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Author: Subject: Chemistry of Terephthalic acid
kmno4
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[*] posted on 26-10-2017 at 16:10


Nitration of TPA
Trial run.

To 100 cm3 Erlenmeyer flask with 30 g of conc. H2SO4 and magnetic dipole, 4 g of TPA is added, next 3,0 g of powdered KNO3 is added in portions. The white muddy mass is stired (or rather moved) by Nd-magnet (outside the flask) after each portion of KNO3. At the start, only 15 g of H2SO4 was used, but it gave not stirrable mass, so additional portion was added.
Next, the flask was placed on the water bath (exactly like in nitration of phtalic acid given in separate topic).
Heating at ~80-90 C during several hours gave nothing interesting - white mudd, slightly less dense on hot.
Heating was paused for 15 hours and continued the next day, but before heating, additional 0,5 g of KNO3 was added.
Soon after it was placed in hot water bath, I noticed the the white mass turn to bright yellow mass and became markedly less
dense. The clear yellow colour soon turn to brown-yellow one, but still bright. During another several hours of heating, the
colour did not change any more, but the mass started to increase its volume (foaming). The flask was removed from the bath and
set aside to cool down. The muddy mixture was less dense even at r.t., it may indicate that "something happened" (good or not good, hah).
The mass was transfered to 100cm3 of chilled water, giving intesively yellow solution with white foamy suspension, some
nitrogen oxide is generated but in so small amount that no hood is needed. It was set aside to cool down again and filtered*.
The white powder (I was almost sure it is unchanged TPA) in the funnel was washed with 2 x 10 cm3 of warm water and dried to
constant weight (4,1 g). I wanted to discadr it, but I noticed (during washing the equipment) the the powder is rather soluble
in water.
The powder was added to ~130 cm3 of water and heating was started. It dissolved gradually as temp. was increasing giving green solution. Several minutes of boiling caused complete dissolution and clear liquid, so it is not TPA !!!
On slow coolig, the solution deposits nothing during hours, but later small globules set on the bottom the flask, but in very small amouts. Keeping this for some time more at r.t. causes precipitation of white powder, supersaturation in this case is very stable, even if solid is already present.
Fitering gave 3,5 g of white** solid and pale green liquid which was kept for another run.

Another run :)
This time it was 15 g of H2SO4, 4 g of TPA, 3,1 g of KNO3. Procedure conducted as above, the white mud stirred from time to time with Nd-magnet (but I think it has no effect on the progress of reaction; no solid cake is formed during whole time of heating). First, 5-6 hours of heating, pause ~15 hours, and againg heating during ~8 hours. After ~6 hour of the second period, I noticed partial yellow coloration of the white mud and in a short time (<0,5 hour), the white colour turn to brown-yellow, as earlier. Heating was continued for additional ~2 hours (I had no mor time , it was late at night).
It was cooled, transferred into 75 cm3 of water. cooled and filtered. Obtained cake (~8 g), without washing and drying (it is
loosing the time and the product !) was dissolved on hot in green liquid from earlier run.
This time, some (however it looked like miligrams) solid was still present even on boiling, adding 10 cm3 of water gave nothing.
It was filtered on hot, green solution (but not completely clear) was quickly cooled down by immersing the flask in cold water.
Again, no solid was obserwed even cooled below r.t., solution became more turbid only (most possibly unreacted TPA). Suddenly, the solution turn milky and full of white precipitate.
Filtered, washed (~10 cm3 of ice cold water) and weighted: 4,7 g.
The most possibly it is desired nitro-TPA, unfortunately my melting point determination apparatus cannot be used (too low
temp.). I performed titration of this nitro-TPA, to estimate its purity: 0,44 g was titrated with 0,1 M NaOH. The result is 205
(as molar mass), it fits almost perfecly to 209 (theory).
The synthesis is dirty cheap, long-time heating is no problem, beacuse it can be paused at any time and continued later.
Of course, given proportions are surely not optimal but it works :)

* the filtrate deposits no solid on cooling (even below 0 C)
** it is purely white in LED-light, colour observed in day-light or incandescent lamp has faint yellow coloration




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[*] posted on 28-10-2017 at 04:58


@kmno4; magnificent! I am still away from home so I can't try this myself at present but when I get home I will have a go. This opens up a whole new avenue for derivatising terephthalic acid. It can be reduced to amine, acetylated and further nitrated and reduced to give the the dicarboxy-p-phenylenediamine. It may also be possible to replace the amine group via the diazonium salt with various groups. I wonder how reactive the bromo group would be in this position.

