Catalysts for carbonyl + amine condensation reactions?

[Mr Cool]

Does anyone know of any catalysts that can be used to speed up the reaction between a carbonyl compound and an amine?

Specifically, I'm interested in making guanidine from urea and ammonia. Someone said it could be done by heating urea with an ammonium salt; they also said that they had heard that someone tried it and it blew up. Perhaps I could try using ammonium sulphate...

At the moment I'm wondering about using urea, with excess ammonia and a greater excess of sulphuric acid; something like 1 mole of urea, 1.5-2 moles of ammonia and 1.5-2 moles of sulphuric acid. Any comments on this? The idea is to have a large excess of ammonia to help prevent condensation between two urea molecules, and enough H2SO4 to have some free even after it has reacted with the ammonia. Then the excess could catalyse the reaction...

So, if you have any info on making guanidine from urea, either with NH3/NH4+ and a catalyst or NH3/NH4+ and heat (or a catalyst and heat), please post it. I'm not really interested in the method with cyanamide, it seems labour intensive and dangerous to me.

It'd be nice if it'd work; urea and sources of ammonia are very cheap and plentiful, and GN/NG can then be made with low conc. HNO3 (and H2SO4, if NG is made), and they're quite powerful.

[PrimoPyro]

Personally, my opinion is that acidic environments inhibit aminal formation, which of course is directly responsible for condensation here.

My reasoning is:

In the ammonium ion, the polar character favors the proton bearing the charge, not the nitrogen like is so in tertiary amines. That proton exhibits electrostatic attraction toward the =O atom, and this gets us nowhere. The two will never combine because this just sucks up energy to form a very unstable compound that regurgitates that proton in a couple nanoseconds to put us back at square one.

However, consider a nonionic ammonia such as in a basic anhydrous environment where actual NH3 is the main constituent phase of the ammonia molecule, not NH4+.

The lone pair on the ammonia exhibits that same sort of electrostatic attraction toward the carbocation attached to the =O. They meet and form an aminal, C=O + NH3 --> C(NH2)(OH) by proton shift from the ammonia to the oxygen. This aminal loses water to form the desired imine, C=NH + H2O

Imine formation is of great importance in "other" clandestine laboratories I have some experience with, and is beaten like a dead horse. Imine formation is reversible, and anhydrous conditions favor imine formation, so do dehydratory conditions. Imine formation DOES proceed in acidic environments, but I personally think it works better using neutral NH3 species.

NH4+ + heat --> NH3 + H+

Ammonium ions easily lose their protons when energy is added, such can be witnessed in an ocillating color phenolphthalein solution with NH4OH --> NH3 + H2O.

So if acidic environments are to be used, don't perform it at cold temps where ammonium ions predominate the solution.

Just my $.02......

PrimoPyro

<small>[ August 06, 2002, 11:09 PM: Message edited by: PrimoPyro ]</small>