Sciencemadness Discussion Board

aqua regia + ethanol

garage chemist - 16-8-2004 at 14:27

Hello everyone, I'm now finally registered at this awesome Forum.

I've got a problem with a chemical reaction that I tried without knowing much about how to conduct it properly because I found no information about it.

I read somewhere that when ethanol is added to aqua regia, chloropicrin is formed.
So I wanted to try it out. I mixed 1,5 ml of 37% HCl with 0,5 ml of 90% nitric acid (selfmade, had lots of NO2 in it) and added 1ml of Ethanol.
At first, nothing happened. Then it began to bubble a bit. The bubbling increased very fast and I realised that it would get out of control. I cooled the reaction vessel in cold water, but the bubbling increased further, so I dumped the mix into a large amount of water to stop the reaction.
Now I wonder, what was the gas that was being given off in such large amounts?
It had a very strong smell that I've never smelled before.
There was definately no chloropicrin produced, the smell was very different. (I know how chloropicrin smells)

Maybe some of you know what the gas was.

BromicAcid - 16-8-2004 at 18:16

Lots and lots of possibilities for your unknown gas.

Any of the insitu oxidizing agents formed in aqua regia; NOCl, NO2Cl, NO2, NO

Oxidation byproducts from ethanol; CH3COOH, CO2, CH2O

Nitrated byproducts of ethanol and its oxidized states. N2 but I doubt it. HCl was probably evaporated off too if the mix heated up as stated. It could have spewed off any of a number of things.

JohnWW - 16-8-2004 at 20:52

There is no way thay ethanol + aqua regia (HNO3 + HCl) could possibly form chloropicrin, which is an aromatic compound. Picric acid is 2,4,6-trinitrophenol.

The reaction is unlikely to produce ethylene, or something like vinyl chloride or dichloroethylene, which usually requires concentrated H2SO4 at 180ºC.

The most likely main products would be probably acetaldehyde and then acetic acid, in view of the usual oxidation products of alcohols, although for this Mn(VII) or Cr(VI) is usually used. In addition, the decomposition of excess HNO3 would result in some atomic oxygen and hence liberation of Cl2, which could result in some chloroacetaldehyde and chloroacetic acid.

Another significant product could be ethyl chloride, in view of the usual reaction of alcohols with concentrated HCL or HBr, although some H2SO4 is usually added for this reaction. In addition, nucleophilic substitution by the Cl- present would result in some acetyl chloride, CH3COCl, although the usual reagents for this are PCl5, PCl3 or SOCl2. This could in turn react with another ethanol moleule to form trace amounts of ethyl acetate, although an alkaline environment is usual for this to occur to any major extent.

In the same way as in the nitration of glycerol to produce the explosive nitroglycerine (glycerol trinitrate), HNO3 can also react with ethanol to produce ethyl nitrate, CH3CH2ONO2, also explosive. However, this reaction would compete with oxidation by HNO3 to produce acetaldehyde and acetic acid, and would be expected to predominate only at low temperatures (where oxidation is less favored) and in the presence of H2SO4 to take up the water produced in the reaction.

The peculiar strong smell you noticed is most likely to be a mixture of CH3CH2Cl, CH3COCl, CH2ClCOOH and CH2ClCHO.

John W.

chemoleo - 16-8-2004 at 21:08

Just as a note, I don't think CH3COCl does exist in solution with H2O. Indeed, this is one of the main precursors for acetic anhydride synthesis (acetyl chloride hyrolyses happily).

A 'peculiar smell' is unfortunately very non-descript - making it hard to discern what you got.
Nonetheless, I agree that you will probably get a mixture of things, amongst them chlorinated hydrocarbons, albeit not chloropicrin (if it is the aromatic type ;) ).
Also, I noticed that your EtOH is in excess (compared to 0.5 ml of HNO3); conditions like these almost certainly encourage the formation of many products.
If you want a clean product (whatever its nature may be) I'd better start off with an excess of oxidising acid.

[Edited on 17-8-2004 by chemoleo]

Polverone - 16-8-2004 at 22:23

HALOGENATION. XII. THE FORMATION OF CHLOROPICRIN AND TETRACHLOROQUINONE BY THE ACTION OF AQUA REGIA ON ORGANIC SUBSTANCES.
Rasik Lal Datta, Nihar Ranjan Chatterjee;
J. Am. Chem. Soc.; 1916; 38(9); 1813-1821.
Quote:
In a previous communication it has been shown that chloropicrin is formed by the action of aqua regia on several organic substances. It has now been found that whenever an organic compound breaks up destructively under the influence of aqua regia, chloropicrin is invariably produced. The chloropicrin formed is sometimes accompanied by very small quantities of other chloronitroderivatives of methane. In the case of certain aromatic compounds, tetrachloroquinone is formed as the end product of the action of aqua regia on them. As tetrachloroquinone itself breaks up to chloropicrin under the influence of aqua regia, the latter is also formed under such decompositions. The formation of chloropicrin has been noticed only in isolated cases by previous workers. Stenhouse showed that chloropicrin is formed by the distillation of nitro compounds with bleaching powder solution. Kekule obtained it by the distillation of alcohol with sodium chloride, potassium nitrate and sulfuric acid. Later on, he again prepared the substance by the action of strong nitric acid on chloral. The action of the same reagent on chloroform was found to give chloropicrin.
...