By the way, which way did you titrate the acid? I have found that for terephthalic acid it is better to use a back titration by dissolving the acid in a known volume of standard NaOH and then back titrating with standard HCl in the presence of phenolphthalein. This seems to give more accurate results.

Anybody got any info on pKa values for nitroterephthalic acid?

I am still trying to get my head round the paper you posted on the phase relationship of nitrophthalic acids and water.
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[*] posted on 28-10-2017 at 07:57


KMnO4, did your NaOH have more sig figs than just 0.1M? If not, I hate to say it, but I don't think the titration shows much.

However, the reaction is sweet (something I've wanted to do) and the thread a sa whole is great!

[Edited on 10-28-2017 by The Volatile Chemist]




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[*] posted on 28-10-2017 at 11:49


Well the molecular weight of the terephthalate ion is 164 verses 209 for NTPa so that suggests about 91% NTP acid. however, in direct titrations against a slurry of terephthalic acid I found that the colour change occurs and then reverses and each time take a little longer to reverse. The pKa values for terephthalic acid are 3.51 and 4.82 so I would expect nitroterephthalic acid to have values more like 2.6 and 4.0 and I wondered if this might be a cause of some error. I have a nifty bit of software that save a lot of time and brain power on my personal computer unfortunately its a long way away!
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[*] posted on 31-10-2017 at 10:30


Which piece of software? I'm curious, as I'm thinking of making a piece of lab-assistance software soon if I ever get free time myself.

I just meant that if you titrate with 0.1M NaOH, you have one sig fig to work with. Your MW, if it is calculated as 205g/mol, will be effectively anywhere between 150 and 249g/mol. Unless I'm reading something wrong.




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[*] posted on 1-11-2017 at 07:04


@Volatile Chemist, the software used here was BATE pH calculator. Its not free but I like it, its about 35 GBP for the download or you can buy the complete package of 4 pieces of software for about 120 GBP and that includes a formula balancing app that export into HTML so you can paste your equation into an SM post directly. I've been using the latter for years but only recently have I played with the other 3 pieces of software. You can download them for free and use them for a month to see if you like them. You can't save the charts in the free trial version but they are pretty interesting to play with.

I don't quite understand your second point. The NaOH titration gives you an equivalence number, which is the molecular weight per mole of ionisable hydrogen. So if you started with say 0.25g of acid and the titer was 33ml of 0.1M NaOH then 0.25g = (33 x 0.1)/1000 = 0.0033 moles if it's a monobasic acid or a MW of 75.76. If its a dibasic acid then there are only half as many moles present so 0.00165 mols = MW151.51.

When I do this type of titration on phthalic acids I dissolved about 0.2 g in 50ml of 0.1 or 0.2 M standardized NaOH solution and then titrate the excess NaOH against 0.1M HCl. I make up my own standard NaOH solution but I buy the acid a commercial standardized 1M HCl and dilute it to 0.1M and also use it to standardise my NaOH solution. So the accuracy of your MW determination is usually down to two main sources of error, one is the volume of titer which you can only read to an accuracy of about 0.05ml with my burette and the accuracy of the Molarity of the standard solutions but with my terephthalic acid work I was getting a MW of within 1% of the theoretical value, with the terephthalamidic acid there was a difference of 10+ % indicating that it is rather impure. So kmno4's 205 or MW of 207 indicates a mixture of about 91% nitroterephthalic acid 9% terephthalic acid assuming both are anhydrous and the titration is accurate.
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[*] posted on 1-11-2017 at 12:41


