Sciencemadness Discussion Board - p-nitrosophenol purification

p-nitrosophenol purification

Keras - 10-4-2022 at 23:22

Folks,

I attempted yesterday to synthesise p-nitrosophenol (path to paracetamol) from phenol and sodium nitrite + HCl. Modus operandi adapted from Vogel in experiment 6.103:

To x mol of NaOH dissolved in a liberal amount water (100 mL in my case) are added x mol of phenol with constant stirring until all the phenol dissolves (in my case, I used only a couple of grams of each). The solution is then brought to near 0°C using an ice bath and added ice cubes. Sodium nitrite (1.1x mol) is then added, whose endothermic dissolution helps cool the mix further. Then a dropping funnel filled with 6% HCl (1.3x mol, in my case 50 mL) is set up and the acid is allowed to drip down at a low rate, while keeping the stirring on (800 rpm) and the temperature always below 5 °C.

With the first drops of HCl the liquid, which is so far clear and colourless, acquires a yellow tinge. This pale yellow colour doesn’t evolve much until the solution reaches a neutral pH, after which it steadily increases in intensity until it becomes dark brown. At the end of the reaction, small floating blotches of dark liquid can be seen sticking to the beaker.

After all the HCl has been added, the beaker is taken out of the ice bath, and, with continued stirring, left to warm for ½ hour or so.

It is then filtered at the pump, resulting in a mass of dark brown powder and a very dark collected liquor.

Question: is this supposed to be pure p-nitrosophenol? Or a mix of product + tar?

This compound doesn’t seem to be widely used, and it is even difficult to know what colour it is supposed to be. Purification of Laboratory Chemicals says it gives yellow crystals from xylene. I thus attempted a hot recrystallisation with boiling xylene. Apart from clogging my Büchner filter (which I had to clean afterwards with piranha solution to get back to whiteness), I got only a disappointed result: there was, it is true, a paltry yellow deposit on the walls of the flask, but there was also a lot of what I describe – for want of anything better – as brown powder. When diluted in cold xylene, the whole thing appears dark red, but when wiped out on a paper towel sheet, it leaves definitely yellow stains.

At this point, I have dissolved what I could in a small beaker (25 mL) of xylene, decanted it (and saved the rest), and I plan to let it evaporate at r.t. just to see what comes of it. I was thinking to attempt steam distillation, but it is mentioned nowhere, contrarily to 2-nitrophenol. I could also try TLC to assess the purity of the product (with xylene as a solvent, I suppose).

If anyone has any experience with p-nitrosophenol, I'd gladly hear from them. Vogel says about 1-nitroso-2-naphtol that the nitrosation reaction gives almost the pure product, which is yellow but then turns to dark brown upon drying, so I might simply be trying to flog a dead horse. Any help appreciated!

PS: As mentioned, the product stains filters and beakers as much as iodine does. Fortunately, it is very soluble in acetone. I suppose bathing the funnels in water for a few hours would've been enough to clean them up, but I was in a hurry, so I used piranha solution. By the way, for those who, like me, do not want to use up precious hydrogen peroxide in cleaning efforts, sodium percarbonate + hot sulphuric acid works very well, too.

[Edited on 11-4-2022 by Keras]

Tsjerk - 11-4-2022 at 00:25

I don't know if it helps, but here is some discussion on this preparation:

https://core.ac.uk/download/pdf/29155005.pdf

The brown stuff indeed seems to be tar, and the formation of it doesn't seem to be necessary.

Keras - 11-4-2022 at 02:56

Quote: Originally posted by Tsjerk

I don't know if it helps, but here is some discussion on this preparation:

https://core.ac.uk/download/pdf/29155005.pdf

The brown stuff indeed seems to be tar, and the formation of it doesn't seem to be necessary.

Thanks!

Eventually, I found out that, due to the tautomerism nitroso- ↔︎ oxime, 4-nitrosophenol is soluble in alcaline water. (NOH → NO⁻, probably). That could be a good way of purifying it from tar.

Fery - 11-4-2022 at 09:24

Hi Keras, all methods use H2SO4. Is there any advantage in using HCl as you did?
In the pdf from the link posted by Tsjerk, see the bottom of page 19:

Quote:

When nitrosophenol was boiled with dilute aqueous hydrochloric acid, the nitroso group was replaced by a chlorine atom with the formation of chlorophenol, which could be recognized by its odor.

