Panache
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Registered: 18-10-2007
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Mood: Instead of being my deliverance, she had a resemblance to a Kat named Frankenstein
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Purifying toluene and drying ethyl iodide/iodoethane
Toluene can be a pain to purify. The presence of sulphonated compounds,such as methyl thiophenes, bp 112-113, is common, especially in solvent grades.
HPLC grades and the like have these cleared out but not all have access nor the inclination towards the higher cost. Ordinarily these impurities pose
little real problem. An exception however is if one is trying to sodium dry the toluene. it turns turbid and bright orange and gives one little
confidence in the solvent/reagant.
Vogel advocates thrice treating the toluene with 10% of its weight in conc H2SO4. I have tried this and its not a method one would turn to readily,
especially given the chance of sulphonating your ring.
Anyhow, quite by chance i have discovered another method for removing the S compounds. It is analogous to the method for distilling ethanol, under
high vacuum, such that the azetrope with water is broken (if anyone hasn't tried this, it works very well). My wiz-bang electronic vacuum gauge never
works reliably so i prefer to quote my vacuum readings via a bp of the solvent. If the toluene is distilled at full vacuum on my system, it comes
across the stillhead over a very narrow range within a degree at -9C. This toluene appeared to me quite sparkling clear, compared with the solvent i
use (which i normally distill once at STP).
On a hunch i dropped some sodium into the toluene (it was quite dry already) and got no orange turbidity, the sulphur cmps had not distilled over,
analagous i guess to the ethanol (which distills at ~-20C if i recall) distilling dry at high vac.
Any physical chemistry devotees may want to explain this observation, i know there is one.
On a second note i had a small portion of ethyl iodide which i intended to simply store. After drying it overnight over CaCl2, i stored it in my
ultralow fridge, bottom shelf, so around ~-90C. A few days later the ethyl iodide had developed a turbidity and upon inspection i could see ice
crystals in the reagant, i filtered these and the solution cleared. After checking references the only method advocated for drying alkyl halides in
CaCl2 or Na2SO4 but this obviously wasn't sufficient. Can anyone shed any light on this or perhaps recommend a effective way to dry this reagant.
And yes i realise i should probably try distilling it at high vacuum, lol.
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not_important
International Hazard
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Registered: 21-7-2006
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Both CaCl2 and Na2SO4 are best used as preliminary drying agents, magnesium or calcium sulfate do a better job. For liquids with very low freezing
points intense cooling does work, so by accident you picked another useful drying method.
See this chart for comparisons
http://www.mallbaker.com/techlib/documents/americas/3045.htm...
or the shorter chart near the end of this PDF
http://ciencies.udg.es/ciencies/CienciaSenseFronteres/Extrac...
or the not so well laid out guide attached.
[Edited on 29-5-2009 by not_important]
Attachment: Desiccant Guide.pdf (552kB)
This file has been downloaded 1084 times
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Fleaker
International Hazard
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Toluene is easy enough to dry with azeotropic distillation. After that, sodium treatment and careful fractionation then storage over molecular sieve.
As far as your EtI, sodium sulfate works fine for bulk removal of water. Sieve can also be added. Consult Amarego's text on purification of laboratory
chemicals in the references section.
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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panziandi
Hazard to Others
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Registered: 3-10-2006
Location: UK
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Mood: Bored
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Panache (I always giggle when I see your name since it reminds me of a nightclub in Bristol) anyway... Very nice idea of distilling at a lower
pressure. I'll consider this when I run out of sulphur-free toluene. As to the EtI, drying it overnight with CaCl2 is what I have done before
performing a Grignard etc. Certainly CaSO4 or MgSO4 would be better because they can hold more water, but CaCl2 is good since it removes any entrained
ethanol also. Could you try Molecular Sieves? I'm very impressed with your freezer and the idea of freezing out water in that is quite promising.
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