12-03-01 18:31
No 243881
Best Method to Make Methylamine Gas from 40% aqueous solution.
Background
Methylamine (MeNH2) is commercially produced for sale in three major forms:
•Compressed gas
•Methylamine hydrochloride salt
•40% aqueous solution
Handling the first two forms has been well discussed in Hive forums. However, a review of TFSE and the Rhodium chemistry page reveals unsound information regarding how best to extract methylamine gas--MeNH2(g)--from the 40% aqueous solution.
40% methylamine solution is commercially produced by the absorption of MeNH2(g) in water. Each liter of solution contains 360 g of MeNH2. This solution is not explosive, but it is flammable and should be handled with the usual care taken for toxic flammable liquids, especially regarding exposure to open flame or to an ignition source.
According to the write-up by Sunlight on Rhodium’s chemistry page, MeNH2(g) is best produced by dripping the 40% solution over solid NaOH. While this method produces gas, unfortunately it has disadvantages: it is low yielding, slow, and does not lend itself to larger scale production because a waste product containing solid NaOH and a thick solution of MeNH2 forms in the bottom of the reactor flask and cannot be easily removed. In fact, this method is almost unworkable. The person using this method is faced with the choice of either throwing away the reaction flask after use, or actually digging in and trying to extract the stinking NaOH cake and MeNH2 liquid in the bottom. Not a pleasant task.
In response to a recent post, Rhodium suggested converting the 40% solution to MeNH2•HCl (aq) by addition of muriatic acid, boiling off the water, and then reacting the dry methylamine hydrochloride salt with NaOH to generate MeNH2(g). This method works, but reacting concentrated 40% MeNH2 solution with acid is time-consuming and uses more acid plus NaOH than the method proposed here. In addition, it leaves a substantial amount of NaCl solution, contaminated with MeNH2, that must be removed and discarded. It does not allow for easy larger scale production.
Through experimentation it was determined that a third method was superior to either of the previous two. This third method is simple and was derived from experiments based on the suggestions of the members responding to a recent post on the subject. Thanks go to them and especially to Rhodium for the information provided. So that nobody will have to suffer through all the trial and error and gas inhalation SWIA endured by naively following the Sunlight write-up and other unsound posted suggestions, and in the interest of establishing clear and definitive information for novice chemists, I offer the following “best” method for converting 40% aqueous solution of MeNH2 to MeNH2(g) with 99% efficiency and in a manner that allows for larger scale production.
Discussion:
Best Method
Through experimentation it was determined that the best method for extracting the MeNH2 from the aqueous solution is to raise the temperature of the solution while stirring. Gas is produced immediately upon stirring at standard temperature and pressure and the solution begins to boil at 60ºC. Copious amounts of MeNH2(g) can be obtained by gradually increasing the temperature of the solution between 60ºC and 80ºC at normal pressure. A reflux condenser and a gas washing tube filled with anhydrous MgSO4 to pre-dry the gas and 3A molecular sieve to provide a final drying are sufficient to remove any water vapor. (Note--use of NaOH, as has been suggested in Hive posts, to dry the gas is not recommended for the following reason: NaOH will form a hard moist cake at the vapor/NaOH interface. This thin cake will eventually impede gas flow, raising line pressure, and causing a joint to pop or explode--escaping toxic noxious MeNH2 fumes will quickly render the workplace uninhabitable. This is a fact based on experience, not idle speculation, so avoid the posted suggestion to use NaOH to dry MeNH2 gas: that suggestion is unsound and although a person can “get by” in the short term, in the long term it will eventually lead to catastrophe.)
