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#1 2011-10-03 14:00:10


Why are bromination reactions so selective?

I believe to understand selectivity of this reaction is to understand the molecular energy levels. Let me draw an analogy. Let us say you have ten different nuts and you wish to crack them to recover their meats. If you placed them on an anvil and hit them with a 16 lb sledge hammer, you would succeed in crushing every one of them and the meat inside. The 16 lb hammer would have too much energy to just break the shell of the nut. If you hit each of them with a much lighter tack hammer, you could break some, but not others. The difference is the tack hammer would not possess enough energy to break the shells of the more resistant nuts.

The energy of halogen radicals can differ significantly and the more energy a radical possesses, the less selective the halogenation. For example, fluorine is a very reactive gas. The fluorine radical is very reactive (high reaction energy) and can form bonds to replace virtually any hydrogen. A chlorine radical is less reactive. It will react more quickly with C-H bonds that result in more stable radicals than with those that result in less stable radicals. However, because it has more than enough energy to react with methane to give a methyl radical, it can also react with methyl groups of alkanes. The rate of reaction is not as fast as that of a secondary or tertiary radical because they take less energy to remove. I would liken this to either sometimes the reactions fails to take place or the intermediate falls back to the starting materials.

Bromine is less reactive still. A bromine radical cannot overwhelm a methyl group. It is only just sufficient to form a primary radical, therefore it is much slower. A secondary radical is more stable and the rate increases. A tertiary radical is the most stable and thus a bromine radical is best able to form a tertiary radical.

Iodine can also form radicals, but the iodine radical is more stable still. It does not possess sufficient energy to break any of the C-H bonds of alkanes.



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