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#1 2010-05-07 13:59:05


Effect of increasing methane concentration in radical chlorination

Yahoo!Answers in chemistry wrote:

In the halogenation of methane, if we increase the concentraion of methane then the formation of should increase. However, should not the concentration of the ethane increase because its more stable? (1st step of chain propagation)

I agree with part of this thinking about increasing methane concentration increases the amount of chloromethane. It will also increase the amount of ethane that forms, but that will only be a minor effect at best.

How do we know that? Let us assume that the ratio of moles forming product in the propagation reaction is 9x that of termination steps (1x). Therefore, 10% of the products forming would occur from termination steps.

In the reaction, ethane can only form from two CH3 radicals combining in a termination step. However, the termination can occur with methyl radicals to ethane or chloro radicals to chloromethane. Initiation forms two moles of chloro radicals. A chloro radical can form a methyl radical or be involved in a termination step. Therefore, formation of a methyl radical also forms a chloro radical. Therefore, one might expect that termination would be approximately equally likely to form the chloromethane or ethane. However, that would only occur to 10% of the product or at most 5% ethane.

If the propagation/termination ratio is much greater than 9/1, then ethane will smaller still.

Increasing the methane concentration or decreasing the chlorine concentration will decrease the propagation/termination ratio. However, it will also slow the reaction. The rate of the reaction will depend on the formation of radicals from chlorine, either propagation or initiation. Should they disappear, the reaction will stop. Therefore, while these conditions can increase the amount of ethane formed, it would be difficult to do so without simultaneously increasing the chloromethane to a greater extent.


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