As the temperature of the solution increases to 80ºC water vapor is observed condensing in the lower half of the reflux condenser. After a time at the same temperature the production of MeNH2(g) begins to decrease. At this point the partially spent solution is allowed to cool and is pumped out of the reactor flask (a peristaltic pump is ideal) and into a plastic (HDPE) carboy for further treatment to recover the remaining MeNH2. The reactor is reloaded with fresh 40% solution (again, a peristaltic pump is ideal--if this type of pump is unfamiliar see http://www.masterflex.com) and stirring and heating are applied, as above, until the temperature reaches 80º and gas production diminishes, whereupon this partially spent solution is added to the contents of the carboy, and the reactor reloaded. In this fashion, a substantial amount of 40% solution can be processed, without having to take the gas apparatus apart, nor expose the work area to MeNH2 fumes, nor with any solid residue remaining in the boiling flask.
The partially spent solution can then be further treated to obtain practically 100% of the remaining MeNH2. This is accomplished by addition of muriatic acid according to the reaction MeNH2(aq) + HCl(aq) = MeNH2•HCl(aq). The reactor should be maintained in an ice bath during acid addition because there is substantial heat generated by acid addition. Upon neutralization, the MeNH2•HCl solution is brought to a boil, the water and any MeNH2 vapor recovered by condensation, and the dry MeNH2•HCl can then be reacted with saturated NaOH solution to generate MeNH2(g) according to the reaction:
MeNH2•HCl(s) + NaOH(aq) = MeNH2 (g) + NaCl(aq)+ H2O
There is an advantage obtained by first boiling off the major part of the MeNH2(g), prior to acid addition, since substantially less acid is consumed and substantially less hydrochloride salt is produced, therefore less NaOH is needed to convert the hydrochloride to a gas, etc., and less mess and hassle overall. In fact, a person could produce all the MeNH2 he or she might need by simple stirring and boiling of the initial 40% solution, easily unloading and reloading the reactor, and save the task of reacting the spent solution with muriatic acid for some later date.
Absorbtion of MeNH2 in MeOH
It is assumed that members reading this post understand that one of the reasons for generating MeNH2 gas is to absorb that gas in cold, stirred MeOH. By weighing the MeOH before and after gas dissolution, the amount of MeNH2 recovered can be calculated, which is required for additional syntheses. Please note that several posts on the Hive have suggested using a dispersion tube when absorbing MeNH2(g) into MeOH. That advice is not warranted and is unsound, because it will increase line pressure on the system which could lead to catastrophe. Do not follow those suggestions--do not use a dispersion tube. MeNH2(g) is readily absorbed in cold MeOH. The b.p. of MeNH2 is -6ºC, so a acetone/dry ice bath is sufficient to condense vapors--a water/ice bath will also work, if dry ice is unavailable. Besides, MeNH2(g) is absorbed in MeOH at just about any temperature short of hot. 1/2”OD polyethylene tubing from the hardware store without any sort of dispersion device on the end of the tubing is sufficient for this purpose.
Suck-back control
Suck-back occurs when the amount of gas being generated is insufficient to compensate for the amount of gas being absorbed. When the production of gas diminishes, suck-back will occur. The suck-back of MeOH with MeNH2 gas can be rapid and violent. Continual suck-back indicates that it is time to replace the spent solution in the reactor with fresh 40% solution. Suck-back is controlled by stop-cocks to relieve line pressure. However, a trap must be installed between the gas drying tube and the receiver for those inevitable times when the operator looks the other way and valuable MeNH2/MeOH solution is sucked into the system. The trap must be larger than the volume of MeOH in the receiver, so that nothing is lost and nothing can reach the reactor. If MeOH were ever to be sucked back into the hot reactor where the temperature is above the b.p. of MeOH, the resulting explosion of glassware will get everybody’s attention. But be assured that this is not possible with the described set-up.
