Hexavalent
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Problems with Claisen condensation
As some of you know, I recently embarked on a Claisen condensation on ethyl acetate to yield ethyl acetoacetate.
Last night, I ran the reaction on a small scale - using 24.5g of the EtOAc in a 50ml RBF - to test it out before I did it on a larger scale in a 250ml
flask. However, I encountered some problems, namely that, during reflux, a large amount of solid formed at the bottom of the flask and a gel-like
substance adhered to the walls of the headspace in the flask.
My sodium was bought as 99% pure and was shiny when I cut it and put it into the liquids. The ethyl acetate (and the extra ethanol added) were dried
using anhydrous MgSO4 - before anyone suggests this, it was not CaCl2, which would have complexed the ethanol and potentially
caused the problem - and all the liquids were tested with CaC2 and anhydrous CuSO4 on a spotplate before using. All tests were
negative for water.
My flask and condenser were visually clean - I thoroughly cleanse equipment after each experiment with tap water, soapy water, tap water and then DI
water, and I only used the standard minimal amount of grease on the joints. I was also magnetically stirring the flask slowly and the solvent, when
heating, was boiling gently. The colour at this point was yellow/tan, and not the red I was expecting.
Any ideas?
[Edited on 29-10-2012 by Hexavalent]
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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Magpie
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Not really. But I can retell from my notes my experience in making this reagent. Although I seemed to have had many surprises and one major faux
paux my product was produced in good yield and was used successfully in a later synthesis.
I made a 1/2 scale batch from Brewster, 1960, my old school lab manual. I used 117 mL EtOAc and 9.4g Na in a 250mL RBF. Refluxed on a steam bath for
2 hrs. Mix turned a brown-orange. Then added 2.5mL EtOH and heated for another 30 min. But to get everything dissolved I ended up heating and
shaking for another 1 hr. Then added 12.5mL EtOH in 4 portions through the condenser. Everything turned solid again, brown in color. Added 110mL of
50% acetic acid (twice what procedure called for) to get just faint acid color on pH paper.
Now for my major fuck up: I added 200 mL of saturated CaCl2 solution instead of the specified 200mL of sat NaCl soln! This caused an immediate gel
formation in the 500mL sep funnel. I salvaged this by adding enough water to get the gel to go into solution, made a phase separation, then added the
NaCl soln. Dried the organic over powdered CaCl2. Purified the ethyl acetoacetate by vacuum distillation at ~50mmHg where it boiled at ~100C. My
yield was 13mL which according to Brewster was twice what could be expected using distillation at atmospheric pressure. %yield was only 16.7%. Hope
this helps.
[Edited on 29-10-2012 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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Hexavalent
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Do you mean EtOAc?
Either way, lacking a vacuum pump, could one simply distill off, at atmospheric pressure, everything boiling below, e.g. 100*C, and just keep what is
left in the stillpot? With a few washes, how pure would the ethyl acetoacetate be now?
Also, is there a substitute for the acetic acid? Could formic acid or a mineral acid be substituted instead?
[Edited on 29-10-2012 by Hexavalent]
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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UnintentionalChaos
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Quote: Originally posted by Magpie  |
Now for my major fuck up: I added 200 mL of saturated CaCl2 solution instead of the specified 200mL of sat NaCl soln! This caused an immediate gel
formation in the 500mL sep funnel. I salvaged this by adding enough water to get the gel to go into solution, made a phase separation, then added the
NaCl soln.
[Edited on 29-10-2012 by Magpie] |
Sounds like my major issue using the orgsyn prep...
They have you dry the crude organic phase over CaCl2, which contains ethanol, apparently, as it swelled up into glop overnight, entrapping most of the
solution...so I added water and proceeded as best I could.
As for you Hexavalent, I expect water is your problem, even though everything seemed to be dry. The orgsyn prep specifies that "If the ester is dry
enough to use in this reaction it will not give a gelatinous mass of sodium hydroxide when treated with a little sodium." I got tons of solid with
just a few droppers full of the 50% acetic acid at the end. The solution was initially cloudy from a little NaOH, but this dissolved as the reaction
proceeded.
[Edited on 10-29-12 by UnintentionalChaos]
Department of Redundancy Department - Now with paperwork!
'In organic synthesis, we call decomposition products "crap", however this is not a IUPAC approved nomenclature.' -Nicodem
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Magpie
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yes
| Quote: Originally posted by Hexavalent |
Either way, lacking a vacuum pump, could one simply distill off, at atmospheric pressure, everything boiling below, e.g. 100*C, and just keep what is
left in the stillpot? With a few washes, how pure would the ethyl acetoacetate be now? |
Brewster says that cut III (175-185C) is ethyl acetoacetate. Cut II (95-175C) has some ethyl acetoacetate. So keep cut III. Redistill cut II,
keeping that boiling in the range 175-185. I can't guess what your purity would be if you merged cuts II & III. Do the redistill of cut II if
you want the purity.
I think that using acetic acid maintains the ethyl specie. If you add formic you may get some methyl replacing ethyl in your ester product. HCl
wouldn't do that but you would not have the mass action of the added ethyl that you get from using acetic acid.
The single most important condition for a successful synthesis is good mixing - Nicodem
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Hexavalent
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Okay, thanks a lot.
I asked DJF90 earlier and he hypothesised that water was the problem too.
I'll buy some more ethyl acetate at some point and try again, but I'll dry it on its own with CaCl2, dry the ethanol with magnesium sulfate
and mix the two. I'll test with sodium before using, and dry further if necessary.
I haven't a thermometer that is graduated past 150*C nor a vacuum pump, so distillation is impossible for me unless I buy a new thermometer or a pump.
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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