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Author: Subject: Strange purple compound when oxidizing p-aminophenol
woelen
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[*] posted on 25-8-2010 at 02:02
Strange purple compound when oxidizing p-aminophenol


I have received some old photographic developer agent and its label says "Rodinal (p-aminophenol HCl)". This is HO-C6H4-NH3(+) Cl(-). So, it is both a phenolic compound and an aniline compound, with the aniline protonated like ammonium ion.

It is a crystalline grey solid, which when added to water moderately easily dissolves and gives (nearly) colorless solutions, with small black particles, which easily can be filtered. I think that pure p-aminophenol HCl should be white and that the grey color is due to oxidation products and that these small black particles are the oxidized material. Anyhow, when filtered, a clear nearly colorless solution is obtained.

When this material is added to a base, then a dark almost black and turbid liquid is obtained. The material then quickly is oxidized by oxygen from the air.

However, in acidic solution, the material is much more resistant to oxidation. A solution in 1M H2SO4 or 2M HNO3 remains colorless for hours, even when in contact with air. When a small amount of H2O2 is added, then the liquid also seems to remains colorless, but very slowly, the liquid turns purple/blue. First there is just a slight hint of this color, but after ten minutes or so, especially if the liquid is heated somewhat, the liquid has a really beautiful deep blue/purple (indigo) color. This color is very bright and intense and I like it very much.

I also tested with other oxidizers and the beautiful color also appears with other oxidizers, so the special color is not due to the peroxide, but it really is some oxidation product of p-aminophenol.

This particular experiment raised my interest and I also did similar tests with plain phenol, but when this is oxidized, e.g. with H2O2 in acidic solution, then I only get a brown liquid, which is not interesting at all. I did a similar experiment with anilinium sulfate and this also does not give such fantastic blue/purple colors. So, it really must be something specific to p-aminophenol.

Finally, I did a last test with metol (this is the sulfate salt of monomethyl p-aminophenol, HO-C6H4NHCH3). Very similar to p-aminophenol, but with one of the hydrogens on the -NH2 replaced with a methyl group. When this is oxidized in acidic solution, then also a brightly colored solution is obtained, but now it is purple/red instead of blue/red.

The nice colors are not stable. The bright purple/blue or bright purple/red solutions slowly decompose over a time-period of hours. After one day, the solution has turned dark brown/black with solid particles floating around in the liquid.

Are these beautifully colorled compounds well-known compounds? I have looked around on Internet but could not find any information about these. At many places, mention is made of the easy oxidation of solutions of metol and p-aminophenol in alkaline conditions, but no mention is made of the colors in acidic solution. I can imagine that this lack of information is because normally these compounds are used in (mildly) alkaline solutions as black and white photographic developers.




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[*] posted on 25-8-2010 at 02:51


The same colours can be obtained by dissolving a paracetamol tablet in water with a few drops of hydrochloric acid, filtering off the excipient and warming the solution to hydrolyse the paracetamol to p-aminophenol hydrochloride. The solution is then cooled and a few drops of hydrogen peroxide are added to form the colour.
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[*] posted on 25-8-2010 at 05:23


I believe this is the well-known variety of aromatic amine oxidations, modified by the presence of the hydroxyl group which changes the conjugation (color) as well as the product. For instance SH mention bis-dimethylamninobenzene (CH3)2NC6H4N(CH3)2 oxidized to a stable 'blue' cation radical by abstraction of electron - in this case the OH group due to its lone pairs will have a similar stabilizing effect. Eventually the radicals polymerize - and this seems to be what is happening here as well
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[*] posted on 25-8-2010 at 11:06


I wonder if it's related to this
http://en.wikipedia.org/wiki/Mauveine
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[*] posted on 25-8-2010 at 11:07


Partial oxydation of p-aminophenol leads to p-quinone like molecules that reacts with the amino moety of unoxydised molecules (kind of Schiff's base made between an amine and a keton (quinon are related to ketons))...even anilin does form such polymerisation compounds responsible of the coulour of anilin upon air exposure (orange to brown and then to black).

H2N-C6H4-OH -ox-> HN=C(CH=CH)2C=O + H2O
HN=C(CH=CH)2C=O + H2N-C6H4-OH --> HN=C(CH=CH)2C=N-C6H4-OH + H2O
HO-C6H4-NH2 + HN=C(CH=CH)2C=N-C6H4-OH --> HO-C6H4-N=C(CH=CH)2C=N-C6H4-OH + H2O

A slight change in the structure leads to variation into the colour shade!




