THE REACTION BETWEEN METHANE AND CHLORINE

THE REACTION BETWEEN METHANE AND CHLORINE A Free Radical Substitution Reaction This page gives you the facts and a simple, uncluttered mechanism for the free radical substitution reaction between methane and chlorine. If you want the mechanism explained to you in detail, there is a link at the bottom of the page. The facts If a mixture of methane and chlorine is exposed to a flame, it explodes - producing carbon and hydrogen chloride. This isn't 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.
	Note:  These reactions are sometimes described as examples of photocatalysis - reactions catalysed by light. It is better to use the term "photochemical" and keep the keep the word "catalysis" for reactions speeded up by actual substances rather than light.
CH4  +  Cl2CH3Cl  +  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 One of the hydrogen atoms in the methane has been replaced by a chlorine atom, so this is a substitution reaction.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 doesn't 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.
	Species:  a useful word which is used in chemistry to mean any sort of particle you want it to mean. It covers molecules, ions, atoms, or (in this case) free radicals.
The over-all process is known as free radical substitution, or as a free radical chain reaction.
	Note:  If you aren't sure about the words free radical or substitution, read the page What is free radical substitution? Use the BACK button on your browser to return quickly to this page.
Chain initiation The chain is initiated (started) by UV light breaking a chlorine molecule into free radicals. Cl22Cl Chain propagation reactions These are the reactions which keep the chain going. CH4  +  ClCH3  +  HCl CH3  +  Cl2CH3Cl  +  Cl Chain termination reactions These are reactions which remove free radicals from the system without replacing them by new ones. 2ClCl2 CH3  +  ClCH3Cl CH3  +  CH3CH3CH3

FREE RADICAL SUBSTITUTION

FREE RADICAL SUBSTITUTION Substitution reactions These are reactions in which one atom in a molecule is replaced by another atom or group of atoms. Free radical substitution for A' level purposes involves breaking a carbon-hydrogen bond in alkanes such as methane CH4 ethane CH3CH3 propane CH3CH2CH3 A new bond is then formed to something else. It also happens in alkyl groups like methyl, ethyl (and so on) wherever these appear in more complicated molecules. methyl CH3 ethyl CH3CH2 For example, ethanoic acid is CH3COOH and contains a methyl group. The carbon-hydrogen bonds in the methyl group behave just like those in methane, and can be broken and replaced by something else in the same way. A simple example of substitution is the reaction between methane and chlorine in the presence of UV light (or sunlight). CH4  +  Cl2CH3Cl  +  HCl Notice that one of the hydrogen atoms in the methane has been replaced by a chlorine atom. That's substitution. Free radical reactions Free radicals are atoms or groups of atoms which have a single unpaired electron. A free radical substitution reaction is one involving these radicals. Free radicals are formed if a bond splits evenly - each atom getting one of the two electrons. The name given to this is homolytic fission.
	Note:  If a bond were to split unevenly (one atom getting both electrons, and the other none), ions would be formed. The atom that got both electrons would become negatively charged, while the other one would become positive. This is called heterolytic fission. Warning!  It is important that you get these terms the right way round. "Fission" is obvious - it just means "splitting". "Homo" and "hetero" are used in the sense of "same" (homo) or "different" (hetero). This is just like their use in "homosexual" or "heterosexual". So, homolytic fission is splitting a bond to produce two particles which are the same in the sense that they both have a single unpaired electron (both are free radicals). Heterolytic fission produces two particles which are different because one is a positive ion and the other a negative ion.
To show that a species (either an atom or a group of atoms) is a free radical, the symbol is written with a dot attached to show the unpaired electron. For example: a chlorine radical Cl a methyl radical CH3
	Note:  If you wanted to be fussy, the dot showing the electron really ought to be written next to the carbon atom in the methyl radical, because that's the atom with the unpaired electron - in other words as CH3. This isn't normally done unless the radical gets more complicated. Examples of this will crop up when you look at the mechanisms.