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Author: Subject: Preparation of Mononitrotoluenes (o-, p-)
Fery
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[*] posted on 17-4-2021 at 22:49


S.C. Wack do you mean using 91% IPA for recrystallization of mixture of dinitrotoluene isomers, or recrystallization of 4-nitrotoluene?
For removing unreacted toluene I suggest vacuum distillation, without vacuum you have to heat the mixture much higher than b.p. of toluene which caused darkening of obtained dinitrotoluenes. For separation of mono and dinitro the distillation in vacuum is must to go, literature claims that in industry explosion disasters were reduced when heating dinitro only upto 190 C (and b.p. of mononitro isomers are in range approximately 220-230 C at atmospheric pressure). It would be nice to have lab. scale preparation without dinitro formation. I do not see usefulness of 2,4 dinitrotoluene in home lab - maybe oxidation with dichromate+H2SO4 to 2,4 dinitrobenzoic acid, then decarboxylation into meta dinitrobenzene (which could be synthesized form nitrobenzene but the nitration requires fuming nitric acid so this preparation is not trivial) and then reduction to 3-nitroaniline.




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[*] posted on 18-4-2021 at 14:55


IPA from the store is so good at crystallizing things with little solubility in water (usually better than other alcohols) that I've run out of it and have to buy some more. I had 4 separate solid layers and good crystals with the 60% acid product but have not taken the mp's. It's mostly p-NT. I have an idea for oxidation with Cr to try out.

If one has little toluene left, with good vacuum it can be difficult to condense, ending up in the pump. For sure the SOC direction can be followed even if one substitutes the 1.44 acid with weaker acid (without increasing the sulfuric) or adds toluene to acid, as it also calls for distilling only the first 40%.

PS the relevant part of Houben-Weyl (vol 10 part 1) (BTW this mononitration of monoalkyl aromatics section starts out talking about 1 eq HNO3 in mixed acid of 10-15% water at 20C, not the 20% water of the Belgian reference 2 that starts this and does not give that yield with that nitric ratio IME...PS2 note the workup hint):

houbenweyl.gif - 54kB

[Edited on 19-4-2021 by S.C. Wack]




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[*] posted on 21-4-2021 at 15:19


Quote: Originally posted by S.C. Wack  
It's mostly p-NT.


I'm beginning to doubt this, as a large fraction recrystallized 3x from n-propanol has a mp of 66C (2,6 dinitrotoluene)...all the less solubles give a strong positive for DNT...It's entirely possible that the 91% IPA gave a positive DNT test because of acetone in it...not sure if it was made from acetone or what...not sure what caused dinitration either, maybe with the water and magnetic stirring, the nitrotoluene was better able to dissolve in the acid than the toluene. If I had nothing else to do I'd try adding something to get better contact and see what that does.




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[*] posted on 22-4-2021 at 00:53


S.C. Wack - I have some mixture of dinitrotoluenes from mother liquor after etanol recrystallization of 2,4 dinitrotoluene (identified by m.p. which is the same as lit. m.p.). So do you suggest to recryst my remainder mixture from 1-propanol to obtain 2,6 dinitrotoluene which is enriched in my mixture (after I obtained 2,4 dinitrotoluene)? Or 91% IPA?



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[*] posted on 22-4-2021 at 14:03


It's rather hard to say as I don't have any IPA, or mixtures of isomers it seems...the entire less soluble fraction has a mp of 66C regardless of how many times it's been crystallized, whether it's (faintly) chlorine yellow or brown. It should be mentioned that these odorless needles are insoluble in water with or without base, the thermometer was tested with boiling acetone, (PS the toluene was pretreated per Vogel, probably unnecessarily) and the mag stirring was as intense as mag stirring can be...it is the 2,6 version, and a lot of it. This explains why there was no o-NT in the waste acid, and not much anywhere else; it was nitrated preferentially over the p. A hint for why this might be was found in IEC 785 (1955) (Mononitration of o and p-nitrotoluene), where it is said that both are nitrated with ease, but not so much the p-NT, with smaller amounts of sulfuric acid.

[Edited on 23-4-2021 by S.C. Wack]




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[*] posted on 2-5-2021 at 13:22


PS: 91% IPA (still more expensive than 2 years ago, could be permanent) + refrigeration gave better crystals than n-propanol under the same conditions, especially p-nitrotoluene. That slight brownness to my DNT was cleared up perfectly by boiling for a couple minutes in IPA with decolorizing carbon. This gave long and hair-thin, colorless (until it turned chlorine yellow on drying in the window) transparent needles. The p-NT, already melting correctly and yellow, gave considerably thicker and slightly longer needles, which are also different than the DNT in being pretty much white (opaque), with the slightest hint of chlorine yellow, without decolorizing. And of course the strong smell of MNT.

With second thoughts about using sulfuric acid as solvent for oxidizing the p-NT (w/ Cr+6) because of all the OS warnings already with more dilute acid, KMnO4 was used...the o- would be more convenient either way, as some solid p-NT will be found in cold reflux condensers...Zn/bisulfite will be tried for PABA. (edit: wow...that was easy; the whole off-topic sequence from after isolating the p-NT to before isolating the PABA)

[Edited on 3-5-2021 by S.C. Wack]




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