Equipment drawing
A drawing of a gas generator follows. Two options are not shown. What is not shown is that an equalizing addition funnel can be added to the gas generator at the joint on the reactor used for the pump in/pump out connection by adding a three-way claisen adapter to the joint (if these items of glassware are not familiar to you, see the post on “best method for HCl gas” for a drawing of an equalizing addition funnel and three-way claisen adapter--or go to http://www.aceglass.com for descriptions of lab glassware). Also not shown is that a special two-way adapter with a stop-cock take-off can be inserted between the reactor and the reflux condenser to convert the vertical reflux condenser into a regular condenser by opening the stop-cock. With this addition funnel and special adapter for the condenser, muriatic acid can be added to spent MeNH2 solution to form MeNH2•HCl (aq), without changing condensers. The spent solution can then be boiled dry to recover solid MeNH2•HCl--all without taking the shown apparatus apart, thereby avoiding any release of MeNH2 fumes into the workplace. The reason a kettle is preferred over a RB flask as the reactor is precisely in order to recover solid MeNH2•HCl from the spent solution. (The kettle opens up for removal of the solid, the RB flask does not.) However, a kettle is not necessary--a 3N RB is perfectly adequate, if only liquids are being processed and the final hydrochloride solution is not being boiled dry in the RB flask. The reader can be assured that the described set-up is based on experience, not idle speculation. By pumping 40% solution in and out, a 10L reactor (kettle or RB) and 5L receiver can easily process 20L of 40% MeNH2(aq) in a day, with corresponding production of MeNH2/MeOH solution. This is more than adequate for most laboratories.
Not shown on the drawing is that the receiver flask and magnetic stirrer and acetone bath are all supported on a lab jack/stand. This allows for the receiver to be removed and replaced between batches, in order to weigh the contents and replace MeOH, without adjusting the rest of the set-up. If using a kettle, either place an asbestos or ceramic cloth pad between the hotplate and bottom of the kettle, or slowly heat the flask, in order to reduce the possibility of uneven glass expansion at the base and possible breakage. An RB flask would require a heating mantle.
Glassware should be clamped or secured to a lab frame. All standard taper joints must be secured with Keck clamps: all hoses must be fastened with hose clamps. The contents of the reactor and receiver must be stirred at all times: magnetic stirrers are best.
Many members will think this set-up too elaborate or too expensive, but should they ever be in the position of having to extract the MeNH2 from a substantial amount of 40% solution, experience will quickly teach them that anything less invites disaster. The equipment shown is based on an actual set-up and is guaranteed to work as described-- efficiently, rapidly, safely.
Conclusion
I realize that most members will never face the task of processing quantities of 40% MeNH2 solution, but I hope this information will prove useful to the few who do.
Regards
Argox
The drawing for this file and the previous hydrochloric acid file could not be posted with the machine I am using. I have sent them to Rhodium, hopefully he can post them for me.
(Newbee)
12-03-01 20:28
No 243929
Hey MaDMax--great suggestions! SWIA hasn't had to basify much MeNH2.HCl (as you can tell from his struggles with 40% solution). So what is the best way to get the condensed MeAM into the MeOH? Just make sure both flasks are -10C and pour it in?
Did you try a regular condenser, say an Ahlin, or is the metal coil (SS, right?) best. You use the vapor pressure from the MeAM generator to push the condensed liquid through the coil and up into the flask, or is one rigged above the other?
You must have wrapped your flask with tape (the stringy packing tape is best) or had it plastic coated for safety before you pressurized it. I wouldn't want newbees to think that glassware can be pressurized without any thought given to explosion. Because any nick or scratch WILL cause it to fail under pressure and all the chem guys can tell horror stories of what flying chards of lab glass can do to the human body. Tell me if you believe I'm exaggerating this point, but I think not.
Why not use a metal container or plastic (not PVC)? Off top of the old head, it would seem that polypro or polyethylene tubing (or stainless steel) could be rigged with valves and such (like the HCl generators that Rhodium posted in General Discourse a couple of days ago) to handle pressure and MeAM corrosion. What do you think? Hell, SWIA might even rig something up overnight in the shop and check it out. Thanks for the ideas, I'll pass them on to SWIA, I know he'll be excited to throw away the alcohol bubbler, which is such a pain in the ass. You're his new best friend.
