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Chlorination of Methane

  • Page ID
    3758
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    If 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.

    \[ \ce{CH4 + Cl_2 -> CH_3Cl + HCl} \nonumber\]

    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 One of the hydrogen atoms in the methane has been replaced by a chlorine atom, so this is a substitution reaction.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 \(\rightarrow\) 2Clelectron.GIF

    • Chain propagation reactions : These are the reactions which keep the chain going.

    CH4 + Clelectron_aqp8.GIF\(\rightarrow\)CH3electron_6lqc.GIF + HCl

    CH3electron_y0y8.GIF + Cl2\(\rightarrow\)CH3Cl + Clelectron_0ocf.GIF

    • Chain termination reactions: These are reactions which remove free radicals from the system without replacing them by new ones.

    2Clelectron_mwp1.GIF\(\rightarrow\)Cl2

    CH3electron_hmbh.GIF + Clelectron_92k4.GIF \(\rightarrow\) CH3C l

    CH3electron_emft.GIF + CH3electron_oz31.GIF\(\rightarrow\)CH3CH3


    This page titled Chlorination of Methane is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Jim Clark.