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[*] posted on 25-8-2010 at 17:51


I dont know about this, oxidation to quinones corresponds to a loss of aromaticity, and should only be favoured under stronger/longer term oxidation.
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[*] posted on 25-8-2010 at 23:01


@unionised: I do not think that the compound I made is mauveine or something related to it. I do not get black goop when p-aminophenol is oxidized in acidic solution, the liquid really obtains a beautiful blue/purple color and it also remains clear. The appearance of the color also is slow, the liquid slowly goes from colorless to deep blue/purple (indigo).
Also, the blue color is unstable. Even after an hour or so, you can see that it lost some of its brilliance, the color becomes more dull somewhat dark grey/blue/purple. After one day, all color is lost and just a dark brown, almost black insoluble compound remains.

As a test, I have done the oxidation of anilinium sulfate with sodium hypochlorite solution. When this is done, then the liquid becomes turbid and brown/black, but one also can see a hint of a purple color. When the liquid is decanted, then a lot of black tar-like stuff sticks to the glass (ugly-looking). When acetone is added, the tar-like stuff dissolves and you get a very dark purple liquid, but this liquid really is purple, not the blue/purple color I obtained with oxidation of p-aminophenol.

--------------------------------------------------------------------------------------------------

The explantion of len1 sounds plausible, but how could this be tested? For a radical species this is very stable (hours of existence). The polymerization, described by PHILOU Zrealone certainly could occur, but I think that is more something of later stages or something in alkaline solution. I know the effect of polymerization of polyphenols and mixed aniline-like/phenol-like compounds (e.g. catechol, hydroquinone, pyrogallol, metol, p-aminophenol). It is a bad effect, known by every amateur photographer who makes his own prints. But this polymerization never leads to bright and vivid colors, it always leads to dark brown goop and ugly/dirty looking stuff.

I suggest, that if someone has p-aminophenol or one of its salts, then just try this simple experiment yourself (or try Sciencesquirrel's experiment, I know of the hydrolysis of paracetamol, I actually even used this for developing black and white prints). The color of the oxidation product is really remarkable, so bright and clear.

[Edited on 26-8-10 by woelen]




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[*] posted on 26-8-2010 at 22:51


http://pubs.acs.org/doi/abs/10.1021/ac60200a019

This would suggest the color is due to Fe3+ ions in solution complexing with the oxidation products of PAP, moreover its sensitive to traces of Fe3+ and is used to detect said. Maybe you should try it in the absence of Fe3+ and if you still have same result you might be on to something new

[Edited on 27-8-2010 by len1]
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[*] posted on 27-8-2010 at 09:50


I have done the experiment with very pure chemicals, where there is no Fe(3+) present. I even tested with thiocyanate and not even the faintest red color can be observed.

The p-aminophenol is a photograde chemical and I hardly can imagine that a photographic developer would contain any iron. Iron is very bad for further processing (such as toning) and definitely is not present in developers. So, I am quite sure that the color is not due to the presence of Fe(3+).




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[*] posted on 27-8-2010 at 11:58


In experiments with reduction of nitrosophenol I observed the same colors.
When nitrosophenol was reduced with Na2S2O4 I got white precipitate. This white something (p-aminophenol) immediately turn black in the air. When I put it in acid I got very intensively coloured solutions and this colour was not stable. I do not remember this sequence of colours exactly but probably it was red->green.
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[*] posted on 27-8-2010 at 18:25


Pharmaceutical grade paracetamol will contain almost nothing in the way of contaminants.
If iron were present then it would be at the fractions of a part per million level, reagent grade hydrochloric acid will also contain only low levels of iron.
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[*] posted on 28-8-2010 at 18:06


The idea that the color is due to complex formation with metal ions was my interpretation of the article, the other interpretation is that different ions catalyze oxidation to different products.

Guilbault does however state that of all oxidants he tested only the three above gave a purple product, which he considers to be a phenoxizine derivative


O=Ph(N)(O)Ph-NH2

These he mentions are stable only in strongly acidic solution, which agrees with what you got. Over time the phenoxizine is oxidized further, and so colour disappears.

But he did try many other oxidants: S2O8, WO4, Cr2O7, and states onle the three lead to purple oxidation product. maybe he did not try peroxide..?

microchemical journal 24, 23-32 states things somewhat differently.

a) it is the oxidation product of PAP with H2O2 that is coloured purple and fe3+ catalyzes this

From this it follows that adding Fe3+ in mM concentration should increase reaction manifold.



b) other oxidixers cu2+, cro4, cr4+ are also catalysts.