Regards
Argox
(Old P2P Cook)
12-03-01 22:54
No 243976
Your HCl post was not bad but I have to disagree with a lot of what you say here. For example this:
According to the write-up by Sunlight on Rhodium’s chemistry page, MeNH2(g) is best produced by dripping the 40% solution over solid NaOH. While this method produces gas, unfortunately it has disadvantages: it is low yielding, slow, and does not lend itself to larger scale production because a waste product containing solid NaOH and a thick solution of MeNH2 forms in the bottom of the reactor flask and cannot be easily removed. In fact, this method is almost unworkable. The person using this method is faced with the choice of either throwing away the reaction flask after use, or actually digging in and trying to extract the stinking NaOH cake and MeNH2 liquid in the bottom. Not a pleasant task.
Your objections to this procedure are not valid. This is a good procedure and certainly not at all unworkable. Again, everything you say here is wrong.
And on a different point. When absorbing methylamine gas into alcohol there is absolutely no reason to use dry ice. A plain ice bath is the best choice not the poor second choice.
(Pioneer Researcher)
12-04-01 11:53
No 244136
Thanks terbium, you really know the real thing.
a) the method was taken from scientific literature but with KOH, Rhodium suggested NaOH should work, I tried it and it worked fine. I doesn't post theoretical procedures, they are from experimental works.
b) yield, weighting MeOH solution after addition of the gass, dried (not strictly necessary) trough a filled with NaOH pellets tube, is always 80-90 %, so 350 cc of aq. methylamine are enough to produce a methanolic solution to make a high yielding LabTop NaBH4 rdxn of 350 gr. of ketone, and yield based in pure ketone is always 84 + %.
c) I've never scaled it more than said, but it seems absolutely scalable. It's easy, and if you adjust the gass flow, an ice bath is enough to avoid bad smells. Absortion in methanol is very good.
d) cleaning the flask is a job of kids, take care with the caustic NaOH and yes, there are a few seconds of strong methylamine smell (like when you open the bottle), but nothing unworkable.
e) your method is similar to the one described in fester's books. NaOH is easier and faster.
(Hive Addict)
12-05-01 07:18
No 244482
This has been working well for a friend of mine:
1. calc # mol MeNH2 needed for the rxn+excess
2. calc g MeNH2 in # mol MeNH2 (1)
3. calc g MeNH2*HCl needed to produce 2 when neutralized w/NaOH
4. calc g NaOH needed to neutralize 3 (= # mol fr. 1, maybe # mol + 10% to be sure)
5. calc g MeOH needed to make an 8% soln when combined w/2
6. stir 5 on ice/salt bath, when <0C, add 3
7. slowly add 4 to 6, keeping temp <0C
8. decant methanolic MeNH2 soln fr. NaCl produced in the neutralization into pre-cooled rxn vessel
9. wash NaCl w/a little fresh MeOH and add to rxn vessel
Yes, some H2O is produced in the neutralization. This can be removed w/your favorite amine-friendly drying agent. However, my friend took the advice of an old P2P cook and ignored the water. Yields did not suffer when producing MDMA in NaBH4 or Al/Hg rxns.
z
"And if we don't get some cool rules ourselves, pronto, then we'll just be bogus too!"
(Chief Bee)
12-05-01 15:15
No 244622
I'll include this on my page after this discussion is over, and the original author has included the valid comments into the procedure description to make it as complete as possible.
(Hive Bee)
12-05-01 21:09
No 244799
Kitty has a few ideas to offer:
1.) Suck-back can be avoided by using ice-maker-sized PE tubing found at the local hardware store connected to those perfectly fitting anti-suck-back (ASV) valves found at any pet-store in the aquarium section (they are used to prevent suck-back of water into fisk-tank air-pumping systems). These contraptions are pretty inert to both acid and basic gases. They are not impervious to solvents however (DCM, Toluene). Better more durable versions of ASVs are available through plastic-products companies. Look in the index of their catalogs under "valves". These things are VERY common and have many uses.