You performed the reaction at +5 C, not at boiling temperature. But couldn't the side reaction occur at least at some extent even in the cold?

Keras - 11-4-2022 at 12:15

Quote: Originally posted by Fery

Hi Keras, all methods use H2SO4. Is there any advantage in using HCl as you did?
In the pdf from the link posted by Tsjerk, see the bottom of page 19:

Quote:

When nitrosophenol was boiled with dilute aqueous hydrochloric acid, the nitroso group was replaced by a chlorine atom with the formation of chlorophenol, which could be recognized by its odor.

You performed the reaction at +5 C, not at boiling temperature. But couldn't the side reaction occur at least at some extent even in the cold?

Good point. I used HCl just because the nitrosation takes places as long as the nitrous acid forms, which doesn't require any particular acid besides a strong one. I didn't think about side reactions. I’m going to think about it, thanks about the suggestion!

[Edited on 11-4-2022 by Keras]

kmno4 - 11-4-2022 at 13:44

You may read this:
http://www.sciencemadness.org/talk/viewthread.php?tid=12677

Keras - 11-4-2022 at 21:18

Quote: Originally posted by Keras

Think of it, it is unlikely that the chlorine ion competes with the nitroso group for electrophilic substitution, given that the phenol ring is highly nucleophilic, NO⁺ is very electrophilic and Cl⁻ rather nucleophilic.

I would expect any substitution NO ↔︎ Cl to happen in a second time, because the combined effects of OH and NO group balance each other. But even then, I’m very surprised that the NO “detaches” from the ring. There must be some sort of specific mechanism at work here.

Fery - 12-4-2022 at 03:24

I'm not good in theory, but p-nitrosophenol is in equilibrium with quinoneoxime, they are tautomers. If the first one does not react by a side reaction maybe the second one does. Strange that you got so discolored product. Chemists in the link posted by Tsjerk got product of quite good quality. But they used H2SO4, not HCl. They also explored various concentrations, temperatures etc.

Keras - 12-4-2022 at 07:55

Quote: Originally posted by Fery

I'm not good in theory, but p-nitrosophenol is in equilibrium with quinoneoxime, they are tautomers. If the first one does not react by a side reaction maybe the second one does. Strange that you got so discolored product. Chemists in the link posted by Tsjerk got product of quite good quality. But they used H2SO4, not HCl. They also explored various concentrations, temperatures etc.

Alright, I will redo the experiment ASAP using sulphuric acid and see what comes of it. I retain the usage of an alkaline solution to dissolve the product, should it be extracted from tar.

Keras - 2-5-2022 at 12:40

Gah. This is really disappointing. This weekend I tried:

• A 0,1 mole PhOH experiment in, say, 600 mL water kept under 10 °C, nitration using 37% sulphuric acid. I got a ton of fluffy brown, dirty powder. Seems to be a bit less messy that with HCl, which tends to confirm that there may be side reactions with the Cl⁻ ion, although I don’t see how the aromatic ring of p-nitrosophenol could be electrophilic (IMHO, the withdrawing effect of the -NO group doesn’t offset the activating effect of the -OH one).

But okay. This was anyhow far from the yellow crystal the literature mentions. So I thought I could maybe steam distill it. Alright, I took some, put in a 250 mL round bottom flask together with, say, 200 mL of water and Bob's your uncle.

First 100 mL or so I got a nice, clear, yellow solution (no crystals). Figured out I could evaporate the water to get the product → nothing was left at the end (it probably co-distilled again with the water vapour, though I didn't push the temperature over 80 °C). Dud #1.

Meanwhile, I had the second batch of steam distillation running (I managed to leave like 10 mL of water in the r.b.f, just to ease the cleaning). When I returned to it, wow, surprise: the water I'd collected wasn't yellow anymore. It had turned a deep, beautiful blue, as if indigo had distilled instead of p-nitrosophenol. I transferred it into an evaporating dish and put it on a window sill exposed to the sun. At the end of the day, it had lost its lustre and become blue-grey. It’s still not fully evaporated, so we’ll see. Meanwhile, I also left the other portion containing the original deep blue water to evaporate in the beaker it was in. Both are in a cool place, away from direct sunshine. I still have plenty of dirty crystals.

