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Author: Subject: preparation of barbituric acid (question about the mechanism)
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[*] posted on 31-8-2021 at 15:56
preparation of barbituric acid (question about the mechanism)

Hey there, so here's the preparation:
Screenshot_5.jpg - 199kB

I don't understand the purpose of the NaOEt.

I considered:
1. NaOEt being a catalyst for the reaction (e.g. proton carrier). but won't the urea be enough for this purpose? it's weakly basic after all (even water are often used as a proton carrier).
2. NaOEt is needed for neutralizing the product (otherwise the urea would form hydrogen bonds with the protons preventing it from reacting). but couldn't we just use excess urea instead? why go through the troubles of making NaOEt?

I also thought about a few downsides of strongly basic environment:
1. the diethylmalonate getting deprotonated thus becoming less reactive towards nucleophilic attack.
2. water contamination are more likely to screw things up react with the diethylmalonate.
3. decomposition of the urea into ammonia.

to focus on my questions:
1. what is the purpose of the strong base?
2. would the reaction work without it?
3. would the reaction work with excess of urea instead?

much thanks ahead,

[Edited on 31-8-2021 by B.D.E]
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[*] posted on 1-9-2021 at 12:47

1. Neutralize the product and catalyze the reaction
2. unlikely, possibly to some degree with a hydroxide base.
3. very unlikely
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[*] posted on 2-9-2021 at 10:00

Deprotonation is one of the steps in this mechanism of amide formation, and urea is not a strong enough base to do that.

[Edited on 2-9-2021 by Tsjerk]
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