2.) Three flasks should be used whether for a basic or acid gas set-up: The first flask producing the gas should be connected to one PE tube leading to an ASV. This is then connected to another PE tube which leads to the second flask and is submerged in the solvent. The second flask is two-holed stoppered and has an exit PE tube connected to a second ASV. This ASV is connected to another PE tube leading to the third flask (not-enclosed and open to the atmosphere) full of neutralizing solvent (for acid gas, sodium carbonate [bicarbonate foams too much]; for basic gas, water or dilute HCl). In this way, the system is totally enclosed with no danger of suck-back or gas leakage. For acid gas pumping, after the process is over (say for crystallization), connect the ASV between the first and second flask to one of those common fish aquarium air pumps--that the aquarium ASVs are actually designed for--to pump out the excess acid gas (like HCl) so that when the entire contraption is disassembled, there is no acid gas left to leak out into the atmosphere.
3.) As for the reflux condenser/aqueous 30% MeNH2-heated-and-stirred-in-a-flask set-up, this is nothing new. Fester described this same set-up in any one of his SOMM editions for use in synthesizing N-methylformamide (the MeNH2 gas was pumped directly into formic acid and refluxed). A much better way (theoretically, of course) to perform this reaction would be to have, in the flask or the gas-producing reflux set-up, a solution of NaOH (perhaps about 50%). This could be attached to a claisen adapter which could have a pressure-equalizing funnel, or any variation thereof, which would add drop by drop an aqueous solution of MeNH2-HCl to the aqueous solution of NaOH. Gas would be produced and could then be funneled into the receiving flask. Heating and stirring would be commenced to drive off the remaining gas. A reflux condenser would of course be in place to ensure that the heated water would not vaporize into the receiving flask holding the solvent. IMHO, theoretically, this is a more convenient process which takes advantage of the more ready availability of easily synthesized MeNH2-HCl, as opposed to the less available 30% aqueous MeNH2. WARNING: This process has not been tested.
4.) Last suggestion is for making a slightly aqueous (about 13% water) 10% or so dilute MeNH2 isopropanolic solution:
Add NaOH to as little water as necessary to make a saturated solution. Add this to a 70% solution of store-bought OTC isopropyl alcohol (IPA; rubbing alcohol). The NaOH aqueous phase will salt-out the upper IPA phase which will contain approximately 87% aqueous IPA. Dissolve the calculated amount of MeNH2-HCl in water to form a saturated solution. Cool the IPA-NaOH biphasic mixture and then add to it the saturated aqueous MeNH2-HCl solution. Swirl lightly while still cooling and allow enough time to elapse for the neutralization reaction and salting-out of the now basic MeNH2 to occur (anyone care to guess how long?). At the end of the reaction phase, allow the biphasic mixture to come to room temperature and then separate the phases by any means convenient to produce--in theory--a dilute slightly aqueous MeNH2 isopropanolic solution useful--IMHO--for any reductive amination using activated aluminum and alcohol solvent. This proceedure is theoretical only and has not been tested.
Hope this has been of some help.
(Newbee)
12-06-01 02:14
No 244898
I thank MaDMAx, terbium and sunlight for responding to this post. Terbium’s insights are always appreciated--when he says a post is “not bad”, is that a compliment?
However, I stand by absolutely everything in this post, with one small exception--the 40% MeNH2 solution will begin to boil at 48ºC, not 60ºC as originally stated. Everything else is accurate and sound.
Throughout the method I describe, the reader will notice that the emphasis is on production. The method I describe seems to be the only way to process larger quantities of 40% MeNH2 solution, a problem that few members will face, as I said. Describing the Sunlight method as “unworkable,” I mean on a larger scale, as can be seen from the context in which I make that statement. At the level Sunlight describes in response, 350 cc of 40% MeNH2 (aq), of course! a person could just open up the little flask, stick it under the faucet to clean it out with hot water, and endure “a few seconds of strong methylamine smell.” But opening up a much larger reactor, the smell would blast the bee and the neighborhood back into last week.