• Retry #2. I thought the problem was in the concentration. So I tried again with the same quantities, but using a 2 L beaker, at first half-full, and diluted HCl. Here too, I regularly added chunks of ice in the solution so as to check the temperature under 5 °C. At the end, I got a 1,8 L red-wine coloured solution. I was quite embarrassed what to do with it. I took a sample and tried to extract it with dichloromethane. Alright, most of the colour went into the organic phase. Let it evaporate → no crystals, only an oily tar. Dud #2. With the rest of the beaker, I ended up with the silly idea of boiling it to reduce the volume of water. Bad luck: the colour deepened as the solution warmed, and when it began boiling it was almost black. Besides, it almost overflew, so I got fed up and ditched everything. Silver lining: the stirring hot plate I just bought can boil a 2L beaker full of water while being outside with temp < 10 °C.

• Retry #3. I was being annoyed with throwing away not large, but still significant quantities (~ 10/20 g) of both phenol and sodium nitrite, so I decided to go miniscale and redid the experiment with sub-gram quantities of material (0,01 mol Phenol) and 50 mL water in a 250 mL beaker, using about 10 mL of 37% sulphuric acid. Solutions had been either cooled overnight in the fridge (for NaOH + PhOH) or in the freezer (for the acid). Added circa 0,9 g of NaNO₂ and dripped the acid (good thing, since I love miniscale, I have a 10 mL dropping funnel which did wonders) while stirring with once again temperature under 10 °C all the way (ice cubes added). Procedure took well over one hour. At the end of it, about 180 mL of whisky like liquid. Extracted with three portions of dichloromethane, which resulted in the water being quite discoloured, and the CH₂Cl₂ almost pitch black. Strangely enough, blobs of dichloromethane inside the sep looked like crystals. Anyway. Let the dichloromethane evaporate → nothing much. Maybe some yellow crystals, but really very few of them, all covered in tar → bin. Dud #3.

At that point, I stopped. I’m wondering if what was sold to me a sodium nitrIte isn't in fact sodium nitrAte (or maybe it got oxidised), which would explain why I get tarry substances, the yellow product would be o-nitrophenol and maybe the blue one p-nitrophenol?? However, Wikipedia states all nitrophenols are yellow, but Vogel (p. 978/979) speaks of ‘almost colourless crystals'. So I’m once more at a loss here, any further suggestions welcome. Also, I took some pictures, I can post them if requested.

[Edited on 2-5-2022 by Keras]

Fery - 2-5-2022 at 21:44

o-nitrophenol is yellow
p-nitrophenol colorless (if pure enough)
if your nitrite was in fact nitrate you could produce nitrophenols instead, in that case steam distillation would separate o-nitrophenol (volatile with steam due to intramolecular H...O bonding) while p-nitrophenol stayed in solution in the distilling flask
https://www.sciencemadness.org/whisper/viewthread.php?tid=15...

Keras - 3-5-2022 at 04:29

Quote: Originally posted by Fery

o-nitrophenol is yellow
p-nitrophenol colorless (if pure enough)
if your nitrite was in fact nitrate you could produce nitrophenols instead, in that case steam distillation would separate o-nitrophenol (volatile with steam due to intramolecular H...O bonding) while p-nitrophenol stayed in solution in the distilling flask
https://www.sciencemadness.org/whisper/viewthread.php?tid=15...

Well, steam distillation ended up, as I said, yielding a clear yellow solution, but I suppose I had put too few crystals into the water for the yellow product to crystallise out from the distilled water. I still have a bunch of that brown stuff left, I might try again with more. Then I don’t know why from yellow it then turned blue with the second batch. I suspect something else distilled from tar. I don’t know what it is, but it’s nice to look at.

Something else I don’t really understand: If I’m not mistaken, nitrite in presence of acid forms nitrous acid, which is itself protonated into water and the nitrosonium NO⁺ ion. That ion attacks the nucleophilic aromatic ring forming the p-nitroso product. Therefore, at the end of the reaction, there is no more H⁺ in the solution, and the pH should be about neutral. But, each time I checked, it was strongly acidic, which tends to prove that the reaction that took place was not the one I expected.