Boffis, I made this titration with phenolphthalein as indicator, because it changes its colour at pH around 9, so it seems to be good choice. I think that purity you calculated (91%), is too low, but without HPLC I cannot prove it :)
I did reduction of this nitroacid with Fe. I chose this method because it is cheap and almost always works. However, I expected problems caused by the nature of reduced product.
With Fe powder, reduction is quick and takes 30 minutes to complete (with 4 g of nitroacid, 5 g of NaCl, 8 g of Fe, 75 g of H2O).
Unfortunately, prepared solution of aminoacid contains larger amounts Fe(II) - filtration gives clear solution, but after 1 minute it becomes turbid.
It is extremely hard to remove this iron from the solution (for me). I tried careful oxidation with 3% H2O2, but the solution gradualy turns red, most possibly some quinone is formed from the aminoacid. Additionally, formed Fe(III) hydroxide has very bad filtering properities: it comes through every filter I have.....
I have another reduced sample, without H2O2 treatment. It has dirty-pale-yellow colour, with reddish sedimemnt on the bottom.
Filtration gives clear pale greenesh solution, becoming turbid during almost seconds... I checked this solution for Fe(II) content with K3[Fe(CN)6] - the test is strongly positive. So far, I have no idea how to remove Fe from it in a simple way.
But it is really solution of desired aminoacid - it gives strong fluorescence (white-blue) under black-light lamp.

I am going to use Na2S2O4 as reductor the next time, but first I have to prepare some nitroacid (for now, nothing more left).
-------------
Addendum
The turbid solution, with Fe(II) contamination, has pH=5,5.
I was going to add some HCl(aq), but with bottle of this acid in my hand, voices in my head started to shout " Don't do this !".
Yeah, It was stupid idea, so instead of HCl I took NaOH solution and added some to the solution. The effect was terrific: suddenly whole solution turn blue-black for a moment, then some suspension started to float inside, after few seconds I got clear water-like solution with sediment looking like good old "Fe(OH)3".
It was filtered off (small amount) without problems, the filtrate was clear, with pale green-yellow colour, without tendency to become turbid in the air and pH=7,5.
Test for Fe(II) gave nothing, so damned Fe was removed completely.
The solution of amino-TPA is under further investigation.

ps.
0,1 M NaOH means what it means: 0,1 M NaOH
If you want to know more, the manufacturer says that concentration is between x mol/dm3 and y mol/dm3
But can I/you belive it ?
Discussion the difference between 0,1 and 0,14, accuracy of a balance made in PRC, selection of indicators for titration, pK of weak dibasic acid and its starting concentration.... is very interesting, but for separate topic(s).
I just wanted to know if I have TPA or nitro-TPA, so results 205, 210 or 199 are equally good estimations comparing to 166.
In general, performing accurate calculations for some substance with unknown impurities, is pointless.

[Edited on 2-11-2017 by kmno4]




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[*] posted on 1-11-2017 at 15:45


Quote: Originally posted by Boffis  

I don't quite understand your second point. The NaOH titration gives you an equivalence number, which is the molecular weight per mole of ionisable hydrogen. So if you started with say 0.25g of acid and the titer was 33ml of 0.1M NaOH then 0.25g = (33 x 0.1)/1000 = 0.0033 moles if it's a monobasic acid or a MW of 75.76. If its a dibasic acid then there are only half as many moles present so 0.00165 mols = MW151.51.

When I do this type of titration on phthalic acids I dissolved about 0.2 g in 50ml of 0.1 or 0.2 M standardized NaOH solution and then titrate the excess NaOH against 0.1M HCl. I make up my own standard NaOH solution but I buy the acid a commercial standardized 1M HCl and dilute it to 0.1M and also use it to standardise my NaOH solution. So the accuracy of your MW determination is usually down to two main sources of error, one is the volume of titer which you can only read to an accuracy of about 0.05ml with my burette and the accuracy of the Molarity of the standard solutions but with my terephthalic acid work I was getting a MW of within 1% of the theoretical value, with the terephthalamidic acid there was a difference of 10+ % indicating that it is rather impure. So kmno4's 205 or MW of 207 indicates a mixture of about 91% nitroterephthalic acid 9% terephthalic acid assuming both are anhydrous and the titration is accurate.

Hmm...

Do you know what I mean by 'sig figs' or is this just a newer concept that not everyone has heard of? Nothing personal, just surprised - either you're missing something or I am.

If you buy an acid that says 1.M on it, it is likely pretty close to 1M. But how close?
To be able to write 1M on the bottle, it would need to be within the range of 0.5 to 1.4, or thereabouts. Now if the bottle said 1.0M, then we can rest assured it contains a solution between 0.95M and 1.04M.

