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Free-radical bromination of propane in the presence of chlorine

When the free-radical bromination of propane is carried out in the presence of chlorine, the bromination selectivity (ratio of 1-bromopropane to 2-bromopropane) is much lower than it is when chlorine is omitted from the reaction (In other words, chlorine changes the observed ratio of 1-bromopropane to 2-bromopropane). In fact, the bromination selectivity is similar to the chlorination selectivity. Explain.


Attempt at solution:

I know that 2-bromopropane is the major product in bromination (without the addition of chlorine), because it has the more subsituted carbocation intermediate. I also know that chlorination won't occur because the enthalpy for that reaction is around -10 kcal/mol. However, I'm not entirely sure as to how the chlorine lowers the ratio. I thought it was something along these lines: without the chlorine, 1-bromopropane is the least favorable product; in the presence of chlorine, a chlorinated product becomes the least favorable product, therefore moving the 1-bromopropane up to be the second least favorable product and thereby lowering the ratio.

But I don't think this accounts for a difference in the ratios. Can anyone explain, or give me a hint as to why this occurs?
 

Answers and Replies

127
0
Ooh I like this question. It really gets at whether or not you're thinking about the mechanism.

I'm not sure where you're going with that answer attempt. Both reactions are exothermic in the end, but you don't need to think about the stability of the products to answer the question.

Here's a hint: which part of free radical bromination makes it selective? How might the presence of chlorine mess this up? The same thing that makes bromination selective also makes chlorination non selective. Remember that all the reactants are in a big soup, and an intermediate from one reaction can bump into a reactant from a "seperate" reaction.

Write out the whole mechanism and think about it. If you haven't already, read up on the hammond postulate and its implications for free radical halogenation.
 
83
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An old thread, but I found it in a Google search so it deserves an answer for the sake of anyone else who might find it.

Bromine and chlorine will combine to form bromine monochloride. This compound has properties somewhere between Cl2 and Br2, and will brominate, rather than chlorinate hydrocarbons. (the chlorine preferentially attaches to the hydrogen to form HCl.) Since chlorine is less selective than bromine, bromine monochloride is thus also less selective than bromine.
 

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