Bmoore55 - 3-5-2022 at 05:41

The nitration of phenol using the method you mentioned will produce both the ortho and para products along with some polymerized products in the form of tar. You should be able to separate the ortho product using steam distillation and the remaining organics in the distillation pot can be used to isolate the para product. To do this dilute the remaining organics in some distilled water and add some decolorized charcoal and boil this mixture for a couple of minutes. Then, while hot, pour the mixture through a gravity filtration and collect the filtrate. This can then be left to cool and the para product should precipitate out of solution. Place this in the refrigerator if you want to maximize the amount of the para product you want to collect. I hope this helps.

Keras - 3-5-2022 at 06:18

Quote: Originally posted by Bmoore55

The nitration of phenol …

Unfortunately, that’s nitration using sodium nitrate. I was attempting nitrOSation using sodium nitrIte, which has the selective advantage of not producing a mix of isomers, but almost entirely p-nitrosophenol, which is the product I want.

Fery - 4-5-2022 at 04:29

Keras, you wrote: I’m wondering if what was sold to me a sodium nitrIte isn't in fact sodium nitrAte
So I posted you a link with nitration, not nitrosation. It could help you to guess whether you have nitrAte or nitrIte salt. Because in case of nitrAte you produced o-nitrophenol and distilled it out with steam as you wrote in one of your posts but you did not obtain yellow o-nitrophenol in the distillate.

[Edited on 4-5-2022 by Fery]

Keras - 4-5-2022 at 05:39

Quote: Originally posted by Fery

Keras, you wrote: I’m wondering if what was sold to me a sodium nitrIte isn't in fact sodium nitrAte
So I posted you a link with nitration, not nitrosation. It could help you to guess whether you have nitrAte or nitrIte salt. Because in case of nitrAte you produced o-nitrophenol and distilled it out with steam as you wrote in one of your posts but you did not obtain yellow o-nitrophenol in the distillate.

TBH, I steam distilled really a few grams of product in 200 mL of water. I obtained a yellow distillate (at first), which could be nitrophenol (which one?).

Interestingly enough, the French version of Wikipedia mentions another way to separate o- from p-nitrophenol with n-pentane, as well as mentioning that o-nitrophenol is a pH indicator: from colourless to yellow as the pH increases.

There was a noticeable colour change in the reaction vessel when the pH went from basic to acid, so ? In any case, I didn't obtain any crystals after steam distilling, but that might be because I didn't exceed the solubility limit.

draculic acid69 - 12-5-2022 at 16:14

Quote: Originally posted by Fery

Is this the same way that O toluidine seperates from P, toluidine?

Keras - 29-5-2022 at 11:00

Quote: Originally posted by draculic acid69

Is this the same way that O toluidine seperates from P, toluidine?

I have no idea…

So, I tried again today using a different process. I suspected tar formation was due to the excess of nitrite. So I added about 1 g of phenol dissolved in 20 mL of water to 4 g of 37% sulphuric acid in a 100 mL erlenmeyer flask – placed this in a freezer at -24 °C → it froze – and dripped the sodium nitrite (ca. 1 g dissolved into 10 mL of water) into it. As usual everything was put in an ice bath and the temperature swung between ca. -5 (the mixture melted around -10 °C) and 5 °C (ice cubes were added when the temperature rose above 5 °C).

Contrarily to the other experiments, this time the solution went gently from colourless to yellow, orange then dark orange. At this point, it was clear that something precipitated, like yellow dust. At the end the solution had turned almost reddish/ochre. Stopped at that point, filtered. Got a few milligrams (didn't weigh it) of brown powder. No tar, just brown. Probably yellow inside but coated?

I figured I could remove the brown cover by boiling the precipitate with activated carbon, but no luck. With heat, all the powder dissolved and the solution turned very dark red. Adding activated charcoal didn't change anything. I removed the flask from the hot plate, waited for the solution to cool down, but nothing precipitated. Too much water (though I think there were no more than in the synthesis phase), maybe. At that point, I discarded everything.

So, lesson learnt:

1. Phenol and probably sulphuric acid must be in excess all along. I did it wrong this time and put in more nitrate than necessary; that’s why, I think, I ended up with a slight tar deposit.

2. p-nitrosophenol cannot be heated above, say, 50 °C without significant degradation. Isolation cannot be achieved by boiling, liquid-liquid extraction with a low-b.p. solvent must be used instead (diethyl ether, dcm) and the solvent must be left to evaporate at r.t. or distilled in a rotovap.

Next weekend I hope to be able to do it again, this time with the proper procedure.

[Edited on 29-5-2022 by Keras]