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Magpie
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[*] posted on 11-1-2007 at 22:04
methylamine


A search brought up a huge number of hits for "methylamine" but no thread devoted to it per se.

As mentioned earlier I intended to convert my recently synthesized acetamide to methylamine via the Hofmann rearrangement. This is a very interesting reaction thought to occur through a bridge from the alpha carbon to the nitrogen. (This is apparently the latest thinking as my older books talk about an electron deficient nitrogen and a methide migration.) The overall reaction is:

CH3-(C=O)-NH2 + Br2 + 4NaOH --> CH3-NH2 + Na2CO3 + 2NaBr + 2H2O

Attached is a picture of the apparatus I used for the 2nd step in the synthesis where the acetobromoamide (which is yellow) is being added to aqueous NaOH held at 80-90C. A lot of small bubbles were evolved. I assumed this was CO2.

Once the acetobromoamide was added the pot was heated to evolve gaseous methylamine which I absorbed in some water and then in 6N HCl. During this step I removed the stirrer hotplate and used a bunsen burner to get better heat control. When I get the methylamine hydrochloride isolated as crystals I'll post a picture.

[Edited on 13-1-2007 by Magpie]

methylamine.jpg - 121kB




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[*] posted on 12-1-2007 at 04:28


The setup is so so so nice that I would like to hug it :P.



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[*] posted on 12-1-2007 at 04:51


I have to agree with that... Lots of cred to you, Magpie :)

Anywho... You're distilling off the methylamine freebase and trapping it in concentrated HCl (aq)... It's funny that you succeed in this approach of isolating the methylamine HCl, as I've tired the very same thing, and failed... Why did I fail, you ask? Well, it seems the methylamine just didn't want to stay in the aqueous solution - it forms MeNH2*HCl and espaces the HCl (aq) in the flask in form of a fish stinking mist.

Have you experienced the same problem, and if so, what precautions did you take? If not, why are some chemical phenomena unique for my laboratory? :-P

Sincerely,
Furch
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[*] posted on 12-1-2007 at 04:53


Very nice! I see that you used the lab procedure with bromine instead of the ghetto procedure with bleach. Respect.
I am curious how you made the bromine necessary for it, and what was your sorce of bromide? Did you find OTC NaBr, or did you buy a bromide salt from Ebay?

I would say that the bubbles you saw were the methylamine gas, since CO2 cannot be evolved from such a strongly basic solution.
It could also have been nitrogen, from side reactions like oxidation of ammonium ions by hypobromite.

No amateur chemist that I have heard of has prepared methylamine this way- they always use the hexamine/HCl or formaldehyde/NH4Cl route, which are notoriously messy and are known for the very tedious workup necessary in order to obtain a reasonably pure product.

But you will also have to do a selective dissolution of methylamine-HCl from ammonium chloride, since side reactions also produce some ammonia. Inform yourself in Rhodium how to do that, or ask Organikum.

A simple test to confirm that your product consists primarily of methylamine is to heat some of the hydrochloride with NaOH solution to evolve the freebase gas and try to light it.
Contrary to ammonia, methylamine gas is flammable, and the commercial 40% aqueous solution bears a "flammable" warning sign. I tried lighting a few drops of it, and it indeed burns with a blue flame similar to that of methanol.




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[*] posted on 12-1-2007 at 09:27


I prepare bromine by mixing NaBr from spa suppliers (local OTC) with H2O2. Then in a setup very similar to the one for methylamine I carefully and slowly add con H2SO4 from a dropping funnel. This produces bromine very nicely as a distillate and all under good control. I then place it in a bottle with TFPE cap liner. I place this in my freezer and try to use it up soon as I just don't like having it around. The ratios I use are: 51.5g NaBr, 83mL H2O2 (6%), and 20mL con sulfuric. This is stoichiometric IIRC.

GC I'm sure you are right about all those bubbles. They probably were methylamine and I lost a lot that way as I didn't have my absorbtion trap set up until I had added all of the acetobromoamide. I did capture some as a gas this way, however, and presented it to family members for a quick smell. Both thought it smelled like the seafood section of our local grocery. :D

I followed my old school procedure on this but next time will be capturing the methylamine from the beginning. I don't know how much I actually captured and won't know until I weigh the methylamine HCl. The workup is by drying followed by extraction with absolute ethanol and seems fairly straightforward. I'll let you know how it goes.




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[*] posted on 12-1-2007 at 11:32


You should have added the N-bromoacetamide solution to the room temp. NaOH solution, and then heated to induce the reaction and drive over the methylamine gas.
As I read above, you should definately catch it in water (but don't stick the tube into the water, because of suckback- let the tube end a cm above the water, and swirl the water from time to time) instead of HCl, because HCl vapors would combine with the gaseous methylamine and form a smoke of methylamine-HCl instead of an aqueous solution of it. You can always neutralize the solution later.




