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Author: Subject: bromination of phenyl group
chemrox
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[*] posted on 18-12-2007 at 20:49
bromination of phenyl group


I assume the bromination of benzene et al. with Br2 and a catlytic amount of Lewis acid is irreversible but I don't see why. It's important because if it's not reversible the evolved HBr can be captured in water to make the mineral acid. If it were reversible one wouldn't want to push on the reactants side of the equation by making back pressure of any amount.



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Drunkguy
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[*] posted on 19-12-2007 at 09:36


The bromination of benzene is typically done with Br2 and pyridine ala Vogel textbook of organic chemistry. However the cost of just buying yourself some makes it shady why anybody would go to the trouble of DIY chemistry for this simple compound.

It's not reversible.
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Nicodem
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[*] posted on 19-12-2007 at 09:50


Actually it is kind of reversible when catalyzed by Lewis acids. The problem is that the equilibrium constant is so high, that the reversibility has no role in the product constitution. Unlike sulfonation, acylations or alkylations of aromatics, which are highly reversible thus giving high para selectivity if enough time/heat is given to the reaction, electrophilic substitutions with halogens do not allow for such thermodynamic control. Though there are exceptions and there actually exist cases where the reversibility of aromatic compounds bromination can be used for preparative purposes (for example, see the patent Eclectic provided in 2-bromonaphthalene from naphthalene thread).

Edit: Otherwise, reading again Chemrox's question, I must admit I do not understand what is being asked so maybe this is completely off topic.

[Edited on 19/12/2007 by Nicodem]




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maniacscientist
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[*] posted on 19-12-2007 at 14:04


It´s a radical reaction mechanism, one wouldn´t need a Lewis acid for bromination of the ring or the OH group, although both might and will probably happen.though I don´t know about Bromobenzene synths... gimme more...
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[*] posted on 19-12-2007 at 14:25


The bromination of benzene or aromatic bromination is not a radical mechanism. Look up electrophilic substitution. I don't understand the point of your post anyway, maniacscientist.



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chemrox
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[*] posted on 19-12-2007 at 14:28
mechanism & another related ask


It's not a radical reaction its an electrophilic substitution. Vogel's use of pyridine isn't necessary. With Fe you probably get insitu generation of FeBr3 that then makes a complex [ph-Br]+[FeBr4]- -> ph-Br + HBr + FeBr3

@Nicodem: thanks, very interesting and answered my question. The foirmation of dibrom products is a sideraction that requires workup. I've seen steam distillation recommended and I don't like the idea vey much. It would mean distilling twice and a lot of water removal. Why not simply do a fractional distillation? Do you see any reasons for steam and not frax first time through instead?

[Edited on 19-12-2007 by chemrox]




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chemrox
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[*] posted on 19-12-2007 at 14:30


Quote:
Originally posted by Drunkguy
However the cost of just buying yourself some makes it shady why anybody would go to the trouble of DIY chemistry for this simple compound.


oh please... been drinking again?




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JohnWW
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[*] posted on 19-12-2007 at 14:31


It would be different if it were to be done under UV light, which creates Br• atoms (free radicals), and similarly if chlorine were to be used instead of bromine.
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chemrox
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[*] posted on 19-12-2007 at 14:43


Quote:
Originally posted by JohnWW
It would be different if it were to be done under UV light, which creates Br• atoms (free radicals), and similarly if chlorine were to be used instead of bromine.


I quite agree-different mechanism. I don't know how to set that one up although I'd like to and have no idea what the yields would be like. I guess one would have to be very careful about reaction terminating impurities...what kind of light source for example and for what length of time? Until the color goes clear I would suppose. And then there's the worry about exothermic overrun. I get bored and leave the room and whooo haa! Bromine and benzene, two nasties, all over my lab...




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[*] posted on 19-12-2007 at 14:47


pull the monochromator out of the spectophotometer we're not using and try that as a light source .. I'm intrigued. We can have Doug sit on it with a book or something. Who the f**k is Drunkguy anyway? Ready for coffee?
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[*] posted on 19-12-2007 at 14:47


You can't brominate an aromatic position radically. You can however do that with a benzyl carbon, but that's a very different reaction, etc.

And let's keep this civil before I pull the plug on this one.




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Magpie
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[*] posted on 19-12-2007 at 15:36


I've made bromobenzene using Br2 and Fe. IIRC the reaction was very smooth with good yield. I believe I described this on the forum in another thread.



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chemrox
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[*] posted on 19-12-2007 at 18:52


Yes, you're right about keeping it civil. Magpie's experience same as mine about the bromination going smoothly. @Magpie, what about the workup? How did you isolate the mono-Br product? BTW I made several liters when I couldn't get any locally and needed it post haste. I used fractional distillation but also had some problems after the Gignard reaction with excess elemental Bromine in the final product .. I think steam distillation might have picked off the Br and dumped it in the aqueous phase but I didn't want to do it although it was recommended.



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[*] posted on 19-12-2007 at 19:19


Chemrox, here's my story:

http://www.sciencemadness.org/talk/viewthread.php?tid=6229&a...




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Drunkguy
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[*] posted on 20-12-2007 at 09:46


Assuming one actually wanted 1,4-dibromobenzene though.

Does anybody have a procedure for this?
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[*] posted on 20-12-2007 at 09:59


Yes, you brominate bromobenzene.
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maniacscientist
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[*] posted on 20-12-2007 at 10:19


Right, with the catalyst e.g. FeCl3 you get heterolytic ions of Bromine and the catalyst, the catalyst being regenerated by a second bromine-ion yielding HBr anf Br2 again and so forth... with radical substitution one would yield most like a hexa-substituted cyclohexane. reationconditions for the former: cold, catalyst, core, the latter heat/light, sidechain/substitution
Quote:
Originally posted by chemrox
It's not a radical reaction its an electrophilic substitution. Vogel's use of pyridine isn't necessary. With Fe you probably get insitu generation of FeBr3 that then makes a complex [ph-Br]+[FeBr4]- -> ph-Br + HBr + FeBr3

@Nicodem: thanks, very interesting and answered my question. The foirmation of dibrom products is a sideraction that requires workup. I've seen steam distillation recommended and I don't like the idea vey much. It would mean distilling twice and a lot of water removal. Why not simply do a fractional distillation? Do you see any reasons for steam and not frax first time through instead?

[Edited on 19-12-2007 by chemrox]


[Edited on 20-12-2007 by maniacscientist]
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JohnWW
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[*] posted on 20-12-2007 at 18:51


Irradiation of a stoichiometric mixture of benzene vapor and chlorine by UV light (which produces Cl• radical atoms) produces a mixture of isomeric (and in one case enantiomeric) 1,2,3,4,5,6-hexachlorocyclohexanes, C6H6Cl6. The principal of these products is lindane (having the 1,2,3 chlorines equatorial, 4,5,6 chlorines axial), used as an highly potent insecticide, formerly used against mosquitos, grass grubs, Colorado beetles, caterpillars, etc. along with DDT (but now banned except for use on untreated timber against borer and termites). No HCl is produced. The free-radical bromination of benzene should follow similarly, although with more difficulty due to steric factors.
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[*] posted on 21-12-2007 at 17:02


I've always heard you did'nt want uv light of any kind in benzene mono-bromination you would wind up with the hexabromide since it changes the mechanism from se2 to free radical

[Edited on 22-12-2007 by jon]
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