MagicJigPipe
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Oxidation of Benzaldehyde, Prevention Of
I have noticed that, over time, benzaldehyde is slowly oxidized to benzoic acid. This is well known.
I don't understand though. Oxidation of benzaldehyde requires oxygen if for no other reason than conservation of matter/mass (unless the O
would somehow come from other BzH molecules).
In a small vial there has been sitting 20 mL of BzH for approx. 4 months. After filling the vial, Ar was blown over the top of the BzH for about 30
seconds and then the cap was quickly placed on. The vial was wrapped in aluminum foil, placed in a refrigerator and kept at about 5*C for the entire
time. Still, there seems to be a steadily increasing amount of benzoic acid collecting on the bottom of the vial. Why is this and how can it be
prevented? Is it from dissolved oxygen? Is it some sort of self oxidation? If so, what would be the mechanism for such a thing?
EDIT:
By the way, when I first smelled BzH, I almost crapped my pants. It smells SOOOOO good! Cherry is my favorite flavor--especially marischinos--and
this smells just like them except better!
I can imagine that if one were attempting to synthesize BzH, smell would be an easy way to tell if it is working.
[Edited on 11-26-2010 by MagicJigPipe]
"There must be no barriers to freedom of inquiry ... There is no place for dogma in science. The scientist is free, and must be free to ask any
question, to doubt any assertion, to seek for any evidence, to correct any errors. ... We know that the only way to avoid error is to detect it and
that the only way to detect it is to be free to inquire. And we know that as long as men are free to ask what they must, free to say what they think,
free to think what they will, freedom can never be lost, and science can never regress." -J. Robert Oppenheimer
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Rosco Bodine
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Hydroquinone is an antioxidant useful as an inhibitor / stabilizer for benzaldehyde (and IIRC also useful for other sensitive aldehydes or ketones) A
small amount like 0.01% is effective.
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bbartlog
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| Quote: | | unless the O would somehow come from other BzH molecules |
Well, the Cannizzaro reaction is sort of like what you're describing... but it should require a strong base to proceed. Do you actually know that the
accumulating precipitate is benzoic acid? Seems like it could also be benzoin. In either case though it seems like impurities (acting as catalysts)
would have to be involved.
[Edited on 26-11-2010 by bbartlog]
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MagicJigPipe
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You know, I just assumed it was benzoic acid due to the fact that I was almost certain that there were no impurities. This BzH is reagent grade and I
didn't expect there to be enough of any catalyst to cause this reaction to occur at a significant rate. I will attempt to collect the precipitate
Monday and see if it is an acid.
Thanks.
[Edited on 11-28-2010 by MagicJigPipe]
"There must be no barriers to freedom of inquiry ... There is no place for dogma in science. The scientist is free, and must be free to ask any
question, to doubt any assertion, to seek for any evidence, to correct any errors. ... We know that the only way to avoid error is to detect it and
that the only way to detect it is to be free to inquire. And we know that as long as men are free to ask what they must, free to say what they think,
free to think what they will, freedom can never be lost, and science can never regress." -J. Robert Oppenheimer
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garage chemist
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Oxidation of benzaldehyde by air is rapid and cannot be inhibited.
If your benzaldehyde has already precipitated some benzoic acid, then much more of it is in solution.
I have some old benzaldehyde, the bottle of which I thought of as being airtight. There was no precipitate on the bottom, but when I vacuum distilled
a 20ml portion prior to use, I was surprised at how much benzoic acid was left behind in the distillation flask (over 1,5g).
Benzaldehyde must always be vacuum distilled directly before use. No exceptions. Material from the stockroom and even directly from the supplier
contains wildly varying amounts of the acid.
A colleague of mine has once received an old partially full 1L bottle of benzaldehyde containing some precipitate, from which he has isolated about
300g of benzoic acid upon vacuum distillation.
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Rosco Bodine
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Benzaldehyde can be made storage stable in a closed jug using hydroquinone as an inhibitor and perhaps a bit of toluene or benzene to provide some
vapor pressure as added protection. There is your exception and it as old as the hills and it works.
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entropy51
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Old benzaldehyde sometimes gives a positive test for peroxides. Although I have never heard of them forming crystals, it could be
another candidate for what you're seeing.
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mr.crow
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If there was benzoic acid left behind wouldn't there be an equal amount of benzyl alcohol (cannizzaro reaction)? How could that much air leak in
through the cap?
