Jor
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black byproduct tribromoaniline synth.
Today I started the synthesis of 2,4,6-tribromoaniline. I used this procedure:
http://www.versuchschemie.de/topic,10463,-2%2C4%2C6-Tribroma...
I used the exact same amounts, but still there was a slight excess of bromine, as after all Br2 and Br2(aq) was added, there was still a orange
solution above the precitipate.
I filtered the mixture, washed the precitipate once with dH2O, twice with sat. NaHCO3 solution, and one more time with water.
Next the precitpate, containing some black lumps was suspended in 30mL of MeOH, briefly heated to boiling it is now left to stand overnight. Tomorrow,
hopefully all the black stuff will have dissolved.
Now I was wondering, what is this dirty black byproduct? Is it some kind of polymer? And if yes, via what mechanism is it formed, and why does it
dissolve in MeOH while the tribromoaniline does not?
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not_important
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The tribromoaniline is soluble in the common alcohols, SFAIK. For example this prep http://scripts.iucr.org/cgi-bin/paper?S0567740869002780 recrystallises from EtOH, as does the one
herehttp://unesco-nigeriatve.org/download/instructional_materials/nd%20slt/Semesters/Semester%204/STC%20221%20Org%20Chem%20II%20Final-P/Organic%20%20I
I%20Practical%2028%20Nov/STC%20221%20PRAT.pdf
The black stuff is likely polymeric/condensation products, including those formed by any oxidation products in the aniline. Quinones and semiquinones
often given condensation products under aqueous acid conditions. Some procedures appear to give less crude than others, generally they seem to
dissolve the crude product in alcohol, add charcoal, filter, and crystallise.
BTW - the following may be of interest
Oxidative bromination of aniline and its derivatives
http://www.springerlink.com/content/6277q055511j7278/
[Edited on 16-1-2011 by not_important]
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Nicodem
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That "dirty black byproduct" is not necessarily a compound, but could be a complex mixture of compounds (aka crap). Anilines can be N-brominated and
the resulting products can decompose via several paths (aniline oxidation products, e.g. aniline black type of mixtures). Try doing a TLC of that
black stuff to see if it is a compound or a mixture of compounds. Most likely you will get a strong starting spot that refuses to move in any mobile
phase and a plethora of spots above (this is typical TLC behaviour for what we scientifically call tarry products - also called crap in everyday lab
parlance).
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Picric-A
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I once tryed producing 2,4,6-trichloroaniline by passing chlorine through a solution of aniline hydrochloride in water. All i got was a black tar,
impossible to remove from the glassware and literally stained everything it touched....
Will never try that one again!
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Jor
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I added the bromine as a solution in 1,5M KBr. As long as this solution was added, no black tar formed. When the leftovers pure bromine (wich did not
dissolve) were added, black tar formed.
I don't know why the author states that the black tar can be removed with MeOH. Anyway, I think I will boil the solution and add EtOH untill all
tribromoaniline has dissolved, followed by hot decantation (or filtration) and cooling down to crystallise the tribromoaniline.
Yes Not_important, I also found a book after my post that states it can be recrystallised from MeOH. I will now start adding EtOH, as this is less
expensive, and I need less probably.
Nicodem, i don't have TLC plates, but I will soon get some. By the way, are light petroleum ethers essential for performing TLCs, or are solvents such
as ethyl acetate, ether and dichloromethane also fine? I know almost everytime I see an TLC being done, hexanes are used.
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Picric-A
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Quote: Originally posted by Jor  | | Nicodem, i don't have TLC plates, but I will soon get some. By the way, are light petroleum ethers essential for performing TLCs, or are solvents such
as ethyl acetate, ether and dichloromethane also fine? I know almost everytime I see an TLC being done, hexanes are used. |
With the TLC eluent, a polar and non polar solvent mix is normally used to control the rate the solvent moves up the plate and ensure proper
sepration. pet ether is only normally used as the non poalr solvnt becuase it is cheap nd very non polar. Other non polar solvents can substitute it
with no problem, its just a cas of finding the optimum solvent mix for your sample, similar to recrystalisation.
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