So if someone wants to do a titration, and they use a solution they call 0.1M, then this is not a very accurate value to work with. For all we know, it could be 0.14M. However, if you refer to 1.00M NaOH, I know this solution is very trustworthy indeed. However, 0.1M, for a titration, is not trustworthy, and because we only have one digit of accuracy, the result of the titration can only have one digit of accuracy. And thus the molar mass calculated by it is not particularly reliable.




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[*] posted on 3-11-2017 at 07:14


I know exactly what you mean VC. What you are saying may be correct nomenclature in mathamatic but take a look at a bottle of commercial standard acid it says 1M, 2M, 0.2M etc not 1.00 or 2.00M. On a bottle of commercial 2M HCl that I purchase not so long ago it says in the small print that the acid is standardized to +/-0.5% of the stated value ie somwhere betewen 1.99 and 2.01 while on my Analar 1M acid it claims +/- 0.2% so somewhere between 0.998 and 1.002M. But I have no way of knowing the exact value so, like everybody else I use the stated value. I, like everybody else in the real world, only express more significant figure on solution that have been made by myself and compared against the reference solution, hydrochloric acid in this case.

So, please stop being a nit-picking-bastard and if you have nothing constructive to add to thus thread then don't post anything.
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[*] posted on 8-11-2017 at 08:48


TBH I think VC has a valid discussion, but i think this is completely the wrong thread. I found his argument thought provoking but it detracted from the main thread. So please start a thread on this, i would really enjoy something like that. But please not in this topic.

@kmn04

You made me go cut up milk bottles!! I got enough experiments running, i dont need/want another!! But its fascinating what your doing.

Up until now I confess to being in the 'meah' group of people when TPA is mentioned, to me its a white fluffy inert do nothing substance i have no use for.

But seems I got it all wrong lol, keep up the good work! I have my milk bottles in NaOH as we speak.
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[*] posted on 16-11-2017 at 09:39


@kmno4. I decided to try a "preparation scale" nitration of terephthalic acid based on your observations; here is what I did:-

20.045g of dry, reprecipitated, terephthalic acid were stirred into 70ml of conc. sulphuric acid (96-98%) in a 250ml flask containing a large stirrer bar. To this were added 15.40g of dried and finely ground sodium nitrate, initially in small portions but since there was almost no temperature rise eventually the remainder was added in one go. The flask was placed in a silicone oil bath on a stirrer hotplates and heated to 90 +/-3° C. The mixture proved difficult to stir so a further 15ml of conc. sulphuric acid were added. After about an hour a very slight reaction appeared to be occurring since, as kmno4 described, the mixture became thick and slightly frothy and a further 15ml of conc sulphuric acid were added to thin the reaction mixture. Once the target temperature had been reached the reaction slurry was left in the oil bath for 23 hours before being raised out of the oil and allowed to cool. The reaction mixture never really changed in appearance, it remained almost white and slightly frothy though the fine white suspension did appear to become more granular with time. The stirrer struggled to keep the frothy material stirred into the reaction mixture.

The cool reaction mixture was drown in 150ml of water and 150g of ice. A further roughly 100ml of iced water was used to rinse the flask. The result was exactly as you described, a bright yellow solution and a practically white, fine suspension that looks no different to terephthalic acid. Once the slurry had cooled to room temperature the solid was filtered off and washed with just a little iced water, about 30ml, and sucked as dry as possible before being air dried at 35-40°C. The yield of product was 23.334g of a fine white powder.

4.0635g of the crude product was recrystallised from 130ml of water as kmno4 described. It dissolved completely at close to boil point giving a clear pale yellow solution. It was left to crystallise overnight and cooled to about 8°C. The pure white product is still a fine white powder, which after filtering and drying gave 3.379g.

While I was waiting for the solution to crystallise I decided to analyse the crude product.

0.1774g of crude "nitroterephthalic acid" was accurately weighed out and dissolved in 20ml of 0.1865M sodium hydroxide (Note 1). One drop of phenolphthalein solution was added and the excess alkali titrated against 0.2M hydrochloric acid; 11.7ml were required:

Therefore 11.7ml of 0.2M HCl = 0.00234 Moles of HCl = 0.00234 Moles excess NaOH
NaOH used = 20.0ml of 0.1865M = 0.00373 Moles of total NaOH
Therefore NaOH used to neutralise the 2-ntp acid = 0.00373-0.00234 = 0.00139 Moles
It requires 2 moles of NaOH per mole of 2-ntp acid therefore acid = 0.000695 moles 2-ntp acid = 0.1774g

Therefore molar weight = 0.1774/0.000695 = 255.25

Errrrr that is way too high but is very close to a dinitro terephthalic acid but this is impossible given the ratio of nitrating agent to original acid.

