Sciencemadness Discussion Board - Green Solution

Green Solution

Hawkguy - 14-6-2015 at 14:57

I've noticed a green colour emerging after runaway nitrations, specifically regarding Picric Acid and Nitroglycerin. The colour is very light green present in the solution after the reaction, and it is (sometimes?) destroyed by adding water. Any ideas?

[Edited on 14-6-2015 by Hawkguy]

Hennig Brand - 14-6-2015 at 16:53

The most likely common link responsible is the sulfuric acid you are using which could have a contaminant causing the unusual color. I have noticed the same thing myself at least a few times and was never really sure exactly what was going on. It never seemed to cause any problems as far as I know. If it is drain cleaner sulfuric acid you are using it often has already been used to clean metals and things industrially from what I understand.

markx - 14-6-2015 at 22:29

Most likely metallic impurities from acids or tools (if stainless mixer is used e.g. dissolved Fe, Ni, Cr can all impart a green coloration) that change oxydation state or form complexes due to the released NOx or due to dilution if water is added. Organics can't be ruled out completely, but since the coloration emerges with different nitration precursors I might be inclined towards the option of "metallic contaminants from the acid"....

Microtek - 14-6-2015 at 22:56

It might also be HNO2 from partial reduction of the nitric acid. Solutions of NOx/HNO2 often takes on a light blue colour which could appear green, particularly in conjunction with some of the yellowish run-away products.

kecskesajt - 14-6-2015 at 23:05

It is also noticable after ethanol amd isopropanol nitraton.I'm using technical(not plumber grade H2SO4).Maybe nitroform or TNM.

PHILOU Zrealone - 15-6-2015 at 13:58

Quote: Originally posted by Microtek

It might also be HNO2 from partial reduction of the nitric acid. Solutions of NOx/HNO2 often takes on a light blue colour which could appear green, particularly in conjunction with some of the yellowish run-away products.

I would go for this explaination because I have noticed the green colour while nitrating toluene with only lab grade 69% HNO3 in a glass jar with a plastic (thus no metals presents)!
Nor toluene, nor HNO3 had a yellow or green tinge (colourless at first).
The green colour developped with time and with NxOy.
It disappear when dilluted or when let open for a big while.

In many occasions organic nitroso compounds do form colorized dimers...

GrayGhost - 15-6-2015 at 16:00

Long, long time ago I´m preparate picric acid from nitric diluited, benzene ( was OTC in thats times) and mercury nitrate like catalyst.
Was yellow colour and making stain yellow my finger several days.

PHILOU Zrealone - 18-6-2015 at 12:22

Quote: Originally posted by PHILOU Zrealone

Quote: Originally posted by Microtek

Here is an overview of a time-lapse sequence of pictures of a mix of 1/3 toluen and 2/3 of HNO3 69%, both lab grade. This shows the special green color.

A closer look at the first and last pictures:

The last picture is emerald green, but there is no metallic contamination possible ... so the colour can only come from nitroso compounds.

Conclusions:
In books and at school and at university we are all told that aromatic nitration only occure when NO2(+) is present and for this that concentrated nitric acid is needed or concentrated sulfuric acid with nitrate salt or less concentrated nitric acid.

Here the experiment shows clearly that at contact interface without heating and in presence of a lot of water (31%) there is immediate reaction between HNO3 and toluen with natural diffusion.

--> Nothing is better than experimentation even if books says otherwise!

Formation of nitrotoluens is proven by the bitter almond smell and orange-brown colour.

Big surprise is observed by apparition of yellow-orange crystals....and after a year or so the organic upper layer is completely waxy-solid wetted by nitrotoluens.

By melting point comparison with known melting point compounds it was shown to be more probably para-nitrobenzoic acid.
Thus a one pot synthetic pathway to that compound

...

Note that nitration of benzoic acid needs concentrated acids and yield meta-nitrobenzoic acid.

Further investigation must be done about yield and proportion of the various products and purity.

biomechem - 3-3-2017 at 13:19

A friend of mine who is not a chemist at all told me about one weird method. He weighted about 40 ml of toluene, 50 g of KNO₃and 50 ml of H₂SO₄. He put a KNO₃ over toluene in the fridge, and separately sulfuric acid. When everything was chilled, he kept adding acid in 5 portions, but after second one the mixture was already nice orange and after mixing it started boiling. After third addition everythink turned dark orange and the mixture was boiling and fuming. This was the moment he knew he did a mistake and used 2x too much acid. After about a half an hour he added water and wet organic layer
was about 50 ml.

In his second attempt he used more toluene than needed, which gave him pale yellow sln. After purification by distillation he recovered toluene, and oily yellow sln remained in the flask. Product was then separated by cooling in a fridge. He received almost white crystalls of p-NT (and probably DNT), and oil.

Unfortunately he didn't take any photos, what's worse he couldn't check how pure the product is and what are the sideproducts. What do you think about this method? It seems that yields are almost quantitive, moreover rxn is fast, and cooling seems to be not neccesary.

Do you think that cooling it in dry ice added in the rxn flask is a good idea, or it can promote side-reactions?
And the most important question, does another experiment made in anhydrous conditions denies "classic" mechanism of the rxn?

[Edited on 3-3-2017 by biomechem]