Mechanism of chlorination. Free radicals

It will be worthwhile to examine the mechanism of chlorination of methane in some detail. The same mechanism holds for bromination as well as chlorination, and for other alkanes as well as methane it even holds for many compounds which, while not alkanes, contain alkane-like portions in their molecules. Closely 

Among the facts that must be accounted for are these (a) Methane and chlorine do not react in th dark at room temperature, (b) Reaction takes place readily, however, in the dark at temperatures over 250°, or (c) under the influence of ultraviolet light at room temperature, (d) When the reaction is induced by light, many (several thousand) molecules of methyl chloride are obtained for each photon of light that is absorbed by the system, (e) The presence of a small amount of oxygen slows down the reaction for a period of time, after which the reaction proceeds normally the length of this period depends upon how much oxygen is present. 

The mechanism that accounts for these facts most satisfactorily, and hence is generally accepted, is shown in the following equation  

The first step is the breaking of a chlorine molecule into two chlorine atoms. Like the breaking of any bond, this requires energy, the bond dissociation energy, and ill Table 1.2 (p. 21) we find that in this case the value is 58 kcal/mole. The energy is supplied as either heat or light. 

The chlorine molecule undergoes homolysis (Sec. 1.14) that is, cleavage of the chlorine -chlorine bond takes place in a symmetrical way, so that each atom retains one electron of the pair that formed the covalent bond. This odd electron is not paired as are all the other electrons of the chlorine atom that is, it does not have a partner of opposite spin (Sec. 1.6). An atom or group of atoms possessing an odd (unpaired) electron is called a free radical. In writing the symbol for a free radical, we generally include a dot to represent the odd electron just as we include a plus or minus sign in the symbol of an ion. 

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