Once the recrystallised material had dried it too was analysed in the same way using 0.1705g of this material.
This required 11.75ml of 0.2M HCl to neutralise the excess NaOH solution giving a MW of 247.1

This figure is lower but still too high. The calculated MW of various possibilities are:

2-nitroterephthalic acid = 211.13
any dinitroterephthalic acid = 256.13
nitroterephthalic acid monohydrate = 229.13
nitroterephthalic acid dihydrate = 247.16

This indicates that the product is most likely 2-nitroterephthalic acid dihydrate. Another possibility, though less likely given the dihydrate fit, is that some decarboxylation has occurred and the product is contaminated with 3-nitrobenzoic acid which would tend to increase the apparent MW because it is a monobasic acid.

@kmno4: The liquor left from the recrystallization is only pale lemon yellow and I wondered if you have some contamination of your terephthalic acid with the blue phthalocyanine pigment used to colour some plastic bottles? I only used clear, uncoloured bottles for my source of the acid.

Note 1 The standard NaOH solution was prepared from fine prills which tend to take up water from the atmosphere so although roughly 4g of NaOH were used to make up the 0.5 L of solution it still came out significantly below the target 0.2M. It was standardised against 0.2M HCl solution prepared from commercial Analar 2M HCl.

Late addition: The % yield of nitration product depends on what the product is. Since it now appears that the nitration product is a dihydrate that raised the question as to what the starting material was! So I analysed the raw terephthalic acid as I did for the compounds above and got a MW of 188.3 which is very close to the monohydrate which would be 184.15. Previously I had simply assumed that the terephthalic acid was anhydrous but appears that this is not so.

[Edited on 16-11-2017 by Boffis]
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[*] posted on 19-11-2017 at 12:51


Nitration of TPA
Additional runs and interesting observations.

1) 6 g of TPA, 5 g of KNO3, 40 g of H2SO4 (95,5% by titration), total time of heating 23 hours at 75-80 C (total time of pause
about 37 hours). Even during this long-lasting heating, the "mud" inside the flask remains white. At the very end of heating,
the temperature of water bath was increased to 85 C and after short time, the reaction mixture started getting yellow.
So, this colour is rather some sign of decomposition, not the end of nitration.
2) Input as above, total mass of flask at the beginning of heating was 133,7 g, time of heating is 4+10 hours, with 6 hours
pause, temp. was kept between 70 and 80 C. The mass of the flask at the end of heating was 133,7 g. Cooled, white suspension was added to 100 g of ice-cold water (giving green solution+sediment), filter cake was washed with 2x3 g of cold water and dried. 7,1 g of white powder was obtained.
2a) 8,5 g of TPA, 57 g of H2SO4, 7,11 g of KNO3 (the mass of the flask before and after was 155,9 and 155,5 g), 15 hours of heating... 10,0 g of white powder was obtained.

The mass of combined samples of crude nitro-TPA is 24,0 g. 5 g of this acid was dissolved on hot in 100 g of water, chilled and
precipitated powder filtered. The green filtrate* was used for dissolving another 5 g portion... etc.
During some precipitation, the nitroacid deposited as crystalline powder, much easier to filter than the powder form and containing much less absorbed water.
About 0,5 g of this sample was kept for seeding subsequent solutions**.
Combined powders were combined and weighted: 21,8 g (dried). Titration of 0,32 g sample gave molar mass 214 (not bad).
It was purified in the manner as given above and interesting thing was observed: the hot solution of this partly purified nitroacid was practically colourless,ha.
So, the green coloration does not come from nitro-TPA, but from some highly soluble (fortunately) impurities.
About 21 g of crystalline powder (almost white) was obtained (looks similary to benzoic acid).
* these green solutions look the same as solution of K3[Fe(CN)6], at least for me
** solubility of nitroTPA seems to be 0,3-0,5 g per 100g of water (~10 C) and larger than 7 g (larger amounts were not tested) in boiling water.

