Filemon
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hydrobromination alpha of ketone alpha,beta insaturate
It may hydrobromize the double bond in alpha of ketone alpha, beta insaturate with peroxide? I believe that it may but I have not found any document
it confirms.
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JohnWW
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I suggest that you look up the Markownikoff addition rule for addition of HF, HCl, HBr, and HI across double bonds in alkenes, in textbooks of organic
chemistry, of which I have uploaded many (see the References section).
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Filemon
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It is not so simple. In this case the Markownikoff rule doesn't work. A ketone alpha, beta insaturate hydrobromine in beta.
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JohnWW
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| Quote: | Originally posted by Filemon
It is not so simple. In this case the Markownikoff rule doesn't work. A ketone alpha, beta insaturate hydrobromine in beta. |
In that case, the Anti-Markownikoff Rule, if other existing substituents on the alkene dictate otherwise.
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chemrox
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Might be that carbonyl provides the minute amount of O to make enough peroxide to force anti-Markovnikoff orinetation?? Is there a solvent effect?
Does it matter whether solvent is protic? (I think I'm understanding but I could be way off the question too..) I'd like to see the diagrams and do
the curved arrows..
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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Filemon
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No, because the presence of the ketone it produces the nucleophilic is addition in beta.
http://en.wikipedia.org/wiki/Conjugate_addition
[Edited on 7-12-2007 by Filemon]
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chemrox
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OIC its Sn2 to the nucleophillic C beta to the carbonyl C due to the pi bond. I'm sorry there's a bit of a language barrier here and I sometimes
need the stick figures to see what's going on. I should have drawn my own from what I discerned; shame on me. What ws your native tongue? I
remember you told us but I forgot.
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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Nicodem
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| Quote: | Originally posted by Filemon
It may hydrobromize the double bond in alpha of ketone alpha, beta insaturate with peroxide? I believe that it may but I have not found any document
it confirms. |
I wish you would at least try to make your questions comprehensible. 
I can only guess you want to know which position would the bromine radical add on an enone system?
The hydrobromination of alkenes in the presence of radical initiators proceeds by a radical chain mechanism in which Br° adds to the double bond in
such a position as to form the most stabile C-radical possible. For example, the radical addition of HBr on isobutylene should give mostly
isobutyl bromide and only little or no tert-butyl bromide. This is due to the intermediate
Me<sub>2</sub>C°-CH<sub>2</sub>Br being more stabile than the other possible C-radical,
Me<sub>2</sub>CBr-CH<sub>2</sub>° (see various tables for radicals thermodynamic stability).
In your substrate choice and its radical induced hydrobromination, the addition of Br on the enone position 4 should be prevalent due to the
stabilizing effect of the carbonyl group alpha to the radical site (resonance stabilization). However, the ratio of the regioisomeric products depends
on the groups located at the position 4 of the enone. If these groups are able to stabilize the radical more than the carbonyl group is able to, then
you would get a different ratio of the two regioisomers. For example, in the case of Ph<sub>2</sub>C=CH-CO-Me you can expect the main
product would be Ph<sub>2</sub>CH-CHBr-CO-Me since two phenyl groups stabilize the radical intermediate more than a carbonyl group would.
Many complications can arise with such substrates and only a literature search can tell you if enones even react in radical addition reactions with
HBr.
See also: http://en.wikipedia.org/wiki/Radical_addition
[Edited on 7/12/2007 by Nicodem]
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Filemon
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| Quote: | Originally posted by chemrox
shame on me. What ws your native tongue? I remember you told us but I forgot. |
I didn't say it. My language is Spanish.
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Filemon
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Thank you. Now I have understood that it can make. But it is necessary to choose a peroxide that it doesn't react with the ketone (Baeyer-Villeger
oxidation).
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