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MagicJigPipe
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I can't believe I left this out:
The BzH that was placed in the vial was no doubt saturated with this substance beforehand because it was poured from a bottle in which about an inch
thick layer of white/clear crystals had formed. This simply means that the reaction hasn't proceeded to saturate the solution and THEN cause
precipitation (that would be really bad) because it was already saturated to begin with.
Still, since benzaldehyde is a pain in the ass to come by (apparently even for school laboratories) it would be nice to find something that could be
done.
Would pictures help?
Thanks for all the info.
P.S.
Our stockroom officially has no cyanides or red P due to ... well, you know. Now students are not to be even given the chance to learn hands-on
phosphorous or cyanide chemistry simply because "some" are uneasy about the fact that it could be diverted to users with nefarious intentions?
*Places palm upon face*
"There must be no barriers to freedom of inquiry ... There is no place for dogma in science. The scientist is free, and must be free to ask any
question, to doubt any assertion, to seek for any evidence, to correct any errors. ... We know that the only way to avoid error is to detect it and
that the only way to detect it is to be free to inquire. And we know that as long as men are free to ask what they must, free to say what they think,
free to think what they will, freedom can never be lost, and science can never regress." -J. Robert Oppenheimer
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Methansaeuretier
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The bisulfit-adduct is more stable against athmospheric oxidation.
You can add some add (diluted.!) aq. NaOH to liberate benzoic-acid free benzaldehyde directly before use.
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entropy51
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You
might want to be careful about old bottles of organic liquid that have unknown crystals in the bottom. While benzaldehyde is not usually dangerously
contaminated with peroxides, it does form peroxides. It probably is benzoic acid, but...
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Nicodem
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Quote: Originally posted by MagicJigPipe  |
The BzH that was placed in the vial was no doubt saturated with this substance beforehand because it was poured from a bottle in which about an inch
thick layer of white/clear crystals had formed. This simply means that the reaction hasn't proceeded to saturate the solution and THEN cause
precipitation (that would be really bad) because it was already saturated to begin with. |
The solubility of benzoic acid in benzaldehyde is relatively high, so what you see is crystallization due to storing conditions (solubility in
refrigerator is lower than the solubility at room temperature where the solution was initially saturated). I also always distil benzaldehyde before
use if taken from the bottle. You can do this on the rotavapor (from the flask to the trap), no need to fractionate. Using old benzaldehyde, I had the
same experience as garage chemist chemist described above - plenty of solid remaining after distillation. You can store the surplus of the distillate
under argon or nitrogen and as long as it is hermetically closed it will be just fine. But large amounts, like in a 1 L reagent bottle, pick up oxygen
rapidly if they are often used (frequently opened) or if not hermetically closed, so never trust it as being benzoic acid free.
…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)
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loveoforganic
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For what it's worth, I know benzaldehyde won't undergo a non-base catalyzed Cannizzaro reaction, so you either have other impurities in there or
you're letting atmosphere in. In lab, I distilled about 100 mL of benzaldehyde from the sigma container (which went from vividly yellow to perfectly
clear). Distillation was done under n2, and receiving flask was capped and parafilmed under n2 and put into freezer. Still crystal clear two years
later.
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Ephoton
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I am not sure were you get the idea of Cannizzaro as loveoforganic has stated it
wont happen.
its a simple aldehyde to acid oxidation.
hydroquinone has worked for me in the past.
though I didont know about the toluene being used to blank its vapour pressure.
benzaldehyde is pritty easy to make so I dont worry about its oxidation but
rather make it if I need it.
if following cycloknight's pathway too benzaldehyde I
think smell is not the best way but rather taste then
finaly temp when distilling.
but then again he states that in his posts.
[Edited on 20-12-2010 by Ephoton]
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Methyl.Magic
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BEnzaldehyde is usually stored in black glass bottle (not the standard amber ones) to prevent light passing through the glass. Light is the catalyst
of the oxidation. I think you can add radical inhibitor such as 2,6-di-t-butylphenol to prevent oxidation of the benzaldehyde by oxygen.
2,6-di-t-butylphenol is also added in comercial THF to prevent peroxide formation.
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plante1999
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I just read something pretty interesting about benzaldehyde oxidation, apparently, benzaldehyde first oxidize to give per benzoic acid, which react
quite fast with benzaldehyde to make two molecules of benzoic acid. It is said however that when acetic anhydride is present it form benzoyl acetyl
peroxide and acetic acid. I'm interested to know if it is possible to make a viable acetyl benzoyl peroxide synth with oxygen and benzaldehyde.
Anyone have experience with the said "protection" of the peroxide?
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Akira990
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What about storing adduct and than when you need to use benzaldehyde you break adduct with some base or acid.. ?
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