Hilski
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Aldehyde distillation/purification
I have several hundred ml of an orange mixture of benzaldehyde, toluene and a bit of whatever else may have been extracted with the aldehyde after
several reactions involving the oxidation of toluene. I can distill off the toluene at atmospheric pressure with no problem, but I assume the
benzaldehyde will pretty much be ruined if I try to distill it without vacuum.
I currently do not have the means to do vacuum distillations (no pump, proper vac gague or adapter etc) and I was wondering what else I could do to
clean up the benzaldehyde. I was thinking I might try to make the bisulfite addition product after evaporating the toluene, and then steam distill
the mixture. But I don't know if that would work, since the benzaldehyde would basically still be 'crude product'. I think I read somewhere that in
order for a bisulfite addition product to precipitate, that the aldehyde (or ketone) has to be relatively pure already. Does anyone know if this is
the case?
Anyone have any other suggestions?
Thanks
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Nicodem
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| Quote: | Originally posted by Hilski
I think I read somewhere that in order for a bisulfite addition product to precipitate, that the aldehyde (or ketone) has to be relatively pure
already. |
You read wrong. It just needs to be neutral (devoid of acids or bases). And there is no point in evaporating off the toluene as you would be unable to
do so without evaporating a portion of benzaldehyde as well, not to mention its atmospheric oxidation.
PS: Get yourself an aspirator and a couple of glass joints for a vacuum distillation. At least a basic setup. You can't do simple organic chemistry
without it.
…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)
Read the The ScienceMadness Guidelines!
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Hilski
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| Quote: | Originally posted by Nicodem
You read wrong. It just needs to be neutral (devoid of acids or bases). And there is no point in evaporating off the toluene as you would be unable to
do so without evaporating a portion of benzaldehyde as well, not to mention its atmospheric oxidation.
PS: Get yourself an aspirator and a couple of glass joints for a vacuum distillation. At least a basic setup. You can't do simple organic chemistry
without it. |
Well, I must say I'm glad I was was wrong.
I've just been procrastinating about getting the vacuum adapters and stuff. I guess now I have a good excuse to get em'. But for the moment, I
think I will try to make the addition product and see how it goes.
Thanks.
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Mush
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I hardly find any literature data on aldehyde/ketone purification. Comparison of purity before bisulfite treatment and after that (HPLC, GC etc).
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byko3y
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That's because nobody actually cares in the first place. Second - there can be different scenarios of purification. Until you have a contamination
that is water soluble/slightly soluble at neutral ph and water insoluble at non-neutral ph - bisulfite purification works perfectly. I mean it can
turn 5% solution of benzaldehyde mixed with some shitty crap (except ketones and aldehydes, of course), and get 98-99% pure benzaldehyde in 90-95%
yield in a single operation.
That's because bisulfite adduct is insoluble in aprotic solvents and even in alcohols, while free high mass aldehydes are readily soluble in aprotic
solvents as a rule.
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chemrox
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There are several good books on purification of laboratory chemicals and I have not yet seen one that didn't include purification of benzal via
bisulfite and distillation. Benzaldehyde is pretty stable. The b.p. of toluene is around 60*C lower than benzal so you should be able to flash of the
toluene without a column. You could probably run the remaining liquid through alumina and obtain a reasonably pure product but I'd have to read up on
this boring synthesis first. If your goal is learning chemistry fine. Otherwise I would suggest buying oil of bitter almond from a vendor not limited
by draconian laws or one with a gut. Di substituted and tri subbed benzals are a helluva lot more interesting...I suggest you read the primer; Organic
Chemistry Lab Survival Manual from cover to cover before you try any more organic syntheses.
If you don't have an aspirator send me you address and I'll send you one; sheeeit! Lastly: what nicodem said! Read and learn.
[Edited on 27-1-2016 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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Mush
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I meant structurally more complex molecules (benzaldehyde is too simple).
Thanx for your advices, I found what I was looking for.
On the Isolation of (+)3-Thujone
Canadian Journal of Chemistry, 1971, 49(10) 1762-1764, 10.1139v71-286
BISULFITE PURIFICATION OF AN
ALPHA-KETO AMIDE
US 8,222,427
| Quote: |
Previously published procedures for preparing the compound resulted in about 63% to about 98.5% purity; using the process of the present invention
results in about 97.2% to about 99.8% purity. |
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chemrox
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Now you tell us
"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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