Magpie
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bromobenzene
I didn't see a thread for bromobenzene so I started this.
Using homemade benzene and homemade bromine I prepared bromobenzene today using an Fe catalyst. I set up a 250 mL RBF with reflux condenser and an
HBr absorber. The absorber was of the inverted funnel configuration (see attached photo).
I first added 21mL benzene, then 8mL bromine. I then added about 7cm of 20 guage annealed black wire (mechanic's wire) chopped into pieces about
3-4mm in length.
The reaction started within 30 seconds at room temperature as evidenced by the bubbling of HBr. Although I was prepared to stir via magnetic stirrer
and/or heat by bunsen burner I never needed either of these. In fact the reaction had to be cooled with a cold water bath + some ice to keep
excessive bromine out of the reflux condenser. The reaction was finished in 15-20 minutes. I then heated the flask at 50C in a water bath for
another 10 minutes.
I haven't isolated the bromobenzene yet. I'll report the yield when available.
I don't know how sensitive this synthesis is to the presence of water. My old school procedure did give steps to dry the RBF first so I assumed it
must be somewhat important. This is also why I took the effort to decant my bromine off the ice in the bottle and then dry it by washing with con.
sulfuric acid.
[Edited on 15-7-2006 by Magpie]
[Edited on 30-1-2007 by chemoleo]
The single most important condition for a successful synthesis is good mixing - Nicodem
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Elawr
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Congratulations on your successful synthesis! especially impressive considering your homemade precursors. Great job!! Any plans yet as to what you
might do with your bromobenzene?
1
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Magpie
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Thanks Elawr. Yes, I plan on a Grignard synthesis of some sort. Probably just good old benzoic acid. I'm mostly just proving out my techniques
here.
I've worked enough with bromine now that it no longer scares the piss out of me.  I still don't like storing it, however, so used up my whole supply, as planned.
FYI, I downsized my photo so it would post (as edited in the post above).
I should add that I conducted all bromine handling and experiments in a hood.
The single most important condition for a successful synthesis is good mixing - Nicodem
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The_Davster
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Did not you make your benzene from a benzoic acid decarboxylation?
Circular synthesis! I love it!
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Magpie
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Rogue: But of course! The irony of this has not been lost on me either. 
The single most important condition for a successful synthesis is good mixing - Nicodem
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Magpie
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I did the workup on my crude bromobenzene today.
First it was steam distilled using live steam. I was supposed to first make this alkaline with NaOH solution but forgot. Then the organic layer was
separated from the water. In the bottom of the pot was some waxy looking solid that is likely p-dibromobenzene (~100 mg).
The crude organic was dried over CaCl2 then was to be distilled using a "long-necked flask," or, by fractional distillation. I jury-rigged a
long-necked flask by inserting my Claisen adapter between the pot and the distillation adapter. There was a lot of foam and I was glad to have the
long, open neck. About 5 mL of benzene came over first at 80-82C. Then the bromobenzene came over at 154-155C. Yield was 73% based on the limiting
reactant, bromine.
The single most important condition for a successful synthesis is good mixing - Nicodem
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JnPS
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Do you have an estimate of the mass of iron you added? I'm running a similar reaction using AlBr3/Br2 instead of Fe/Br2 and I've been having trouble
finding a ratio of catalyst to substrate. I would assume its fairly small and I only want a monobrominated product but I've seen 1/10 molar
equivalents in some references and up to equimolar amounts in others. My substrate is fairly deactivated (phthalic anhydride), but I doubt I would
need anywhere near an equimolar amount.
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