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Author: Subject: Phenol + KNO₃ + H₂SO₄ + Cu → ???
Keras
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[*] posted on 6-1-2025 at 04:01
Phenol + KNO₃ + H₂SO₄ + Cu → ???


Folks,

I was (somewhat naively) attempting this morning to nitrate phenol using 37% diluted sulphuric acid and potassium nitrate. My hope was that, despite the low concentration, some nitric acid would be formed and would nitrate the activated aromatic cycle of phenol.

Needless to say, it failed. I let the mix warm up from -5 °C to 40 °C and really nothing happened. At the end, I was still dealing with a clear solution smelling of phenol, which probably was melting.

Given the reactivity of phenol, the only explanation was that no nitric acid was formed. So I checked by dropping a few strands of copper wire, and sure, I got nada. No bubbling, no blue colour, no nitric oxides.

And yet… something did happen, but after a while only. Unexpected. Which I repeated thus:

In a 10 mL beaker, put 2 or 3 tiny crystals of phenol (a few mg). Add 2 mL of 37% sulphuric acid, then a few mg of potassium nitrate, then three short (~ 3 mm) strands of copper wire. Put it aside and wait, say, 10 to 15 min. A rosy coloration appears near the strands, which progressively gets more intense until the beaker looks like holding blueberry juice (or potassium permanganate). There’s a faint smell of nitric oxide, but nothing visible escapes (no red fumes). The picture attached has been taken with a light source directly on the beaker. With normal ambiant light, the solution is too dark, which is surprising given the quantity of reactants I used (max. 5 mg of each).

What can it be? Nitrophenols are not red, as far as I know, they’re rather yellow/orange. So are quinones. This doesn’t look like diluted nitrous oxide. I’ll try to repeat the experiment without the phenol, just to rule out any purely ‘inorganic’ reaction.

Nothing precipitates out when put in cold water, but I assume the amount of product is too low.

[EDIT: The colour changes with the pH: it is yellow/orange at high pH, and turns cherry red at low pH. So it’s definitely some organic compound.]

[EDIT: Actually the colour changes when the compound is diluted with water. It turns from cherry red to yellow/orange, which is more something expected from nitrophenols. See additional pictures.]


IMG_3080.jpeg - 833kB

[Edited on 6-1-2025 by Keras]

[Edited on 6-1-2025 by Keras]

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Fery
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[*] posted on 6-1-2025 at 07:55


Hi Keras, the nitration of fenol runs with diluted HNO3
I did the reaction here:
https://www.sciencemadness.org/whisper/viewthread.php?tid=15...
In shortcut I dropped a solution of circa 80% phenol + 20% H2O (because it is liquid) into 30% HNO3 (because I have a lot of 53 % HNO3 so I wanted to use it, all others use KNO3+H2SO4 instead for some unknown reason for me, so I tried pure HNO3 without alkaline metal cation
The final reaction was very dark, maybe your red color is just diluted product. Addition of NaOH made all the remainders and washing all glass extremely yellow even in very diluted concentrations. I hope it could help you.
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[*] posted on 6-1-2025 at 08:07


Copper acted as a catalyst, perhaps?



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[*] posted on 6-1-2025 at 08:41


Yeah, that might be it. I will redo the experiment on a somewhat larger scale, because it was nano-scale, really. More like proof of concept. But I like the idea of copper being a catalyst for some reason.

I have tried to reduce the product with sodium hydrosulfite. It has worked to some extent. When the pH was rendered neutral after reduction, the colour turned a dirty green-grey and a small quantity of a dark powder precipitated. I tried to collect it on a Hirsh funnel, but there was not enough of it. It just stained the sintered disc. Boiling water did not remove it, nor HCl. Bleach, however, cleaned it immediately. The reduced product in solution goes to very dark grey in highly alkaline solution to yellow in highly acid solutions. There seemed to be small droplets of oil floating.

