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Author: Subject: brominating agents
devongrrl
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[*] posted on 8-6-2010 at 10:33
brominating agents


Would HBr be suitable as a brominating agent if elemental bromine wasn't available ?
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rrkss
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[*] posted on 8-6-2010 at 11:04


What do you want to brominate?
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Nicodem
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[*] posted on 8-6-2010 at 11:51


Please do not post questions that can not be answered. What is the point of doing so? Rather take the a minute and explain, what kind of bromination (electrophilic, nucleophilic, radical), of which substrate, in the presence of what (oxidants, acids...), and so on. Surely you don't expect that it is possible to reasonably answer your question as it is now?

PS: Open referenceless threads in the Beginnings section! Moving there...
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devongrrl
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[*] posted on 9-6-2010 at 00:23


Ack, sorry for the dumb question ?

I was thinking in general but say for example
aromatic sustitution of benzene
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JohnWW
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[*] posted on 9-6-2010 at 01:59


The stuff adds across the double bonds of alkenes according to the Markownikoff rule. Also, it reacts with -OH groups to replace them with -Br, although this tends to be an equilibrium reaction..
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[*] posted on 9-6-2010 at 02:55


Quote: Originally posted by devongrrl  
I was thinking in general but say for example
aromatic sustitution of benzene

There can be no useful general answer, if you don't even define the topic. I'm again and again surprised at members who post questions without defining the topic. It always makes me wander if they really expect an answer or do they post just out of boredom?

Yes, of course you can use HBr for the electrophilic aromatic substitution on benzene. Obviously you need an oxidant to generate an electrophilic brominating species from HBr. Oxidants used can be hydrogen peroxide, various salts of peracids, oxygen (air) in the presence of nitrites or other catalysts, KBrO3, HNO3, etc. Benzene is poorly nucleophilic so the reaction does not always work well. Here are a few references of electrophilic bromination of benzene with HBr(aq) or HBr in the presence of oxidants:

Journal of the Chemical Society, Chemical Communications, 1987, 1421-1422. (Abstract: Quaternary ammonium salts function both as Lewis acids and as phase-transfer catalysts in the bromination of arom. compds. in the presence of hydrogen peroxide, as indicated by reaction rate measurements. Typical halogenation procedures are given.)

Tetrahedron, 65 (2009) 4429–4439 (H2O2 used)

Journal of Chemical Research, Synopses, 1998, 828-829. (using sodium perborate)

DE10026138 (with O2 as oxidant and RuCl3 catalyzed, see Examples 4 and further)

WO8807513 (gas phase bromination)




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