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[*] posted on 12-1-2007 at 12:42


Garage chemist,

THANK YOU for bringit it up as well... I tried explaining that phenomenon in my previous post, but didn't get any response.

Are you positive that bubbling methylamine freebase through water (shouldn't a magnetically stirred flask of water do even better, rather than just letting it sit there and swirl it occasionally?) will dissolve the majority of it? How do you know it doesn't just drift away, out into the atmosphere? I mean, since it's invisible as the freebase... It's hard to tell.


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[*] posted on 12-1-2007 at 12:53


Methylamine is even more soluble in water than ammonia (on a weight basis), and a liter of water already dissolves 700 liters of ammonia.
Why do you think that the strongest ammonia solution you can buy is 25%, while the strongest methylamine solution you can buy is 40%?
The methylamine will dissolve vigorously in water, so much that you should NOT bubble it in because suckback will happen.
Let the hose end about 1 cm above the water surface. Magnetically stirring the water is also beneficial, but it will dissolve fine without it.




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[*] posted on 12-1-2007 at 12:59


Furch - I didn't mean to ignore your problem - I just had not seen any fog with my experiment as I used a submerged tube. And yes GC I did have some suckback but was lucky to catch it before it got very far up the condenser.

A method promoted in a more modern lab manual I have might prove beneficial here, ie, the inverted funnel, for absorbing a gas in water. This is used to absorb HCl and HBr fumes and I don't know why it wouldn't work with methylamine as well. It eliminates suckback by having a small air gap along one edge.

GC what you are say about holding the temperature down to room temp until all the N-bromoacetamide is in makes sense. The reaction is exothermic but a cold water bath would likely do the job. I wonder why my old (1962) procedure specifies holding a temperature of 80-90ºC?

[Edited on 12-1-2007 by Magpie]




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[*] posted on 12-1-2007 at 13:15


GC, thank for the reply... I'm aware of the respective solubility of ammonia and methylamine... All I'm saying is that it might need some blending before it actually *dissolves* Catch my drift? :-)

Magpie, sorry, I didn't mean to criticise you... :) Anyway, that's a great idea with the inverted funnel... What's an even better idea is a couple of glass gadgets I've got lying around... They look like funnels, but where you expect the "mouth" of the funnel to be, there is a glass filter, which finely divides the gass, making tiny tiny bubbles on the outside of the filter, increasing the reaction surface greatly.

This gadget does however not work when leading the MeAm gas through concentrated hydrochloric acid, as stated above ;)

I'll give it a go with water then.

Thanks for the replies, and good luck Magpie!


- Furch

[Edited on 12-1-2007 by Furch]
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[*] posted on 13-1-2007 at 02:26


Quote:
Originally posted by Magpie
GC what you are say about holding the temperature down to room temp until all the N-bromoacetamide is in makes sense. The reaction is exothermic but a cold water bath would likely do the job. I wonder why my old (1962) procedure specifies holding a temperature of 80-90ºC?

The reason is simple. The Hoffman rearrangement readily happens at such temperature while bellow that a 'relatively stable' solution of the deprotonated N-bromoacetamide exists. The reaction is indeed exothermic but not terribly so. However, since acetamide has a relatively low molecular mass you can have a higher molarity in the solution as you would have with other amides, hence considerably more heat is evolved that with other amides. More heat, higher temperature, faster reaction, more gas evolution and so on until a runaway. The reaction mixture could then just start boiling methylamine violently once it would get over about 70°C. The other problem is that the amine can get oxidized by the N-bromoacetamide and this is why it is best slowly adding it to the NaOH solution at the temperature where the rearrangement happens relatively fast (>70°C). This way you assure that the formed amine is only minimally exposed to the oxidation environment since the concentration of N-bromoacetamide in the reaction flask will be negligible all the time while the formed methylamine forms in a controlled fashion.




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[*] posted on 13-1-2007 at 10:51


Thanks Nicodem for that information. I agree that the heat generation was not excessive. I did have trouble reading my thermometer because of fogged adapter glass so I'm not sure I had it under the best control. I should have put some flame heat to it to unfog. Or better yet get a thermometer with a smaller range that more suits this experiment.

My procedure also says to start collecting the methylamine gas after all of the N-bromacetamide has been added. Is there any reason why it could not be collected as soon as one sees bubbles being generated in the pot?




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[*] posted on 15-1-2007 at 14:00


My efforts to obtain methylamine HCl from the Hofmann rearrangement of acetamide were not really successful. I think a fair amount of methylamine was generated but my capture and workups weren't successful.

I obtained quite a bit of solids after drying the absorbing solution on a water bath, which took hours. I then extracted this with hot absolute alcohol. Most of the solids (80%) did not extract. According to my procedure this is NH4Cl. What did extract had a yellowish color and took forever to dry on a steam bath followed by a drying oven at about 100C. Melting point of the poorly defined crystals was 190C vs the literature 226-8C for methylamine HCl. I could post some pictures but I don't see the point.