It’s really curious.
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[*] posted on 6-1-2025 at 11:33


No need Cu cat. In the link I posted there is a method using NaNO3 which could be very likely substituted by your KNO3 (it has somewhat less solubility but should work), 250 g of conc. H2SO4 + 400 ml of H2O is the same concentration as Pb battery acid (around 37%)

Quote:
https://www.prepchem.com/synthesis-of-4-nitrophenol/
A mixture of 94 grams of phenol and 20 ml. water is added dropwise to a obtained solution containing of 150 grams of sodium nitrate dissolved in 400 ml. water and 250 grams of concentrated sulfuric acid. Good stirring is maintained during the addition, and the temperature is kept below 20°C. Stirring is continued for 2 hours. The mother liquor is poured off from the tarry mixture of nitrophenols, and the tar is melted with 500 ml. water with the addition of enough chalk to make the mixture neutral to litmus. The wash water is poured off and the washing repeated. The crude ortho and para nitrophenols, freed from nitric acid, are subjected to steam distillation, using a condenser with a wide tube. About 40 grams of pure o-nitrophenol distills over. The residue in the distillation flask is cooled and filtered after standing for 24 hours. The precipitate is boiled with 1 liter of 2% hydrochloric acid and filtered through a fluted filter. The pure p-nitrophenol crystallizes from the filtrate in nearly white, long needles. The extraction can be repeated if necessary. The yield is about 40 grams each of the ortho-nitrophenol and para-nitrophenol isomers. It is bad practice to treat the crude nitrophenols with sodium hydroxide, as called for in some procedures, because the sodium hydroxide has an immediate resinifying action.
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Keras
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[*] posted on 6-1-2025 at 13:09


Quote: Originally posted by Fery  
[…]The yield is about 40 grams each of the ortho-nitrophenol and para-nitrophenol isomers. It is bad practice to treat the crude nitrophenols with sodium hydroxide, as called for in some procedures, because the sodium hydroxide has an immediate resinifying action.


Thanks for the procedure! I’m surprised though. In Vogel's, the same preparation gives only a reduced yield of p-nitrophenol, although the process is more or less the same.

Update: I was a bit bored, so I acidified the reduced (with sodium hydrosulphite) material. It turned sort of yellow. I put it in the freezer, then when it was frozen, back in the fridge for a slow melting, and got a fair quantity of very pure translucent crystals.

IMHO, this is not aminophenol but probably sodium sulphate: I don’t see how I could get so many crystals given what phenol I put it at start. No time to test tonight.


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[*] posted on 6-1-2025 at 13:30


@ keras; somewhere in my archive of papers I have references to the direct formation of azobenzene derivatives when nitrating with dilute acid. So you have probably generated a mixture of such copounds the main one probably being 4,4'-dihydroxyazobenzene. The copper does not serve as a catalyst but as a souce of nitrous acid throught the reduction of nitrate ions under acid conditions.
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[*] posted on 6-1-2025 at 22:18


@Boffis: Interesting, I hadn’t thought about that, though I suspected that copper, attacked by nitric acid would release NO which, in solution, would nitrosate phenol. Maybe the copper also reduces the nitroso compound to amine, in which case diazotisation can take place.

I did many nitrosations of phenol using sodium nitrite and I find the reaction both reliable and clean. Below 0°C it proceeds smoothly and does not produce tar, at least the way I do it, i.e. adding nitrite to the acidified solution of phenol. Reduction to amine with hydrosulphite is a breeze too.

Unfortunately, the nitrosation of phenol only produces p-nitrosophenol, and for this once I’m interested in the ortho isomer…

[Edit: What is the colour of 4-4’ dihydroxyazobenzene?]

[Edited on 7-1-2025 by Keras]
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[*] posted on 6-1-2025 at 22:45


You can make orto easily by nitration of phenol and steam distillation... extracting the para from the remainder is difficult
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[*] posted on 6-1-2025 at 23:01


Quote: Originally posted by Fery  
You can make orto easily by nitration of phenol and steam distillation... extracting the para from the remainder is difficult


Exactly why I attempted ‘standard’ nitration this time. But it failed. Even at 40 °C, nothing happened…
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