chemrox
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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.
"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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Drunkguy
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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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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]
…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)
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maniacscientist
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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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vulture
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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.
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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chemrox
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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]
"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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chemrox
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| 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.
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oh please... been drinking again?
"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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JohnWW
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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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| 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...
"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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contrived
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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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vulture
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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.
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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Magpie
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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.
The single most important condition for a successful synthesis is good mixing - Nicodem
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chemrox
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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.
"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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Magpie
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Chemrox, here's my story:
http://www.sciencemadness.org/talk/viewthread.php?tid=6229&a...
The single most important condition for a successful synthesis is good mixing - Nicodem
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Drunkguy
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Assuming one actually wanted 1,4-dibromobenzene though.
Does anybody have a procedure for this?
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syntelman
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Yes, you brominate bromobenzene.
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maniacscientist
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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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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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jon
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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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