In a preliminary communication the action of aqua regia on acetone, ether, ethyl, methyl and allyl alcohols, acetic and formic acids have been studied and it has been shown that an almost quantitative yield of chloropicrin is formed in the case of acetone suitable for the preparation of this substance in the laboratory. ...


[Edited on 8-17-2004 by Polverone]

JohnWW - 17-8-2004 at 01:44

There seems to be a confusion of chemical nomenclature here, which I have just discovered - and not by referring to conventional organic chemistry textbooks. "Picric acid" is usually understood by chemists to mean 2,4,5-trinitrophenol, chlorination of which would replace one of the two remaining ring Hs with Cl, to produce chloropicric acid or true "chloropicrin".

HOWEVER, in the military, "chloropicrin" means a quite different nitro-chloro compound, namely CCl3NO2, which is trichloronitromethane, or nitrochloroform. It is not used by the military as an explosive, as its name suggests, but instead as an insecticide, a fungicide, and especially as a poisonous war gas.

To produce it from ethanol, CH3CH2OH, would require a VERY large excess of aqua regia with the reaction mixture kept at a high tremperature, so that oxidation predominates over nitration (which would otherwise form ethyl nitrate). It would have to be a sufficient excess, and at sufficiently high temperature, to cleave the C-C bond, and then both nitrate and and completely chlorinate (with some of the oxygen from HNO3 decomposition displacing Cl- as Cl2) both the -CH2OH and the CH3- parts of the ethanol molecule to give CCl3NO2.

The stated proportions of 1 ml ethanol to 2 ml aqua regia would probably be insufficient, given the amounts of reagent required, the high rate of losses as gases or vapors, and the several other competing reactions (which I mentioned in my earlier posts) which would produce other products. The stoichiometry of the reaction to produce CCl3NO2, if all the ethanol was converted to it, would be:

CH3CH2OH + 6HCl + 2HNO3 -> 2CCl3NO2 + 7H2O

Given the differing densities of ethanol and HNO3 and HCl, at least 4.5 volumes of aqua regia (if 3:1 HCl:HNO3 by mole) to one of ethanol would be required, and probably much more because of losses as gases or vapors. The competing reactions I mentioned before would otherwise predominate.

John W.

unicorn - 17-8-2004 at 01:53

Quote:
Originally posted by JohnWW
....chloropicrin, which is an aromatic compound. Picric acid is 2,4,6-trinitrophenol.


Chloropicrin is not an aromatic compound but rather, CCl3NO2. It can be made from picric acid which is most likely how it got it's name, but do not let this fool you into thinking it has a structure in any way similar to TNP.

(edit) hey never mind you got there ;)

[Edited on 17-8-2004 by unicorn]

garage chemist - 17-8-2004 at 02:05

Acetaldehyde!
Now I know what the smell reminded me of.
That was definately the sweetish smell of acetaldehyde. I've worked with it in school once.
This also explains why it began boiling strongly even when it was only lukewarm, since acetaldehyde has a very low boiling point.
I first feared that it might have produced phosgene, as it also has a strong smell, which I've never smelled before.

Polverone, i read the article you quoted somewhere and this actually led me to my experiment. Maybe I should try acetone instead of ethanol?

JohnWW, yes I wanted to produce CCl3NO2. Thanks for the equation! This helped me a lot.

What could be done to prevent the escape of the acetaldehyde, especially if it has to be kept at a high temperature?

Thanks for all the informative answers!

JohnWW - 17-8-2004 at 03:00

To reduce losses of gases and vapors such as acetaldehyde and chlorine and HCl from the reaction vessel, you could carry out the reaction in a round-bottom flask, heated in a muffle heater, to which is attached a water-cooled condenser (preferably using specially cooled water)mounted vertically so as to reflux the flask contents.

John W.

vulture - 17-8-2004 at 07:03

I don't think a reflux condenser is going to be efficient at stopping acetaldehyde from escaping. You'll need a good one (atleast dimroth or better) running with 0C (or less) water.

Success!

garage chemist - 17-8-2004 at 09:33

I've had success in producing chloropicrin from aqua regia and acetone.

I again mixed 1,5ml of 37% HCl with 0,5ml 90% orange fuming nitric acid.
I mixed 0,5ml of this mixture with 5 drops of acetone and let it stand at room temp, for a while. The solution slowly went from orange to clear, but there was no chloropicrin smell.

I then heated the mixture and it began to boil violently and emit brown NO2 gas.
I cooled the flask (it was a 10ml pear shaped flask with NS 14/23) in water, and the reaction stopped.

Then I carefully smelled the flask and there was a strong smell of chloropicrin!
It smelled exactly like the stuff I got from mixing bleach and picric acid.
I diluted the mix with water and there was a small brown drop of heavy liquid at the bottom. :cool:

So boiling acetone with aqua regia is a new synthetic route to chloropicrin!
Only drawback is that both HCl and HNO3 need to be of maximum concentration.
(I tried it before with 65% HNO3 and it didn't work)
There's also no volatile compound like acetaldehyde produced from acetone .

Samosa, this might be of interest for you.
:)