light.
CH4+Cl2→CH3Cl+HCl(1)(1)CH4+Cl2→CH3Cl+HCl
The organic product is chloromethane. One of the hydrogen atoms in the methane has been replaced by a chlorine atom, so this is a substitution reaction. However, the reaction does not stop there, and all the hydrogens in the methane can in turn be replaced by chlorine atoms. Multiple substitution is dealt with on a separate page, and you will find a link to that at the bottom of this page.
The mechanism
The mechanism involves a chain reaction. During a chain reaction, for every reactive species you start off with, a new one is generated at the end - and this keeps the process going. The over-all process is known as free radical substitution, or as a free radical chain reaction.
Chain initiation: The chain is initiated (started) by UV light breaking a chlorine molecule into free radicals.
Cl2 →→ 2Cl
Chain propagation reactions : These are the reactions which keep the chain going.
CH4 + Cl→→CH3 + HCl
CH3 + Cl2→→CH3Cl + Cl
Chain termination reactions: These are reactions which remove free radicals from the system without replacing them by new ones.
2Cl→→Cl2
CH3 + Cl →→ CH3C l
CH3 + CH3→→CH3CH3