Sciencemadness Discussion Board

Aldehyde distillation/purification

Hilski - 19-11-2006 at 02:05

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

Nicodem - 19-11-2006 at 02:45

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.

Hilski - 19-11-2006 at 06:31

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.

Mush - 23-1-2016 at 06:42

I hardly find any literature data on aldehyde/ketone purification. Comparison of purity before bisulfite treatment and after that (HPLC, GC etc).

byko3y - 23-1-2016 at 09:30

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.

chemrox - 27-1-2016 at 14:08

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]

Mush - 5-2-2016 at 07:45

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.



chemrox - 5-2-2016 at 18:59

Now you tell us