I've UTSE'd this question and even searched all over SM, I even have a 408 page document on Formaldehyde from the ACS written in 1944 during wartime
that doesn't have the answer.
Anyway, I can quickly de-polymerize paraformaldehyde via heat and make formadehyde, then treat with NH3 (aq) to get hexamine. However, this is not so
simple as the paraformaldehyde gas clogs tubing, glass, and makes a big mess.
Now, I can also de-polymerize paraformaldehyde with an aqueous solution of NaOH of about 2% or so, with a PH of 11 very easily.
I can also, then neutralize this solution with dilute HCL or H2SO4, but, then I have either NaCl or Na2SO4 in solution respectively.
So the question is, what is the best way to make hexamine from paraformaldehyde, preferably using a basic solution, instead of the gas route?
Long time watcher, first time poster, thanks and hello,
Billy
Also, I wanted to just take a moment and marvel at the wonders of hexamine.... kristofvagyok - 20-10-2012 at 16:02
The answer is easy. Because of the high solubililty of hexamine in water (nearly a kilo in a liter) I would recommend to use the "gas route", because
getting rid of sodium salts from a hexamine solution is aint so easy, can't be done with crystallization or any other simple way.billy666 - 20-10-2012 at 18:30
Thank you for that. I still want to consider non-gas routes if they are at all possible though. Since my source of paraformaldehyde is not
anhydrous, this presents a real problem with using it in a gas phase.
I was successful in my first synthesis, but, so much para. was lost as solidified rubber tubing, and caked onto the walls of the generation flask
,etc, that I didn't even bother to quantify yield. Plus, had a scary episode where my take off adapter on the generation flask became glued to the
flask itself, then when pressure built up due to a clog in the hose, the take off adapter shot off and hit the ceiling of my fume hood and bounced
around a bit and finally came to a rest on the floor UNBROKEN!
Yeah, that scared the you know what out of me. Any time I hear a loud kaboom while working with Ammonia and Formaldehyde, which is never btw, is
quite unnerving. I had thought the gas evolution had stopped, but that was not the case, the entire system had become solid with paraformaldehyde,
and pressure was building fast without exhibiting any notion of such. Everything was nice and quiet as I thought evolution had ceased but could not
tell because of all the para. which had crusted to the inside of the generation flask making it impossible to see how much solid para. was left.
Considering the solubility of paraformaldehyde in H20 seems to be somewhat logarithmic with respect to solution pH, I would think a warmed solution of
Ammonium Hydroxide at ~28% or 1M would work because the pH would be around 11.
What I am unsure of is exactly how the PH effects the dissolution of paraformaldehyde in H20, I am unsure of the action here. Meaning, I am unsure if
the -OH is a necessary component of this system. Will it dissolve faster in a high pH created by NH3(aq)?
From the ACS Formaldehyde document 1944 pg 279 it mentions that Butlerov first created hexamine by a reaction of paraformaldehyde and NH3 (g),
therefore, I would think that aqueous NH3 should dissolve it.
Perhaps another base could be used that would create an insoluble salt in H2O once titrated.
I'm going to study up more on the action of using a base to enhance solubility. I'm also going to try refluxing some paraformaldehyde in an NH3 (aq)
10% solution, or 0.37M. Well, perhaps not full reflux, but, just with moderate heating and stirring.
If anyone has an insoluble salt forming idea, please let me know, thank you. Perhaps a carbonate which would yield something insoluble and CO2
evolution?
I am nervous about doing a lot of experimenting with this because hexamine and formaldehyde are so reactive.
Billybilly666 - 20-10-2012 at 19:00
One more point, in reality, on the context of hexamine usage, how much effect would a small amount of NaCl have on subsequent reactions? I already no
the answer is "It depends", but, why? Some measure of NaCl contamination is probable in many scenarios, and I can't imagine it would effect a
negative outcome in many reactions. Especially if present in small amounts.
Also, to revisit the gaseous evolution a bit, the clogging is most prominent in the glass tubing submersed in the receiving flask. Perhaps I could
wrap this piece of tubing with some NiCr 60 wire (21 guage) and also replace the rubber tube with just glass and wrap that as well. Then put add the
proper resistor and put around 50 volts to it on a timer of sorts. This would remove any precipitated paraformaldehyde from the tubing as well as the
glass bubbler submersed in the water. I would probably need to insulate the submersed NiCr wire somehow, plus, I wouldn't want Nickel or Chromium
contamination in my hexamine. Maybe I could place the wire inside glass tubing and then wrap the glass tubing around the bubbler with heat?
I have quite a bit of paraformaldehyde and want to build a generator and receiving station to convert it all to formaldehyde, or hexamine (more
stable) if possible. Doing it piecemeal will take FOREVER. Especially with all the problems experienced doing a simple test with the gas phase.
I have quite a bit of paraformaldehyde and want to build a generator and receiving station to convert it all to formaldehyde, or hexamine (more
stable) if possible. Doing it piecemeal will take FOREVER.
Here's what Kirk-Othmer has to say on the
subject:
Quote:
Hexamethylenetetramine is readily prepared by treating aqueous formaldehyde with ammonia followed by evaporation and crystallization of the solid
product. The reaction is fast and essentially quantitative. (*)
Your basic problem with gaseous formaldehyde is that its dew point (which includes polymerization, not just a phase change) is rather high, I'm
guessing above the boiling point of water. So while this would work, I doubt it would ever work well in standard glassware. Your idea to heat the gas
passage is the right one, since you want the wall of the passage at a temperature above the dew point. If it's not, it will always clog eventually.
But that's hard to achieve in standard glassware.
Hexamine sublimes before decomposition. Sodium contamination won't be a problem if you purify with sublimation. The problem is that you'd need to do
it in vacuum, as its sublimation point is above is flash point. Heat is reported to depolymerize paraformaldehyde in solution, though, so you might
not need NaOH to speed depolymerization if you simply heat the paraformaldehyde in ammonia solution. You'd likely need to do it under reflux, though,
to prevent escape of ammonia.
