Sciencemadness Discussion Board

Oxidation of Benzaldehyde, Prevention Of

MagicJigPipe - 26-11-2010 at 09:41

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]

Rosco Bodine - 26-11-2010 at 10:11

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.

bbartlog - 26-11-2010 at 10:46

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]

MagicJigPipe - 28-11-2010 at 00:12

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]

garage chemist - 28-11-2010 at 04:19

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.

Rosco Bodine - 28-11-2010 at 04:27

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.


entropy51 - 28-11-2010 at 09:45

Quote: Originally posted by MagicJigPipe  
You know, I just assumed it was benzoic acid due to the fact that I was almost certain that there were no impurities.
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.

mr.crow - 28-11-2010 at 12:35

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?

MagicJigPipe - 28-11-2010 at 16:01

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*

Methansaeuretier - 29-11-2010 at 02:51

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.

entropy51 - 29-11-2010 at 07:29

Quote: Originally posted by MagicJigPipe  
it was poured from a bottle in which about an inch thick layer of white/clear crystals had formed.
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...

Nicodem - 29-11-2010 at 10:05

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.

loveoforganic - 15-12-2010 at 15:30

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.

Ephoton - 19-12-2010 at 19:12

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]

Methyl.Magic - 22-12-2010 at 02:03

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.

plante1999 - 20-8-2014 at 06:38

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?

Akira990 - 22-9-2014 at 13:35

What about storing adduct and than when you need to use benzaldehyde you break adduct with some base or acid.. ?