reduction of nitrobenzene -> p-aminophenol

Patent US2525515A gives very promissing procedure:
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12.3 g. of nitrobenzene were mixed with 150
grams of water in which 40 grams of oxalic acid
were dissolved. The mixture was heated to and
maintained at about 95°C., and then 4.5 g. of
aluminium fiakes were slowly added with vigor-
ous stirring during about one hour. No nitro-
benzene remained after 2 further hours' heating
and stirring.
The reaction mixture was filtered
hot and then made alkaline to brilliant yellow
with sodium sulfite and sodium carbonate. On
cooling and standnig 8 hours, p-aminophenol
precipitated. There was some odor of aniline but
none separated out. The aluminium did not
precipitate from the alkaline solution as it was
kept in solution as a complex oxalate compound.
The p-aminophenol was filtered off and recrys-
tallized from a small amount of water. It melted
at 184°C. Yield 73 per cent.
---------------------------------------------
I tried this on 1/3 scale.
Everything looks fine, Al dissolves and the solution becomes deeply brown. Some Al (miligrams) remains, steam dist. gives 0,2-0,3g of unreacted nitrobenzene.
To this filtered and steam distilled mixture Na2SO3 was added (~0,2 g), giving strong odour of SO2, pH of solution was below 2.
[This procedure, with slight modifications, was repeated 5 times, with the same results]
Then solid NaOH was added in several portions.....
From this moment all stops looking fine
Results from the latest run:
5,29g NaOH were added in several portions: SO2 odour dissapears, pH rises (to about 4-5). After 24h standing, solution was filtered and 50 mg of black something left.
Next, another 0,5g NaOH was added (and so on...) 1,1g of bright precipitate was collected. Another 0,2g NaOH -> 0,34g of brown something left.
Another 0,2g portions (six) of NaOH give ~1,9g of precipitate at every time. These precipitates are dark when wet, after drying they are bright grey and look like Al(OH)3. Obtained powders, when agitated with water, give deeply brown solutions with sediment.
Remaining solution is still dark brown, with weak aniline smell.
The most striking thing - it was impossible to obtain pH higher than 5,5-6 without Al precipitation. Every portion of NaOH gives temporary rise of pH (during minutes), but then it drops and Al salts precipitates.
I expected to obtain clear cut between precipitation of aminophenol and Al(OH)3 but it was only a dream.

Quote: Originally posted by Nickdul

I'd have to find my notes, but the hydrolysis was carried out at RT for about a week, and it still resulted in a dark-tea-colored soln. due to aerial oxidation. Maybe in an inert environment or under hydrogen it can be heated and refluxed to speed up the cleavage of the -COOH group.

Quote: Originally posted by UnintentionalChaos

citrate

Yes, currently it is under investigation. I still have some nitrobenzene for experiments, citrate (citric acid) can be solution of mentioned problem with Al precipitation, because it forms much stronger complexes with Al. There is interesting, experimental paper about it, but maybe later..... Possibly, direct replacing oxalic acid with citric acid wil work, but now I have to play with freshly cooked (oxalic) solution

Another run:
14 g of oxalic acid dihydrate* + 50 g of H2O + 4,1 g.
Stirring, boiling, filtering....
This time small modification: 4 g of citric acid** was added to final solution.
Slightly more than 5 g of NaOH were added (+0,17 g of Na2SO3), at some point bright-brown particles start to separate, liquid becomes clear but still brown, aniline smell appears but nothing more, pH is around 6.
Brown sediment was filtered and washed with water. After drying, 1.9 g of very delicate, cocoa-coloured powder was obtained. The powder is without any odour and melts ~170 C. During melting, under microscope are visible some transparent particles, not melting in this temperqture - possibbly some inorganic salts. The rest of powder melts to black liquid (decomposition).