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[*] posted on 15-1-2007 at 15:15


Dimethylamine?

Quote:
Originally posted by garage chemist
No amateur chemist that I have heard of has prepared methylamine this way- they always use the hexamine/HCl or formaldehyde/NH4Cl route, which are notoriously messy and are known for the very tedious workup necessary in order to obtain a reasonably pure product.


Messy? Tedious? Not in my experience. Patience & recrystalization from alcohol yielded quite a nice looking, large flake crystalline mass with a yellow tint. Easily seperated from the NH4Cl. No problem. The yellow tinted, stubborn substance was isolated from the methylamine hcl with repeated washings & seems to be dimethylamine, as I had read elsewhere.

The methyalmine hcl now seems clean as can be, white, fishy, hygroscopic snow.

Am I the only one or what???

BTW, Magpie: You've made a very nice setup for yourself there. *green with envy* :cool:

[Edited on 15-1-2007 by Misanthropy]

[Edited on 15-1-2007 by Misanthropy]




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[*] posted on 15-1-2007 at 15:27


I've heard that dimethylamine HCl is soluble in chloroform, so you could remove it that way.
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[*] posted on 15-1-2007 at 15:32


Thanks for that info misanthropy. I had noticed when I did the orginal extraction with absolute ehtanol that a fine white "snow" had formed when the ethanol just began to cool. So, at this very moment, I'm recrystalizing the yellowish solids. This time I will separate those crystals by filtration before drying and see if that helps my melting point.

BTW if you guys want to see some nice glassware setups check out Stefan's (garage chemist) phosphous pentachloride post on versuchschemie.de. Even if you can't read German (like me) it is fun to look around.

[Edited on 15-1-2007 by Magpie]




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[*] posted on 15-1-2007 at 19:38


The yellowish solids were recrystallized in aboslute alcohol, Buchner funnel separated from the yellow mother liquor, and washed with a little cold ethanol. They were white as the driven snow and I was very hopeful (see photo). So I dried them for an hour or so at 100C them determined the melting point. It was in excess of 250C. :( I then place some of the powder in a mL of water (it dissolved instantly), checked pH (it was 6) and then added 10% NaOH to a pH of 8. There was no smell of methylamine, even after heating. So I have to chock this one up as pretty much a failure.

fine white snow.jpg - 13kB




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[*] posted on 15-1-2007 at 20:00


Could pH 8 not be high enough???

What else could the stuff be? - even if NH4Cl it should smell when basified, right?
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[*] posted on 15-1-2007 at 20:26


I added some more crystals and brought the pH up to 10. I now get a faint smell of ammonia. But that is supposed to decompose at 220C. Its times like this that it would be nice to have an FTIR.



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[*] posted on 17-1-2007 at 07:56


Very strange!
Did you check if the HCl you used for neutralization was pure? Does it leave a residue upon evaporation?
Did you boil the reaction solution to expel the methylamine gas?
Did you start collecting the gas from the beginning, as it is most advisable?

You could extract the white solid again with hot 99% IPA, ammonium chloride ought to be insoluble in there as well and it is not so important to have anhydrous solvent and solid as it is with ethanol.
The IPA extract is then reduced and allowed to crystallize.

[Edited on 17-1-2007 by garage chemist]




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[*] posted on 17-1-2007 at 09:58


Yes this experiment produced some expected and some unexpected results. My major mistake was probably not collecting the methylamine gas from the beginning as I was just blindly following the cookbook. Yes, I did boil the basified methylamine HCl. My HCl is just muriatic acid so is likely not the purest. Someday I will do this experiment again to see if I can get good results. Thanks for everyone's comments and suggestions.



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[*] posted on 17-1-2007 at 10:42


You might try the well known hexamine/HCl or formaldehyde/NH4Cl method if it turns out that nothing useful can be obtained from your white solid (but don't throw it away! Try to purify it first).
I have reviewed the Rhodium methods and found that it isn't that difficult to get a good product if IPA is used for the extraction instead of ethanol.
The yields are not so good (around 50%) but the precursors are cheap and OTC.




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[*] posted on 17-1-2007 at 17:07


......This gadget does however not work when leading the MeAm gas through concentrated hydrochloric acid, as stated above ......

Works for me, nothing wrong with a little MeNH2.HCl fog in the
flask. A wide bubbler tube with a large volume just below the surface prevents suckback.

The 15% HCl solution has a pH Strip floating in it.
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[*] posted on 17-1-2007 at 17:12


bio2 Wrote:
The 15% HCl solution has a pH Strip floating in it



Duhhhhh


All those pH indicators leach out of the strip in 30 seconds and you are left with just a Strip

[Edited on 18-1-2007 by Maya]

[Edited on 18-1-2007 by Maya]




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[*] posted on 17-1-2007 at 17:27


Not the ones I use, Macherey-Nagel

They can be rinsed and stored in distilled
water and reused a couple few times as well.
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