I was going to try my hand at making a small batch of RDX -- so of course, I needed some hexamine. I was running low on formaldehyde and wouldn't be
able to get more before the weekend was over, so I bought some of those hexamine tablets. Extraction seemed simple.
I ground a few up in a mortar and pestle (50g) and dissolved in 200mL distilled water. Threw in a stir bar and let it all dissolve on low heat (30C)
for about an hour just to be sure. The wax coating was clearly visible floating at the top, so I filtered it off with a Büchner funnel and some
filter paper to recover that oh-so-fishy hexamine + 1,3,5-trioxane solution.
Realizing the volume was a bit overkill, I decided to boil it down for recrystallization. Placed the flask (containing the hexamine+trioxane sol'n)
back on the plate w/ stir bar and heated until it began to boil. I set this up with a simple distillation apparatus, just in case I messed something
up and needed what I boiled off (but mostly because the smell is repulsive).
Got called away to attend to something that couldn't wait (insurance claim) and ended up forgetting about the whole thing for several hours. When I
returned, the thermometer was reading 40 C and the contents of the pot were orangey-brown, about the consistency of water, and contained some chunky
residue at the bottom.
Can anyone tell me what happened? Obviously, I need to pay more attention (or shut everything down if I need to leave for more than a few minutes),
but I'm mostly wondering what the hell I have here in my pot. The stuff still reeks of that fishy odor, but AFAIK a solution of hexamine and trioxane
wouldn't be that color?
Glassware was freshly cleaned and dried, so no contamination there. The original solution was filtered twice, just in case some wax had made it
through the first pass.
I'd like to try this again, but want to understand what went wrong (besides my own stupidity/forgetfulness) -- and also, what reaction(s) may have
taken place to leave me with this brown liquid.
*Note: thinking I'd perhaps just evaporated the solution way, way too far, I tried adding 150mL water back into the mixture. All I got was a lighter
shade of brown, not the off-white of the initial solution.Texium - 4-10-2021 at 09:37
Apologies if this is in the wrong forum, I'm new here. And to be clear, I'm asking from pure curiosity -- I have absolutely no intention of making
RDX. Ever.
Ha. Hahaha.wmacturk - 4-10-2021 at 09:49
Okay, okay, I should have said "I have absolutely no intention of making RDX in significant quantities with the intent to cause injury, structural
damage, or for any purpose other than good 'safe' fun in small amounts."
You caught me. Fyndium - 4-10-2021 at 12:49
Just remotely within context, if one were to reflux/heat polyacetal in strong ammonia solution, could it generate hexamine, as the basic ammonia
decomposes the polymer and the formaldehyde reacts with it?Monoamine - 4-10-2021 at 16:13
Well, do you know if the temperature was really evenly distributed? What temperature was the hotplate itself. If you just have the thermometer in the
solution, but the hotplate is very hot then you can still burn things on the bottom of your vessel (like in cooking ). Also, there could well be some insoluble impurities that sank to the bottom and
burned. Maybe try using a steam bath, since you're only heating to 40C?
In terms of the extraction, looking at the structure of hexamine, maybe you can try to de-polarize the molecule a bit by putting on some non-polar
protecting groups on the amines? Then you could get rid of any polar impurities by extracting into a non-polar solvent. Here's a paper on how to put
an acyl protecting group on amines by only using acetic acid and ethyl acetate:
Sharley, Daniel D. Sanz, and Jonathan MJ Williams. "Acetic acid as a catalyst for the N-acylation of amines using esters as the acyl source." Chemical
Communications 53.12 (2017): 2020-2023.
I think de-protection would be done by refluxing in HCl for a long time. Tsjerk - 5-10-2021 at 02:45
Hexamine is not going to like any heating with a strong acid around.wmacturk - 9-10-2021 at 20:58
Yeah, there was no strong acid around. Just hexamine, water, and likely a tiny bit of the wax that wasn't completely removed. Turns out the tablets
I'm using are pure hexamine plus what's listed only as "wax" on the MSDS (<0.1%). I assumed trioxane as it's often found alongside hexamine in this
kind of fuel tablet.
@Monoamine, fair, I don't know what the temperature at the glass interface was. I was just heating it on a hotplate (would have to go check the
name/brand/model, but it's the basic hotplate+stirrer you'd find in any intro organic chem lab with two knobs on the front marked 1-10). The heat knob
was set a little below 4, and the flask was insulated by wrapping in a few layers of foil.
Given that I now know there was no trioxane in the tablets, I'm almost more confused. I'm sure it wasn't *pure* hexamine and water -- there was
undoubedly some amount of residual wax plus possibly a tiny amount of whatever I ground up in that mortar+pestle previously, if somehow I forgot to
wash it as I normally do. But hexamine melts at something like 280 C and then sublimes, so I'm wondering what really could've happened.
In any event, I'm repeating the procedure now with brand-new, clean glassware, and tablets which were dissolved slowly over several hours just by
submerging them and cranking up the stir bar (in case the mortar and pestle had some residual somethingoranother on them). It's 1AM here, so I won't
get drawn away and distracted Tsjerk - 9-10-2021 at 23:11
). Also, there could well be some insoluble impurities that sank to the bottom and
burned. Maybe try using a steam bath, since you're only heating to 40C?