quest
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The role of light in bromination of alkene
hello all,
I can't find the reason why we use light as a catalyst when doing bromination of alkene.
for the bromination we want to make the Br2 polar to make the double bond (of the alkene) to attack the positive side of the bromine.
as far as I know using strong light on bromine can make radicals that in turn will make a lot of unwanted by products.
I know heat is a catalyst too in this reaction, is it just to give enough energy for the reaction to occur (like many reactions) or is there a special
mechanism like with the light?
thanks.
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forgottenpassword
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Light of the appropriate wavelength breaks the Br-Br bond and ensures that the reaction proceeds through a radical mechanism. Heat can break the Br-Br
bond too but it will also increase the rate of a non-radical reaction pathway. Of course, you will get a mixture of reaction pathways happening in
either case but the product distribution can be affected favourably by the use, or absense of light.
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Nicodem
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Thread Moved
24-3-2014 at 09:15 |
Nicodem
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Quote: Originally posted by quest  | hello all,
I can't find the reason why we use light as a catalyst when doing bromination of alkene. |
Never heard of any such role of light. Light is used as a radical chain reaction initiator for the Wohl-Ziegler reaction (allylic and benzylic
halogenations), but never for the electrophilic additions of bromine on the alkenes and certainly never as a catalyst (how can light act as a catalyst
for a chemical reaction?).
Why would you ask such a question without providing the reference?
| Quote: | | I know heat is a catalyst too in this reaction, is it just to give enough energy for the reaction to occur (like many reactions) or is there a special
mechanism like with the light? |
Heat acting as a catalyst? You should read about what the word catalyst means.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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quest
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Nicodem, you are right, heat isn't a catalyst - it just accelerates the reaction. bad choose of words.
As for the reference, I will be glad to provide it, but it is part of an experiment in "organic lab" in my university and the lab protocol isn't in
English.
One of the lab guides gave as a "hint" that the light help to form HBr, and HBr acts as a catalyst. is that sound more logical in this reaction?
thanks
[Edited on 24-3-2014 by quest]
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Metacelsus
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The reaction is an electrophilic addition. The mechanism does not proceed by a radical pathway.
https://en.wikipedia.org/wiki/Halogen_addition_reaction
Allylic bromination does, but must be done with NBS, not bromine (to avoid addition).
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Nicodem
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| Quote: Originally posted by quest | | As for the reference, I will be glad to provide it, but it is part of an experiment in "organic lab" in my university and the lab protocol isn't in
English. |
Is it so difficult to translate the relevant part? Currently, we know nothing about what it says, we only have your interpretation of what it says and
even that is not clear to me. For example, you don't say what the reaction is, what the reagent is, what the substrate is, or what the product is. I
currently don't have the time for playing guessing games.
| Quote: | | One of the lab guides gave as a "hint" that the light help to form HBr, and HBr acts as a catalyst. is that sound more logical in this reaction?
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No, not really logical.
If the reaction was the radical addition of HBr on an olefinic double bond (another radical chain reaction), then light can act as a radical
initiator, just like it does in the Wohl-Ziegler reaction. Yet, in such an addition HBr is the reagent, not the catalyst. But enough guesswork, tell
us what reaction you talk about.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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