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Author: Subject: Attempt to make (CH3NH3)2Cr2O7
woelen
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[*] posted on 14-5-2019 at 05:33
Attempt to make (CH3NH3)2Cr2O7


I recently obtained some methylamine and used this in a nice experiment in which I made the perchlorate salt of this by adding it to perchloric acid.

I wondered whether it is possible to make the methylamine analogue of ammonium dichromate. Ammonium dichromate can be made easily by adding excess ammonia to a solution of CrO3 in water. The CrO3 reacts with water, forming chromic acid, dichromic acid and partially deprotonated species, derived from these. With ammonia, you get a mix of (NH4)2CrO4 and (NH4)2Cr2O7. The former is not really stable and easily loses ammonia and water, so in contact with air in the long run, only ammonium dichromate remains.

I did an experiment in which I dissolved some CrO3 in water, and then added excess methylamine. One thing, which I already noticed (and which differs from the situation with NH3), is that the liquid does not become orange or yellow on addition of methylamine. It remains dark red/brown, the normal color of concentrated solutions of CrO3 in water.

I allowed the liquid to stand in a dry warm place for a few days, free from dust. After this period, most of the water evaporated, but not all of it. I had no crystals, but a syruppy, dark brown liquid, which had a very faint smell of methylamine.

I tried drying the material further over NaOH in a closed container. But even that did not help. Some water was absorbed by the NaOH, but still, I had a syruppy brown liquid.

Next, I carefully heated some of this liquid in a test tube. On heating, you can drive off water, the liquid starts bubbling a lot. But the gas, produced by the boiling, is not only water vapor. A lot of methylamine is driven off as well. So much of it is produced, that one can ignite the gas by keeping a flame near the open end of the test tube (be careful with this boiling! Besides the smelly, but relatively non-toxic methylamine, you also get hexavalent chromium droplets into the air due to the vigorous bubbling of the liquid). At a certain point, a violent reaction kicks in, the liquid solidifies, little specs of fire can be seen above the liquid and the brown color changes to a dark green color. The CrO3 apparently oxidizes left over methylamine. The solid, produced in the decomposition reaction is not soluble in water, nor in conc. HCl.

I also tried what happens if the dark brown syruppy liquid is added to ethanol. When this is done, then it sinks to the bottom. It does not mix with the ethanol, which is an interesting observation. A big blob of brown liquid sits under the ethanol and on shaking, it breaks up but after a few minutes, the liquid collects at the bottom again. The ethanol only gets a pale orange/brown color. Next, I added some conc. HCl to the ethanol and swirled a little bit. When this is done, then a violent reaction occurs. The liquid almost erupted out of the test tube, due to formation of a lot of gas and at once the liquid is dark green and clear. In the acidic mix of ethanol and HCl, the CrO3 quickly oxidizes ethanol and in this reaction, the liquid heats up considerably and the aceetaldehyde, produced in this reaction, boils off as a gas (its boiling point only is 20 C or so).

Conclusion: Methylammonium dichromate (or chromate) is not an easily produced salt.

Maybe this salt does not exist at all. On Google I cannot find info about this salt. If the salt exists, then it is not easily prepared by simply evaporating a mix of solutions of methylamine and chromium trioxide.

I find this observation quite remarkable. Methylamine is a stronger base than ammonia, so I would expect it to be easier to make a methylamine analogue of ammonium dichromate, or even the chromate (ammonium chromate is stable, provided it is stored in tightly sealed containers, in air it slowly loses NH3 and turns into ammonium dichromate).

[Edited on 14-5-19 by woelen]


[Edited on 17-5-19 by woelen]




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zed
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[*] posted on 16-5-2019 at 15:05


Any hint of Nitromethane?
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clearly_not_atara
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[*] posted on 16-5-2019 at 16:15


I'm rather glad that you didn't discover a new way to make hydrogen cyanide.



[Edited on 04-20-1969 by clearly_not_atara]
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[*] posted on 16-5-2019 at 18:37


Woelen,

A very interesting set of experiments with some unusual observations. First, just a nit to pick. The formula of your expected product should be:

(CH3NH3)2Cr2O7 (The methylamine is protonated.)

If I may speculate, perhaps the methyammonium ion cannot form a proper H-bond network in order to crystallize. Note that the ammonium ion can potentially form 4 H-bonds and the methylammonium ion can form only 3. In addition, the bulk of the methyl group may further disrupt any crystal lattice formation. Can you find a crystal structure of ammonium dichromate which might provide some information regarding my speculation?

Again, interesting work.

AvB
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woelen
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[*] posted on 16-5-2019 at 22:41


@zed, clearly_not_atara: Methylamine is quite stable with respect to oxidation. It is not easily oxidized. So, i did not expect any redox reaction, and indeed, i did not notice anything of that, except when the liquid is strongly heated to well over 100 C, where it decomposes, just like ammonium dichromate and ammonium chromate.

@AvBaeyer: You are absolutely right about the extra proton, I changed the formula. I wrote the post somewhat in a hurry. I will have a look into crystal structures of compounds and of ammonium dichromate in particular. This may be an explanation of my observation.
Another option may be that the compound is liquid at room temperature. Some salts are liquid at room temperature, the so-called ionic liquids. Maybe ethylammonium dichromate is one of those. I came to thinking of that, because of the fact that the liquid does not mix with ethanol (denatured, azeotropic, 96% ethanol, 4% water). It forms a two-layer system. Only a very small amount of CrO3 makes it into the ethanol layer, this layer becomes light orange/brown.
Normal 40% methylamine can be mixed in all proportions with ethanol and CrO3 is soluble quite well in ethanol (giving dark brown solutions in very cold ethanol, which can be prepared if you throw in a piece of CrO3 in a bulk of ethanol). All these compounds are covalent. It might be that my liquid was ionic and that for that reason only a very small amount dissolves in ethanol.




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