This article

a) does not mention the identity of the product instead being content to use colour concentration for analysis


b) Does not explain why Guilbault failed to find h2o2 giving a purple product, in fact one can assume from this article that other oxidizers would too

So putting these together a semi-complete picture emerges, but neither article by itself is satisfactory. It seem the whole thing has been covered in the literarture, though in fragmented fashion.


[Edited on 29-8-2010 by len1]
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[*] posted on 29-8-2010 at 06:47


Something to read:
Benzoquinone imines. Part 16. Oxidation of p-aminophenol in aqueous solution
DOI: 10.1039/P29790000308
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[*] posted on 29-8-2010 at 09:59


I do not have access to this. Is is possible to put a copy of this article somewhere (e.g. in the reference section of sciencemadness?).

I tried the test also with potassium permanganate as oxidizer. When this is used, then superfically looking at it, it seems as if no oxidation occurs, there only is a shift from purple with a reddish hue to purple with a bluish hue. The new color is of similar intensity. So, apparently the manganese(II) does not affect the outcome of the experiment, or the color of the complex formed is so weak that it is hidden by the strong color of the oxidation product of p-aminophenol.




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[*] posted on 2-9-2010 at 20:39


Does it still form the colour if the solution is heated before adding H2O2?
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[*] posted on 2-9-2010 at 23:04


Yes, it still does, then it does so more quickly.



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[*] posted on 3-9-2010 at 01:23


Hmm perhaps something like violaniline except with OH groups or maybe it's forming azophenol (that's not the proper name of it but you get the idea), aniline gives azobenzene when oxidized, p-OH-aniline could just give the substituted azobenzene.
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[*] posted on 4-9-2010 at 00:25


could it be a compound like methyl yellow perhaps?
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[*] posted on 18-10-2010 at 20:11


I tried to repeat oxidizing p-aminophenol with H2O2.

Most of the time was spent purifiying the organic since all of our stock is terribly oxidized, meaning its a black powder giving a soot-like solution. After playing around with the conditions, I got white/light-purple crystals in two sets of recrystallization first from acidic solution, then from water. The procedure is quite wasteful with a yield of only 50%

Oxidation with H2O2 in 1M H2SO4 proceeded on a timescale of several days, but I definitely got a purple oxidation product.

@Wioelen I note you state blue/purple, and was wondering what that means. Did you not get a pure purple? And is the colour always the same? Len
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[*] posted on 18-10-2010 at 22:34


The color of the product I obtain definitely has a blue tinge. I call it blue/purple, one also might call it indigo.

The timescale of a few days can strongly be reduced by heating the solution to 60 C or so. In that case the time can be reduced to minutes. Maybe the long oxidation time in your experiment leads to oxidation product, while another part already has decomposed again. This could explain the color difference.




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[*] posted on 19-10-2010 at 02:42


Thanks for that. I tried it at 60C and judging by sight the reaction takes about 1min to arrive at a stage where at 20C it takes 24hrs. So thats about 1500x speed-up, an unusual temperature sensitivity - where the average rule of thumb is that rates roughly double every 10C.

Interestingly there is a slightly greater blue tinge in the initial stages of the 60C reaction, I cant be sure till i take a vis spectrum The overriding color to me is still mainly purple.
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[*] posted on 19-10-2010 at 03:24


Yes, for me the color also is mainly purple, hence the title of this thread, but there certainly is a blue tinge. The color always is the same (at least as far as I can tell by judging the solution with my eyes, I have no equipment for taking a spectrum).

I also tried other non-coordinating oxidizers and these oxidizers also give the blue/purple color. So, the color really is something from the p-aminophenol and is not a peroxide-specific thing.




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[*] posted on 22-8-2011 at 22:58


I got interested in this after the discussion here. Here's what transpired: http://pubs.acs.org/doi/abs/10.1021/jp2045806.
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[*] posted on 23-8-2011 at 06:40


Reference Information


Identity of a Purple Dye Formed by Peroxidic Oxidation of p-Aminophenol at Low pH
L. Lerner*
J. Phys. Chem. A,
2011, August
DOI: 10.1021/jp2045806


Attachment: Identity of a Purple Dye Formed by Peroxidic Oxidation of p-Aminophenol at Low pH.pdf (1.7MB)
This file has been downloaded 931 times





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