Sciencemadness Discussion Board

Aminoguanidine bicarbonate from urea, P2O5, and hydrazine

UranylPeroxysulfate - 20-5-2021 at 21:16

I’m working on the synthesis of aminoguanidine bicarbonate. The synthesis requires a source of cyanamide, which is usually provided by calcium cyanamide from heating calcium cyanate to 900C. However, a temperature of 900C is unobtainable for me. So, I plan to synthesize aminoguanidine bicarbonate the following way instead:

1. Mix stoichiometric amounts of urea and P2O5, and then heat the mixture to the melting point of urea (135C) to dehydrate urea to cyanamide phosphate.
2. Neutralize the phosphoric acid with NaOH
3. Add hydrazine sulfate, adjust the pH, and make aminoguanidine bicarbonate as usual (same as the calcium cyanamide method, except filtering CaSO4 is no longer needed)

I attempted steps 1 and 2, but when I tested the mixture for cyanamide using AgNO3 in ammonia, it came out negative. Any idea what went wrong? I was thinking that perhaps the cyanamide got destroyed in the reaction or the phosphate caused the AgNO3 to precipitate as silver phosphate (and is this no longer available to react with cyanamide).

Boffis - 21-5-2021 at 08:00

@Uranylperoxysulfate. I wouldn't bother making calcium cyanamide, it is easily bought. I don't know where you are based but in Europe it is easily available at garden shops and agricultural suppliers e.g.:

And at just over £1 a kg you can't go wrong. I am currently working on preparing cyanamide from it for other purposes using the acetic acid-ether extraction route. I am currently away from my lab so work has ground to a holt for now but I have run a few small scale experiments. The key issue is knowing how much acetic acid is required because you must have a small excess. I have a paper that describes the process somewhere.

I am not sure you can dehydrate urea to cyanamide because the cyanamide would polymerise almost immediately under ether acid or basic conditions. To make urea react with hydrazine you must must first convert it to an O-alkyl-isourea salt. This is possible with dimethyl sulphate and similar reagents but generally the isourea salts are prepared from cyanamide. I have references on most of these procedures but they are not easy for amateurs.

If you search this forum there are procedures for the direct conversion of crude calcium cyanamide and hydrazine salts into aminoguanidine bicarbonate.

[Edited on 21-5-2021 by Boffis]

UranylPeroxysulfate - 22-5-2021 at 01:08

@Boffis. Thanks for the information. Unfortunately, it is pretty much impossible to buy calcium cyanamide of any form in China (where I live). I can buy dicyandiamide quite easily though. Is it possible to make aminoguanidine bicarbonate from it?

Polymerization is definitely an issue with cyanamide. I know for certain that it dimerizes in basic conditions, but I couldn't find any information about it polymerizing in acid, especially weak ones like phosphoric acid. Considering your cyanamide didn't polymerize when you add excess acetic acid to it, I doubt polymerization is the issue.

Making an O-alkyl-isourea salt is not a problem for me. I have some benzyl chloride leftover from my Grignard reaction experiments; it should alkylate urea quite well. Could you give me the reference for the procedures?

clearly_not_atara - 22-5-2021 at 07:57

I agree with Boffis -- I doubt that throwing dehydrating agents at urea is likely to produce any cyanamide. Maybe some interesting semiconducting polymers, though.

Bakibaev and Shtrykova (attached) seem to suggest that conversion to the iodide and rxn in neutral (not basic) media at about 100 C would be a good strategy, but they don't give many details.

Attachment: bakibaev1995.pdf (770kB)
This file has been downloaded 183 times

Boffis - 22-5-2021 at 09:07

Cyanamide is only stable in the presence of acetic acid at low temperature ie <20 C. Furthermore strong dehydrating agents convert urea to triazine derivatives like melamine in a single step. There is a rather old monograph on urea (ACS I think), a bit out of date now but worth reading; the facts haven't changed only the interpretations. This book discusses numerous attempts (all unsuccessful) to prepare cyanamide from urea.

Here are the papers I was referring to:

Attachment: O-Methylation of urea to methyl isourea JCS Werner 1914.pdf (142kB)
This file has been downloaded 174 times

Attachment: Preparation of O-Alkyl iso-urea TransRSNZ Ongley 1948.pdf (185kB)
This file has been downloaded 173 times

There are some interesting references in the paper supplied by clearly-not-atara, particularly No.8 by C. D. Nenitzescu which deal with the use of alkyl iodides and urea.