Sciencemadness Discussion Board

Dimethylamine identification help

LD5050 - 21-6-2017 at 18:33

I made dimethylamine using this procedure I found on a quick search through google:

To 75 g (1.03 mol) of dimethylformamide in a 500 ml roundbottom flask was slowly added, portionwise, 160 g 6N aqueous hydrochloric acid. Caution, this was a very exothermic process! The flask was fit with a condensor and the mixture was refluxed for 7 hours. Dimethylformamide hydrolyses to dimethylamine and formic acid, so the fumes were led outside in case some carbon monoxide formed due to decomposition of HCOOH.

The mixture was vacuum distilled at the fridge pump until a white mushy solid was left in the distilling flask. This was sucked as dry as possible on a Buchner funnel and transferred to a dessicator, where it was dried over CaCl2 for a couple of days. There was obtained 59.3 grams of dimethylamine hydrochloride as sparkling white crystals, which is a yield of 70% .

Everything went according as planned. I ended up with a white crytally powder that was VERY hygroscopic. I recrystalized from MeOH and long needle like white transparent crystals formed. The literature claims dimethylamine HCL melting point is 168-176c. I performed a crude melting point test by adding some crystals to a test tube throwing in a thermometer and placing on my hot plate. I got a M.P. Of around 140-150c which is quite low even for my makeshift M.P. Apparatus.

I'm wondering if there is any other tests I can perform to positively identify that what I have is infact Dimethylamine. Searching on google I can't find much on solubility and other properties of dimethylamine. Any help or information on this would be really appreciated thanks.

UC235 - 21-6-2017 at 18:59

Unless you didn't start with DMF, what you have is Dimethylamine HCl. Sounds like a combination of your terrible melting point determination and possible water contamination, which can dramatically depress melting point of hygroscopic salts. Try using a stirred beaker of mineral oil to slowly heat the thermometer and sample side by side.

alking - 22-6-2017 at 08:20

What he said. 150C is pretty close imo under the circumstances, it's highly unlikely you have anything else just based on the fact that you yielded a crystaline product at all. Combine that with the description and seeming uniformity of the crystals, their hygroscopic nature, and the mp I'd say it's all but confirmed.

Melgar - 23-6-2017 at 07:22

Also, you don't have to worry about formic acid producing carbon monoxide unless you're adding it to a very strong dehydrating agent. The only ones I know for sure will do it are P2O5 and H2SO4. The more energetically favorable decomposition is to CO2 and H2, which palladium catalyzes, for example. And of course, any oxidizers will yield either performic acid or carbonic acid (and then probably rearrange to H2O and CO2).

AvBaeyer - 23-6-2017 at 16:14

I have made dimethylamine hydrochloride by the OP route many times. The crystalline product is extremely difficult to dry completely. Calcium chloride is not good enough. I dried my product in a dessicator using both NaOH and H2SO4 (separate containers, of course!) as drying agents. This works quite well but the time it takes depends on how much water is in the initial product.


LD5050 - 23-6-2017 at 21:02

Ya I can't believe how much this stuff likes to suck up water from the air, I mean it's almost instantaneous. I dried mine pretty well by just puting it in a glass bowl with an air tight lid with a little cup full of silica beads inside.

woelen - 10-1-2022 at 03:57

I have dimethylformamide. I read about synthesis of dimethylamine with that, using HCl and heating for several hours, to get a solution of dimethylammonioum chloride. This seems to be very hard to purify and dry. This thread also demonstrates that.

Why not add an excess amount of a concentrated solution of KOH or NaOH to the DMF and heat this? I tried this on a small scale and it seems to me that this works very easily. The solution quickly starts bubbling and a lot of gas is produced. From 2 ml of dimethylformamide, I could maintain a flame for several minutes, using this method. I did not collect the gas in water for this test, I just wanted to see whether it produces dimethylamine easily in a steady flow of gas. The gas has a strong ammonia-like smell, albeit a little less pungent, and at lower concentration it has an added smell of stale piss as well. I burned the gas at the end of a glass pasteur pipette, just to avoid having the smell of the gas all around the house. This almost certainly must be dimethylamine.

Why is this method not used? It looks much simpler to me than the hassle with the extremely hygroscopic dimethylammoium chloride. Simply lead the gas through a large column of ice cold water to get it dissolved. Does this reaction with NaOH or KOH produce a lot of side-products, which also escape as gas and make the product impure?

I also can obtain N-methylformamide (this has only one methyl group) for a fairly low price. Can this equally easily be used to make monomethylamine gas with NaOH or KOH?

[Edited on 10-1-22 by woelen]

karlosĀ³ - 11-1-2022 at 02:05

I made dimethylamine both with acidic and basic hydrolysis, I preferred the basic one after a while.
The acidic actually can produce an almost dry salt if you quickly store it away, in fact I was surprised I had a few years old slightly mushy bottle of dimethylamine HCl made that way.
Carbon monoxide gets produced though.

The yields are fine for both methods, more or less in the same range of around 70%, but one involes hours of heating and the other an hour or two of closely monitored, well focused on, gassing.
Depends on personal preferences, I guess.
I actually did it in three single gassing episodes when I made a half mol of dimethylamine in ethanol, because it was so annoyingly slow and I really did not wanted to ruin it or gass everything except the solvent with the amine.
It does not smell much at all, though, that was great.
But I used ice cold solvent directly from the freezer, standing in an ice bath.

woelen, yes, N-methylformamide can be used for the production of methylamine and in fact I believe that OP already did this a year or two after starting this thead.

By the way, I wonder if dimethylamine oxalate would be a nicely isolateable salt is, dry and not hygroscopic...?

woelen - 12-1-2022 at 05:04

I think, I prefer the basic route. The material does have a smell for me, but I do not perceive it as that bad as some people make you believe it is. Quite smelly, but I know more nausiating smells.

I wonder, why the yield only is 70%, especially in the basic route. If excess NaOH is used, don't you simply drive off all dimethylamine and keep formiate ions in solution? Or is there some side-reaction, in which 30% goes, forming non-gaseous products, which remain in solution?

woelen - 24-3-2022 at 02:01

I now also have N-methylformamide and compared that to N,N-dimethylformamide.

N-methylformamide has a faint sweet smell, N,N-dimethylformamide has no smell at all (maybe a slight pissy smell, due to impurity of dimethylamine).

I mixed both chemicals with a concentrated solution of NaOH (appr. 2 ml of liquid, mixed with 3 ml of concentred lye).
- N,N-dimethylformamide requires gentle heating to produce gaseous dimethylamine. The reaction is quite smooth.
- N-methylformamide decomposes even without heating. Small bubbles of methylamine are produced. Gently heating makes the reaction faster and quite a strong flow of methylamine can be produced with just gentle heating.

So, now I can make both methylamine and dimethylamine easily. The only thing I still do not know is why the yield would only be 70% for this type of reaction, as mentioned in previous posts. Is this really true? I would expect the reaction to have a yield of nearly 100%, albeit that at the end the reaction probably slows down.

I also noticed that dimethylamine and methylamine have very similar smells at higher concentrations. These gases are not as pungent as ammonia, but still quite irritating at higher concentrations and having an ammonia-like smell. At low concentrations, I perceive a difference in smell. Dimethylamine smells like stale piss, while methylamine has a more fishy smell. I do not perceive the smell of these gases as really nauseating, but having these smells around your house is not what you want. So, making larger quantities of the gases and dissolving it in water requires some care, to avoid having the neighbourhood smell of piss or decaying fish.

[Edited on 24-3-22 by woelen]