Imagine the following hypothetical scenario:
Instead of 350cc of 40% MeNH2 solution as Sunlight describes, a member is faced with anywhere from 1 to 4 commercial pails of 40% MeNH2(aq), each pail containing 17 kg of solution. At that level the Sunlight method would indeed be competely unworkable. Imagine opening up a 22L or 50L reactor to dig out the incredibly solid and tenacious NaOH/NaCl/MeNH2 cake left over from every reaction. Life is too short. If a member has ever tried the Sunlight method, they know that the MeNH2 solution sort of percolates through the NaOH. Without mixing, (and there is no viable means of mixing such a cake on the scale we are imagining here), the mixture never comes into intimate contact and too much MeNH2(aq) is left unreacted. Commercial pails of 40% MeNH2(aq) do not grow on trees, but must be paid for, probably at an exorbitant price, and so every drop counts.
However, using the method I describe and with the equipment I describe, the member facing such a hypothetical amount of MeNH2(aq) could knock it out in a good day, WITHOUT ever smelling the lovely aroma of methylamine. (Saving the task of acidifying the spent solution and boiling off water to recover methylamine hydrochloride for another time.) I mean, why would anybody invest in equipment such as peristaltic pumps as I describe (smaller flasks could simply be hand-poured), if not to increase production and avoid vapors entering the workplace. Which takes me to terbium’s criticism:
As I clearly wrote, an ice/water bath “will also work, if dry ice is unavailable.” However, imagine the case above. Bubbling such a large amount of MeNH2(g) through MeOH generates a great deal of heat in no time flat. Think of the hassle of draining water and adding more ice to the bath, trying to keep the receiver contents cool, when with dry ice, all you have to do is drop in another chunk when the initial one vaporizes. And the absorbtion of MeNH2(g) WILL BE SUPERIOR and the heat of dissolution controlled MUCH BETTER at the colder temperature provided by dry ice. But repeating from the post, “ice/water is fine, if you don’t have dry ice.”
I’ll repeat the Conclusion of my post, emphasizing two key words:
“I realize that most members will never face the task of processing QUANTITIES of 40% MeNH2 solution, but I hope this information will prove useful to the FEW who do.”
Readers should not assume that by posting this information, I have actually done anything like this. I, personally, have never worked at a large scale nor ever engaged in even small-scale production. I am a more-or-less law-abiding citizen. All I am saying is that this method would be the best method, should anybody ever face such a challenge. This method was described to me by a friend, and since I don’t have a life, but live vicariously through the Hive forums, I wrote it up and posted it. [That’s my disclaimer, and I’m sticking to it.]
The equipment drawings have been sent to the address Rhodium mentioned. Hopefully he will be able to post them. A while back I offered to donate something to pay operating expenses. Rhodium replied that donations have not been worked out yet, but that intellectual contributions are welcome. These two “best method” posts are offered to the collective in that spirit. Perhaps this information will even help somebody someday. Who knows?
Best regards
Argox
(Pioneer Researcher)
12-06-01 04:20
No 244933
Yes, you're right. If the method is scaled to big quantities may be it's better your choice, I never imagined scaling it more than 2-3 kilos in a 5 liters flask.
Anyway it's not true that part of the Mehylamine doesn't react, but you need to help with ocasional movement of swirling or changing the flask postition, but yes with bigger reactors it couldn't be controlled.
(Old P2P Cook)
12-10-01 20:55
No 246342
Instead of 350cc of 40% MeNH2 solution as Sunlight describes, a member is faced with anywhere from 1 to 4 commercial pails of 40% MeNH2(aq), each pail containing 17 kg of solution.
True, but if one could purchase 40% aqueous methylamine in these quantities then one could just as well purchase cylinders of the anhydrous material. I suppose, though, that one could postulate the scenario where these drums of aqueous material had fallen off the back of a truck and one had not had a choice of aqueous vs. anhydrous.