Addition.
Boffis, your results and calculations seem to me a little aaa... incoherent. Your "hydrates" may exist only in your posts and the most possibly, it is the case. In this particular case of back titration, I would use methyl orange if your NaOH sol. contains larger amount of carbonate.
Next, I would strongly recommend drying your nitroTPA/TPA at least at 110 C (to constant weight). I have noticed that these acids, in powdery form, absorb rather large amounts of water when filtered off some solution. Vacuum filtration helps nothing, filter cake contains up to 50% water. It has nothing to do with hydrates, because the mass, after drying, does not change when kept in the air at r.t.
My experiments presented in this post were performed independently to yours, just simple coincidence. I have to repeat Fe reduction because it seems that I missed some important thing and the yield of aminoacid was close to zero :P

[Edited on 20-11-2017 by kmno4]




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[*] posted on 26-11-2017 at 12:50


@ kmno4, I been reading up about terephthalic acid and its nitration product again and there is no mention of them forming a hydrates. This is a bit of a mystery to me. Since nitroterephthalic acid is fairly soluble it is possible to titrate it directly so when I am home next I will try this out. I have also ordered some new standard 2M HCl from a reputable supplier just in case my old 2M HCl is not accurate any more (I can't even remember where it came from but I have had it for many years). I will also check the melting point of the nitroterephthalic acid which is significantly lower than the parent acid.

By the way I make up my standard sodium hydroxide solution by dilute a 60% (15M) stock solution because the sodium carbonate formed by absorption of CO2 from the atmosphere is virtually insoluble in strong NaOH and forms a sediment which can be avoided by decanting carefully and diluting. I then standardize it against my 2M HCl (usually dilute to 0.1 or 0.2M). However the last thing I did before leaving home last week was to try titrating my diluted HCl against a primary standard solution made of BDH Analar sodium tetaborate and I did not get good results so it looks like my primary standard my be the problem; hence the new acid.
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[*] posted on 26-11-2017 at 14:59


As a slight aside. This looks like a good candidate for a back titration.


[Edited on 26-11-17 by unionised]
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[*] posted on 26-11-2017 at 23:29


Quote: Originally posted by unionised  
As a slight aside. This looks like a good candidate for a back titration.


[Edited on 26-11-17 by unionised]


What does?

I have discussed my use of back titration above on terephthalic acid due to its low solubility. The nitroterephthalic acid is sufficiently soluble to dissolve 0.2g in 25ml of water (see kmno4's comments above) so I think that it is a candidate for normal titration. Don't you?
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[*] posted on 29-11-2017 at 02:34


Reduction of nitro-TPA (II)
a) Fe
2 g of nitro-TPA, 2 g of NaCl, 2 g of pulverized Fe, ~50 g of water.
The acid is dissolved in the water on hot, NaCl is added. To this solution, the iron is added in one portion.
The solution turns green during seconds, then dirty-green -> more dirty -> dirty-green-brown -> dirty-brown, all the time
sediment is produced. As soon as Fe is added, mechanical stirring must be applied to prevent creation of Fe/Fe oxides chunk,
hard and damned strongly adhering to glass. Stirring with magnetic dipole is not good solution, because its plastic coat will
be damaged by hard particles.During first ~30 minutes, the mixture is hot by self generating heat, after this time it is
externally heated to 70-90 C (mixing with glass rod at minutes intervals is applied) during additional 1,5 h (or longer)*.
After given time, the mixture is cooled and calculated (as for disodium salt + 10-20% more) amount of NaOH is added.
This gives dirty-yellow-brown liquid and brownish sludge. This is carefully filtered, giving nicely looking water-clear yellow
solution and Fe oxides cake. If the solution is reddish/pinkish ... etc, possibly filtering was not perfect and very tiny
amount of Fe oxides are present.
Calculated amount (based on NaOH added) of HCl(aq) is added - this causes precipitation of yellow, very tiny powder, sometimes
semi-colloidal: it is desired amino-TPA. The mixture it then heated to weak boiling, this operation gives better filterable
sediment. The yellow powder is then filtered, washed and dried.Assuming it is pure amino-TPA and pure nitro-TPA was used, the yield is about 90% of theory. Pinhead amount of this powder, dissolved in ~20 cm3 of alkaline water, gives strong, nice fluorescence under UV light.
The strength of the fluorescence depends on amino-TPA concentration and pH. The best results are obtained in highly diluted, weakly alkaline solutions. The yellow amine is also fluorescent (orange) in solid state.
* heated for shorter time and/or bad stirring, may give low yield (>50%) of amino-TPA. However, unreacted nitro-TPA solution can be reduced again with Fe.

