TripleOctet
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Barbituric acid won't precipitate out
I am working on synthesizing barbituric acid to later make violuric acid and many of its colorful salts. I have followed ChemPlayer's synthesis but
aimed to make closer to 10g of product: first, I attached a reflux condenser with a CaCl guard tube to a 3 neck 1L round bottom flask. 100mL absolute
EtOH was added to the flask followed by roughly 6.5g sodium metal. While dissolving, a solution of 11g urea was prepared in 75mL of hot (almost
boiling) EtOH. After the sodium dissolved, roughly 20mL of diethyl malonate was added to the flask followed by the hot urea solution. The solution was
then refluxed for about 4 hours during which the CaCl was changed once. After reflux, 100mL hot water was added and 37% HCl was added until litmus
paper was a dark red.
To precipitate out the solution, I tried cooling the mixture to around 0 C for 12 hours (similar to what was done by ChemPlayer) but received no
precipitate. Looking into I found a couple videos where people had the same issue (https://www.youtube.com/watch?v=5HF8SYnwaSk and https://www.youtube.com/watch?v=kTofPgeP8q4) and the comments mentioned that the EtOH could be preventing the precipitation because it increases
the solubility of the acid. I then evaporated off some of the mixture from about 350mL to about 200mL which turned the solution from clear to dark
yellow. I then cooled the mixture for another 6 hours but still no luck.
Is there a reason why my barbituric acid is being stubborn and not crashing out of solution?
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Rainwater
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I'm not sure if this will work for your products as I've never worked with it before.
But hexachloroplatinate and chloroauric acid are very stubborn to crystallize.
As long as I have a reasonable pure samples this always works.
Take a small sample in a test tube and placing it in a dry ice bath, forcing it to freeze, that should generate a good seed crystal, that you drop
into your motherliquor.
Once neculation starts be mindfull of the heat of crystallization released.
May not be an issue for your product, but if you drop a seed into a super saturated solution, you can expect some heat.
[Edited on 30-7-2023 by Rainwater]
"You can't do that" - challenge accepted
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TripleOctet
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| Quote: |
Take a small sample in a test tube and placing it in a dry ice bath, forcing it to freeze, that should generate a good seed crystal, that you drop
into your motherliquor. |
I'll try this. However, I am not sure if that is the issue. When evaporating off liquid it seems that some crystals started to condense on the beaker
wall then dissipated after cooling the flask down.
Would it be possible for me to evaporate off all the liquid and leave whatever remaining products at the bottom of flask? I am thinking that the only
products left would be the barbituric acid and NaCl from the sodium metal and HCl. Then a little bit of water could be added to dissolve the NaCl and
then filtered to obtain the barbituric acid. I don't see why this wouldn't work but would there be an issue with it?
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Ormarion
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Did your reaction precipitated during heating ? i know that i personally never had any issue with the synth of barbituric acid but following addition
of all chems my reaction mix tend to solidify in a beige solid pretty rapidly before acidification. Also have you dried your diethyl malonate? I
personally didn't had to use a calcium chloride tube but all my chems were dried over sieve. I feel like water contamination might be one of the
issues in most of the cases but im not sure
Made a video bout it long ago (sry its in french) https://www.youtube.com/watch?v=TN8tew9xg6A&t=1151s but you can see how it goes
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TripleOctet
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It looks like I got some precipitate on the side of the beaker I was evaporating on. Could be some salt or the barbituric acid... I'm not quite sure.
The diethyl malonate was purchased from eBay and should be dry enough (>99%). Ethanol was also thoroughly dried with plenty of 3A molecular sieves.
When I added the diethyl malonate to the ethoxide solution, I got a could white precipitate that solidified partially on the side of the flask.
Correct me if I am wrong but I believe this is a result of the deprotonated diethyl malonate. When adding water to this solution after reflux, the
precipitate clears up and the solution was acidified.
Did you add water after the reaction was done refluxing and how did you recover the product after? Will evaporating all the water cause any harm in an
attempt to get the products?
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Ormarion
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I see, i didnt add any water before and after acidification , i directly cooled the solution in the freezer overnight but i already got most of the
product that crystallized even before cooling. Only had to filter and dry it
I dont rly know if evaporating it would be a issue, i might try to reach slightly acidic conditions instead of very acidic eventually but its hard to
say
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TripleOctet
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I watched your video and it looked like we did the same procedure except for the addition of warm water before HCl. That is interesting however when I
attempt again I might try it that way.
I am however going to evaporate all the liquid off a portion of the sample I have. The other portion I will keep and use a small amount to ensure that
there is barbituric acid in solution. Reacting a small amount with sodium nitrite should create a red to purple solution indicative of the reaction
between barbituric acid and sodium nitrite to create sodium violurate.
If anyone has any other tips or experience please share!
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TripleOctet
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Quick update
I evaporated almost all of the remaining solution and began to see crystal growth when I reached a little less than 100mL total. After this, I let it
sit on the hot plate for a while longer then set it in the fridge for a few hours. The precipitate was then gravity filtered, washed with cold water a
few times, then collected as an off-white crystalline powder.
I guess it must have been the small amount of ethanol still left in solution that was preventing it from crashing out.
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