Propagation steps methane monochlorination

You should already be familiar with the mechanism for the thermal chlorination of methane. We will use Figure 1.17 to review briefly the net equation, the initiation step, and the propagation steps of the monochlorination of methane. Figure 1.18 shows the energy profile of the propagation steps of this reaction. 

Fig. 1.18. Energy profile of the propagation steps of the monochlorination of methane with Cl, (enthalpies in kcal/mol).

In the reaction of a 1 1 mixture of methane and chlorine one does not obtain the monochlorination product selectively, but a 46 23 21 9 1 mixture of unreacted methane, mono-, di-, tri-, and tetrachloromethane. Thus, all conceivable multiple chlorination products are also produced. Multiple chlorinations, like monochlorinations, occur as radical chain substitutions. They are based on completely analogous propagation steps (Figure 1.19). 

According to Figure 1.20, analogous propagation steps possess the same heat of reaction, independent of the degree of chlorination. With the help of Hammond s postulate, one concludes from this that the associated free activation energies should also be independent of the degree of chlorination. This means that the monochlorination of methane and each of the sub-... 

Fig. 1.20. Energy profile of the propagation steps of the poly-chlorinations CH3Cl - CH2Cl2, CH2Cl, -> CHCl3, and CHCl3 - CCl4 of methane (n = 1-3 in the diagram), and of the monochlorination CH4 -> CHjCl (n = 0 in the diagram).

A possible alternative mechanism to that shown in Problem 24 for the monochlorination of methane would involve the following propagation steps ... 

This reaction proceeds by a radical chain mechanism analogons to the one observed for methane. As in methane, all of the hydrogen atoms in ethane are indistingnishable from one another. Therefore, we observe only one monochlorination prodnct, chloroethane, regardless of which hydrogen is initially abstracted by chlorine in the first propagation step. 

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