b) Na2S2O4
Seems to be optimal choice for the reduction.
Unfortunately - I was not able to obtain more than 40% yield of amino-TPA. Nitro-TPA is reduced completely, because additional
amount of dithionite gives nothing. First portions of NaS2O4 sol. gives interesting deeply-red solution, but it turns yellow,
when about 2/3 of calculated amount* is added. I have no idea why I get such low yield.
* 3 moles of Na2S2O4 per 1 mole of nitro-TPA

[Edited on 29-11-2017 by kmno4]




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[*] posted on 29-11-2017 at 08:54


@kmno4. I wonder if for the dithionite reduction it would be better to dissolve the Nitro-TPA in just sufficient NaOH to give the neutral salt first as dithionite is unstable in acid conditions. The red colour is probably a transient azo compound formed by the condensation of an intermediate nitroso compound with resulting amine. I got a similar result reducing nitro-phenylpyrazolones but here the compound is so insoluble I was able to isolate the azo compound. Further reduction converts it into a hydrazo-derivative but some of these are fairly resistant to further reduction. It might be better to add a sodium salt of N-tpa solution to the dithionite solution to try and limit the amount of transient nitroso that is present at any time.
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[*] posted on 29-11-2017 at 12:38


For Na2S2O4 reduction it was:
2 g of nitro-TPA, 5,8 g of Na2S2O4 (assuming ~85% purity), 2,3 g of NaOH.The acid was dissolved in water with 2 g of NaOH, dithionite was dissolved in ~20 cm3 of water containing the rest of NaoH (0,3 g). This solution was added, in parts, to the solution of just prepared Na2nitroTPA salt. Results as above: red solution, gradually turning yellow, yield of the amine after careful acidification (HCl) around 40%.
This was tried again, with slightly different proportion, but with the same results. Extremely fine suspension of amine powder, looking like about 5 g or so, gives after filtering and drying dissapointing amount of less than 0,7 g of fine yellow powder (it is not sulfur, hah). The filtrate remains yellow, but I cannot force it to give me another portions of amine. Swinging pH gives nothing, but SO2 formation and its dissapiring.




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[*] posted on 16-12-2017 at 14:18


Quote:
For Na2S2O4 reduction it was:
2 g of nitro-TPA, 5,8 g of Na2S2O4 (assuming ~85% purity), 2,3 g of NaOH.The acid was dissolved in water with 2 g of NaOH, dithionite was dissolved in ~20 cm3 of water containing the rest of NaoH (0,3 g). This solution was added, in parts, to the solution of just prepared Na2nitroTPA salt. Results as above: red solution, gradually turning yellow, yield of the amine after careful acidification (HCl) around 40%.(...)

The yield is increased to slightly above 90%, but rather large amount is needed. It seems that Na2SO3, NaHSO3 and SO2 gives some kind of buffer and really large excess of acid is needed to liberate the amino-TPA.
In this way, my experiments with TPA are finished ;)




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[*] posted on 17-12-2017 at 10:22


Hi kmno4, to get the 90% yield do you mean you had to add a lot more hydrochloric acid? I'll be home in a few days so I'll have a go at this.

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[*] posted on 20-12-2017 at 02:51


Quote: Originally posted by Boffis  
Hi kmno4, to get the 90% yield do you mean you had to add a lot more hydrochloric acid? I'll be home in a few days so I'll have a go at this.

Hi, Boffis
Yes, just a typo, I mean HCl(aq).
Calculations were made according to this:
TPA-NO2 + Na2S2O4 + NaOH -> TPA-NH2 + Na2SO3 + H2O
It seems that obtained solution is too basic or TPA-NH2 gives some salt/adduct with SO3/HSO3 and additional portion HC(aq) is added to liberate almost all SO2. Besides, on 4,5 g TPA-NO2 scale and ~200 cm3 of solution, after some time, on cold, large amount of some crystals are formed, possibly Na2SO3 hydrates.
However, when too much hydrochloric acid is added, TPA-NH2 dissolves again and some sulfur slowly precipitates, possibly from
Na2S2O3 formed by the way, but it can be reversed by adding some NaOH.




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