Additional portion (~0,2 g) of NaOH was added to filtrate - nothing separates but it was left for some time in a cooler.
After few hours, larger amount of precipitate appears, but it looks diffrent than earlier.
During filtration and washing it turn out that it is suluble in water, so it is not wanted product.
A sample of it was collected. After drying, the sample turn to hard mass with brown surface, but it is only tiny layer and whole mass inside is white. The most posibbly, the solution just became supersaturated.

If even this 1,9 g would be pure p-aminophenol (but it is not), it is still far from 70% yield.
Some black tar stuck to mag. dipol but it was max 0,1 g. Seems that the rest of reduced nitrobenzene turn to other by-produsts, causing brow coloration of solution. After standing in open air, this colour becomes more intensive, surely because of oxidation.
Maybe intermediate phenylhydroxylamine is not fully converted to p-aminophenol ?? Hard to say...

* the patent says "oxalic acid"; it seems that it does not matter if I use 13,5 g or 19 g of dihydrate, reaction in both cases goes in the same way, with very not sharp phenol-salts separation during neutralisation with NaOH.
** rater anhydrous, not melting in 100 C, but its particles become a little sticky at this temp.

On the picture: left - mass from the second crop , right - main product.

Quote: Originally posted by Dr.Bob

para-aminophenol is highly unstable to air oxidiation, and will oxidize to imino-quinones readily.

.... and in this way I used up all my nitrobenzene (prepared long time ago and forgotten).
Another raw products on the picture (for comparation, earlier prepared on the right). Generally, they look the same: more or less greyish, bright brown powders.
Last runs were made in H2SO4 (proportions also from patent).
Citric acid was added to shift the pH of Al hydroxide precipitation, but when sufficient amount of NaOH was added, large amount of some inorganic salt precipitaded out on cooling. It was filtered and raw aminophenol precipitated from filtrate by adding NaOH.
Yield about ~50%, as in earlier (oxalate) runs.
When another portions of NaOH were added to remaining filtrate, pH goes above 7. No Al-salts precipitate, there is some suspension, but only small amount (<0,1g) of brown-black sediment was recovered. The final filtrate is still red-coloured, but no p-aminophenol can be precipitated from it.

I also tried to use citric acid instead of oxalic or sulfuric acid.
Unfortunately, reaction is very sluggish, Al reacts very slowly, solution becomes turbid, nitrobenzene layer becomes red.
After 5 hours of cooking, no progress was noticed and mixture went to WC.

In attachment an aticle: "INFLUENCE OF ACETATE, OXALATE, AND CITRATE ANIONS ON PRECIPITATION OF ALUMINUM HYDROXIDE"



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Quote: Originally posted by JAVA

So, why using Al instead of Fe? That's the question..

At least few reasons:
- Al is not ferromagnetic: no problems with magnetic stirring
- Al has lower molecular weight than Fe
- Fe gives more aniline than aminophenol (see patent US2446519A)

Why not HCl ?

Again few reasons:
- no HCl vapours during heating
- HCl attacks Al very quickly, partially without nitrobenzene reduction
- HCl gives large amounts of p-chloroaniline as by-product

Extraction ? Very good idea
With what ?

PS.
I have prepared fresh sample of distilled nitrobenzene (~45g) and I am going to do additional tests, with sligtly different experimental conditions. However, this nitrobenzene freezes below benzene m.p. and incompletely
So, first I have to distill it again to make it purer.

[Edited on 29-1-2014 by kmno4]

