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Topic: Workup after bromination of an alkene with trimethylphenylammonium tribromide  (Read 3902 times)

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Offline Babcock_Hall

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"Bromination of Alkenes. The application of pyridinium, quinolinium, or trimethylphenylammonium tribromide as brominating reagents is known but little used.1 Tetra-n-butylammonium tribromide allows easy double bond bromination under mild conditions and with very good yields (Table 1).2 For example, an equimolar amount of TBABr3 is slowly added to a solution of alkene in CHCl3 to give, after solvent evaporation, a residue which is washed with 5% aqueous Na2S2O3 solution and ethanol, and then filtered and recrystallized. Yields are generally in the range 85–95%."
Michel J. L. Fournier "Tetrabutylammonium Tribromide" DOI: 10.1002/047084289X.rt020

http://dx.doi.org/10.1002/047084289X.rt020

I am not sure how to picture what happens after the removal of chloroform.  I imagine that there is a solid and one washes with an aqueous solution of sodium thiosulfate.  Because an organic solvent is not added back, I don't think that this is supposed to be a liquid-liquid extraction.  Then does the alkene get dissolved in the ethanol?  What solid is removed by filtration?

EDT
I just found a paper from Can J. Chem. from the same author.  They brominated an alkyne and did not remove the chloroform until after washing with thiosulfate and then water.

mod edit tidy link to paper
« Last Edit: February 15, 2018, 01:39:40 PM by sjb »

Offline Babcock_Hall

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This is the Google translation of the article (from French) that discusses the bromination of alkynes, as opposed to alkenes:  "The chloroform solution is then
Wash twice with 20 ml of an aqueous solution containing 5% of sodium thiosulphate, then three times with 20 ml of water, and finally dried over sodium sulphate. The solvent is evaporated in vacuo and the residue is crystallized (MeOH) and recrystallized (MeOH). 1-2 dibromC2 alckne is obtained raw quantitatively, then with a yield of about 95% after purification"
Berthelot J, Fournier M 1986 Canadian Journal of Chemistry 64(3):603-607.
http://www.nrcresearchpress.com/doi/abs/10.1139/v86-097#.WoXOmWbMxg1

I have to wonder whether or not the intended alkene protocol in the opening post was also a standard liquid-liquid extraction.  A paper on the bromination of alkenes by Berthelot and Fournier in J Chem Ed (1986) 63 1011, uses water, then thiosulfate (2X), then water again in a liquid-liquid extraction sequence.
« Last Edit: February 15, 2018, 03:26:03 PM by Babcock_Hall »

Offline pgk

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The first one is just a washing, followed by filtration because there is only one liquid phase and notably, the ethanolic aqueous thiosullfate solution.
Purification occurs because TBABr3 reacts with Na2S2O3, forming minerals, which are water soluble.
 
Bu4N(+),[Br2.Br(-)] +  2Na2S2O3 → Bu4N(+),Br(-) + Na2S4O6  + 2NaBr

On the other hand, the unreacted alkene (if any) and the formed TEAB are soluble in ethanol.
Obviously, this technique can be applied only when solid and/or ethanol-insoluble dibromalkane derivatives are formed, e.g. bromination of stilbene.
In contrast, the other two procedures are extractions because they are two liquid phases, in which the product and the impurities are selectively dissolved, respectively and it can be applied when liquid bromalkanes are formed.
In addition, the other two procedures are "reactive" extractions because the main impurity (excess of TBABr3) reacts with the aqueous solute (Na2S2O3).

Berthelot and Fournier: J. Chem. Education, (1986), 63, 1011, can also be found in the link, below:
http://digicoll.library.wisc.edu/cgi-bin/JCE/JCE-idx?type=article&did=JCE.JCE06311.i0058&id=JCE.JCE06311&isize=XL

« Last Edit: February 15, 2018, 03:12:36 PM by pgk »

Offline opsomath

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IMO, the original procedure is written poorly enough to be wrong. Any time I have halogenated, I have washed with reducing agent aq. soln. while the product is still in the organic phase. It does not make sense to do it after rotovap, since free nonpolar halogen will be trapped in the very nonpolar product grains. Then, ethanol is usually a good solvent for recrystallization of bromides or dibromides, but this is not clear from the procedure. The J. Chem Ed procedure above is a very good example!

IIRC, our prep for aromatic bromination (EAS) can also be used for alkenes.

http://cssp.chemspider.com/753


Offline wildfyr

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I always love those solid state reaction shortcuts. How general have you found that mortar and pestle reaction to be? Does it work with other sodium halide salts? Rings that are less activated than double alkoxy?

Offline OrganicDan96

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I have found many purification procedures don't work particularly well in practice. you could wash the chloroform with sodium thiosulfate  then recrystalize the crude solid with ethanol or even column it if possible.

Offline Babcock_Hall

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We washed with water, thiosulfate, and water.  We ran a silica column using 30% ethyl acetate and hexanes.  For the most part, we are satisfied with this protocol, but there is one peak at 3.75 ppm in the H-1 NMR that we did not identify.

Offline opsomath

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I always love those solid state reaction shortcuts. How general have you found that mortar and pestle reaction to be? Does it work with other sodium halide salts? Rings that are less activated than double alkoxy?

Surprisingly, p-alkoxy is harder than some monosubstituted systems, especially for iodine. Iodine is a "surprisingly" sterically demanding group. Here are a couple of other typical preps using mechanochemistry. We never had any luck with electron-deficient rings, even with NBS or other more aggressive brominating agents. For the ones which were a little more sluggish, adding tetrabutylammonium iodide or cerium ammonium nitrate sometimes would work.

Here's some representative procedures also on CSSP.

http://cssp.chemspider.com/Article.aspx?id=791
http://cssp.chemspider.com/Article.aspx?id=789

Offline Babcock_Hall

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IMO, the original procedure is written poorly enough to be wrong. Any time I have halogenated, I have washed with reducing agent aq. soln. while the product is still in the organic phase. It does not make sense to do it after rotovap, since free nonpolar halogen will be trapped in the very nonpolar product grains. Then, ethanol is usually a good solvent for recrystallization of bromides or dibromides, but this is not clear from the procedure. The J. Chem Ed procedure above is a very good example!

IIRC, our prep for aromatic bromination (EAS) can also be used for alkenes.

http://cssp.chemspider.com/753
Thank you.  My present belief is that the original procedure was written incorrectly.

Offline pgk

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The peak at 3.75 ppm in the 1H-NMR indicates an alkoxy group, which can be explained when using methanol or ethanol in washing/recrystallization, as described in the original procedure (formation of a vic-alkoxy-bromoalkane in presence of remaining TBABr3). But it cannot be explained if washing with water/thiosulfate/water only, as you did.
« Last Edit: March 06, 2018, 12:51:22 PM by pgk »

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