[Grundalizer]

Hi, I'm in my senior year at university.  I am in the hardest (but most fun) lab class I have ever been in, Techniques in Organic and Inorganic synthesis.  One of our first labs was the green benzoin condensation using Benzaldehyde and acetone with Vitamin B1 (Thiamine hydrochloride) as the catalyst.

Here is a link to the procedure http://www.cerlabs.com/experiments/10875407374.pdf  it wasn't the exact one we used, but it is identical in reagents, perhaps not quantity.

We used equimolar amounts of benzaldehyde and acetone, because it said that if there was an excess of one or the other side reactions would take place.

Long story short, while most groups got crystallization, we did not.  We tried everything in the book to force out the crystals, scratching the test tube like mad, ice baths, boiling off excess solvent etc.  Nothing worked.

Our teacher gives us no answeres, and urges us to try whatever we think as long as it is safe.  So...we boiled down the original mixture even further last week, and added 3 drops of acetone, then cooled in an ice bath.  No crystals were observed, just little oil droplets like a salad dressing emulsion filled the test tube. 

We used 1 mL benzaldehyde and 0.366 mL acetone originally, and 0.175g catalyst.  We also added another 0.150g catalyst before we boiled down the second time.

We are running out of ideas as to what to do to try and salvage this solution.  The only thing I can think of doing would be to add a couple drops of benzaldehyde...or maybe more ethanol?

We had to reflux the original mixture for an hour at 80 degrees C....everyone's was in the same sand bath and some people got crystals others didnt.

Just wondering if anyone has any ideas

much thanks

[Doc Oc]

If those really are the amounts you used then your stoichiometry is off.

[MOTOBALL]

I think that if the rxn. has proceeded properly and the xtls. are in there waiting to form, here are two suggestions, both of which in my experience can work.

1) Get your rxn mixture to the stage at which the xtls should form ---FREEZE it solid (acetone/dry ice or Liq. N2). Let the solution come up to ammbient Temps. while scratching with the glass rod.

2) If (1) above is not successful (several attempts) then seed your rxn. mixture with a few xtls of authentic product (should be available from teacher/classmates) and repeat (1) above.

3) If still not successful, heat solution with activated charcoal, filter thru' Celite bed (diatomaceous earth) to remove charcoal and adsorbed impurities that may be preventing xtlln. Then back to (1) or(2).

Please tell uswhat works (or does not)

Glad you find practical organic to be fun--it is !!!

[Grundalizer]

Thanks a lot for your reply MOTOBALL.  I joked about crystalizing with liquid nitrogen but everyone laughed at me.  I might get to do it this time!! And it just so happens that we are doing a liquid carbon dioxide extraction lab on Friday, so we'll have dry ice handy!!!

My partner and I will give all those a try.  I only hope that I did not ruin the products by heating it too high and adding excess acetone the 2nd time.

J-bone: 1mL benzaldehyde = 9.85 mmol 0.366 mL Acetone = 4.98 mmol. pretty darn close 2:1 equimolar

[Doc Oc]

Yes, your math is correct.  But 2:1 is not the same as equimolar.

[Grundalizer]

Ah yes, this is probably a huge source of my problem...Acetone is not needed in the benzoin synthesis!!! I just realized this as I write up the report

[Grundalizer]

Here are some of the pictures we took.

This yellow color is the catalyst being deprotonoated by the NaOH--so we can see it has a higher density than the ethanol/water mixture it is in.

http://img512.imageshack.us/img512/7401/yellowtj.jpg

Now, the next picture is what the solution looks like after sitting in reflex for 1 hour at 80 degrees C.

http://img715.imageshack.us/img715/1121/blueyt.jpg

You can clearly see at LEAST 3 layers, we thought we saw 4.

Yellow (catalyst we assume) on the bottom--then the middle layer which is oily looking, which is benzoin? excess benzaldehyde?) and then the ethanol/water on top.

Now before we crystallized, we stirred it all up and it became a uniform pale yellow color, with a clarity and color of the top layer in that second picture.

Unfortunatley, we got no crystals, so we re-heated it as I mentioned above to about 90 degrees C, added more catalyst, and 3 drops of acetone (my fault).

NOW our solution has an oily layer on the very bottom, a yellowish solution in the middle, and a tiny oil layer on the top.

We tried comparing densities of all the compounds, and it said that benzoin was only slightly soluble in water...meaning it is more soluble in ethanol? acetone?  because it is more non-polar than it is polar.