Quote: Originally posted by kmno4

Quote: Originally posted by JAVA   So, why using Al instead of Fe? That's the question.. ... Fe gives more aniline than aminophenol (see patent US2446519A) ->Thank you, didn't read the title full. Obviously, you believe that every procedures in scientific text are true. This isn't the case, that's also my experience. The grey product seems like Al(OH)3 Why not HCl ? Again few reasons: - no HCl vapours during heating ->use ammonia to neutralize the HCl - HCl gives large amounts of p-chloroaniline as by-product Source? Extraction ? With what ? -> Extraction (3 to 4 times) with toluene for example, but rapidly destill it under high vacuum/low temperatures since it get slightly yellow soon. p-aminophenol is only stable in a acid environment and get red under alkaline conditions. PS. I have prepared fresh sample of distilled nitrobenzene. However, this nitrobenzene freezes below benzene m.p. and incompletely So, first I have to distill it again to make it purer. -> Not at all, wash it 3 times with cold MeOH to remove the ortho - isomer. However not all the ortho form is removed. You might do a recrystallisation of the slight yellow crystals with a great smell. To puryfying read this BUT keep every layer during the extraction because the layers switch (upper/lower) during the purification: READ (lol): 6530kg mixed acid consisting of 30% HNO3, 56% H2SO4 and 14% H2O are placed in a reaction vessel and 3500kg chlorobenzene are added with constant stirring. Temperature is held at 40°C at first, later at 55°C and after the addition is complete the whole is warmed to 70-80°C within two hours, still stirring constantly. The phases are then separated while still warm and the spent acid extracted once with chlorobenzene, the extract is used directly as the feed for the next nitration (Note: you can probably just dilute the acid to separate the dissolved product instead of extracting it). The organic phase is washed with water, Na2CO3 solution and water again and then dried by warming in vacuo. 4850kg crude isomeric mixture is obtained, with a "melting point" of 52°C. The mixture is cooled very slowly and then held at a temperature of 20°C for 25 hours to effect the crystallization of p-nitrochlorobenzene. The crystals are filtered by suction and then washed three times with methanol (2000-4000l MeOH for 14000-15000l of crude mixture). The first methanol portion has already been used twice for washing, the second portion once, and the third washing is done with fresh methanol. The crystals now are, after drying, industrial grade p-nitrochlorobenzene with a melting point of 82-82,5°C. If the melting point requirement is not fulfilled, the washing must be repeated (or the product recrystallized). The liquid filtrate goes to the vacuum fractionation process. I washed thrice with fresh ice-cold methanol, thus I had larger losses, but my product was purer.". [Edited on 29-1-2014 by kmno4]

Quote: Originally posted by Boffis

The 4 aminophenol on the other hand was a fine black powder resembling charcoal, when dissolved in alcohol and filtered no amount of active carbon would remove the colour. After some experimentation I found that a good proportion of the aminophenol could be recovered by dissolving in a significant excess of hot 10% HCl and adding small amounts of zinc powder until the colour was greatly reduced. The result was a violet brown solution that gave abundant dark brown crystals on cooling.

I also got brown crystals from cooled (intensively brown) solution of aminophenol in water, however no violet colour was observed.
These crystals, when smashed appear as light brown powder, very similar to product from photograph in paper about hydrogenation (link above)
I have impression that this crystallization does not give much purer product than raw aminophenol (but m.p. of product rises slightly). Seems that my raw aminophenol is not so bad (bright brown powder) and possibly suitable for my purposes.

I woud buy it, but sellers I know want too much, and import from PRC costs 300$
I can efford it , but it is against my rules - too big money for some plain compound.
Synthesis from nitrobenzene is very cheap and simple, even with 50% yield.

- HCl gives large amounts of p-chloroaniline as by-product Source?
For example see here: J.Chem.Soc.,Perkin Trans. 2 1981; 298
No procedure for reduction of nitrobenzene (with rearrangement) to p-aminophenol I know, uses hydrochloric acid. In ~90% it is H2SO4. Ask yourself, why.

Quote: Originally posted by JAVA

-> Extraction (3 to 4 times) with toluene for example....

Maybe your aminophenol is extractable with toluene, but mine is not. See literature (Ullmann), and also paper I posted - they wash their raw product with toluene to remove soluble impurities.

Besides - why shall I extract precipitated product, when I can do simple filtration ?

Quote: Originally posted by JAVA

-> Not at all, wash it 3 times with cold MeOH to remove the ortho - isomer. However not all the ortho form is removed. You might do a recrystallisation of the slight yellow crystals with a great smell. To puryfying read this BUT keep every layer during the extraction because the layers switch (upper/lower) during the purification: (...)