We are thinking about adding ethanol to try and dissolve the oil layers, but we don't know for sure WHAT the oil layers are.  The solution still smells like benzaldehyde, so I assume there may be some leftover benzaldehyde, since benzoin does not smell like cherry's supposedly.

[MOTOBALL]

Be aware that everybody who does rxns with xs. benzaldehyde will at some point get a nice crop of benzoic acid xtls, which can be mistaken for the expected product.

You should probably separate the upper aqueous/oily layers from the oil at the bottom;  try to xtllze this oil as discussed previously.

Keep having fun !!

[Doc Oc]

The problem is it sounds like you have different products in there now.  The benzoin reaction relies on benzaldehyde adding to itself in an aldol type reaction.  With acetone in there you added another electrophile and now probably have that byproduct in there as well.

[orgopete]

If you check the TLC, you will probably find those that did not crystalize, did not have benzoin present. If you check the mechanism, the reaction requires base to deprotonate the adduct. The problem is as suggested, benzaldehyde will air oxidize quite readily. The benzoic acid will alter the pH of the reaction and cause some to fail and some to succeed. The procedure could include a note about benzoic acid or to check the pH of the solution. (Since you added NaOH, you recorded the reaction pH in your notebook at various stages, right?) I will guess that two different batches of benzaldehyde were used to get some to succeed and others not.

[Grundalizer]

The entire reaction was run in one day using only 1 mL of Benzaldehyde.  We did not check the pH.  The solution remained covered and undisturbed in the dark for a week until our next lab class, at which point it was still a pale golden clear solution (probably with a decent amount of benzoin, but just not enough to crystalize out using our ice bath?).  It was only after heating again to drive off solvent and then (mistakenly) adding acetone that we got 3 layers to develop.

Orgopete, thanks for the tip about benzaldehyde oxidation. This must have been one of the possible side reactions http://en.wikipedia.org/wiki/Cannizzaro_reaction
but I assume because of the small volume and presence of a catalyst and heat, benzoin was highly favored over benzoic acid or benzyl alcohol.

I remember doing TLC 2 years ago, but on a much easier reaction mixture because we knew what our two products were supposed to be, an ortho or para nitroaniline.

I'm not a TLC pro by any means so I'd like to run the idea by you all.  I run a TLC on the plates we have (I don't remember if they are silica or alumina or some mixture) by using 1 drop from each of the 3 layers PLUS take a little of the pure product from a friend and dissolve it in some solvent and run that next to them.  That way...wherever the "pure" product dot runs to, if there is a dot of equal height in any of the other layers, I will know that one contains benzoin.  Sounds good?

My only problem is a solvent for the TLC.  Our teacher is huge on Green Chemistry, in fact that's why we did the green benzoin synthesis in the first place.  We have to calculate our e-factor and waste in everything we do, so busting out dichloremethane might kill our greenness, but at least it would have us going above and beyond and not just giving up to try and get our product.

Is 95% ethanol a poor solvent choice in this case?

[orgopete]

No, I am not talking about a Cannizzaro condensation. i am referring to a simple air oxidation of benzaldehyde. It isn't simply that benzoic acid accompanied the benzaldehyde. Aldrich sells benzaldehyde in Sure-Seal bottles. This is to protect it from air. Did you use a syringe to measure the benzaldehyde? Did you see any crystals on the bottle? I usually found that after a few uses, it is easy for the Sure-Seal itself to degrade and air can enter. You can see benzoic acid at that point. It is time for a new bottle.

The effect on the reaction is benzoic acid consumes NaOH. The reaction needs to be basic for success. If you TLC the reaction, you will find a lot of unreacted benzaldehyde, we did. You can just add more NaOH, heat it, and you will get product. I was suggesting that is why some of the class got benzoin and others did not. Either a different sample of benzaldehyde (read benzoic acid content) or a different amount of NaOH was used. The molecules don't know intent. If you have the right reagents present, the reaction succeeds every time. If it did not, it is due to some aspect of the set up. I'm betting on benzoic acid altering the reaction pH.

[nj_bartel]

Have you run an nmr on the crude to ensure you got product at all?  Lots of effort trying to recrystallize could be going to waste if not. 

[LAN]

The problem is it sounds like you have different products in there now.  The benzoin reaction relies on benzaldehyde adding to itself in an aldol type reaction.  With acetone in there you added another electrophile and now probably have that byproduct in there as well.

Indeed- look for other alpha hydrogens in the mix-
I'd predict a bright yellow product. MW 234.29