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Author: Subject: Fractional Distillation of Nitro-Toluene
RxnJackson
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[*] posted on 16-5-2008 at 07:05
Fractional Distillation of Nitro-Toluene


Does anyone have any experience in the fractional distillation of Mono Nitro-Toluene? Current lab workup results in an almost 50% ratio of O- and P- Nitration with very little M- (2% ish) theoretically. I'm not too worried about the M as there is so little but I would like to separate the O and P. My reserach shows BPs at..........

O - 220C
M- 232.5C
P- 238.5C

I have seen all sorts of reccomendations regarding HPLC and Columns, etc.

Is it possible to just use a Vigroux Column and slowly ramp the temp until the first fraction comes over?

I have also heard some scary things about fractional distiallation of Nitro compounds regarding flashpoints and explosions. Does this NEED to be under inert conditions or is that a reccomendation?

Thanks for any feedback

-RxnJackson
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[*] posted on 16-5-2008 at 07:21


Doesn't matter much how slowly you raise the temperature, if you don't have enough theoretical plates you don't get separation.

You can cool the mixed nitrates to get much of the para isomer to crystallise out; then recrystallise that from a solvent, use fractional melting, and so on to clean it up. But to get the pure ortho from the remaining mix you'll need distillation or some other method.
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[*] posted on 16-5-2008 at 07:39


Look in the Sciencemadness Library, in the book on nitrotoluenes there's a method of separating the mononitrotoluenes without any fractional distillation.

Cooling to -4 - -10°C and centrifuging gives a liquid of nearly pure o-NT, the solid is the para and meta NT.
Steam distillation then gives the pure para, as the meta is only very slightly volatile with steam.

Fractional distillation requires that the temperature during the nitration never exceeded 30°C. If 30°C were exceeded, there is the danger that some dinitrotoluene has been formed, and a risk of explosion then exists.



[Edited on 16-5-2008 by garage chemist]




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RxnJackson
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[*] posted on 16-5-2008 at 07:55


Thanks GC.........

I checked the library but couldn't seem to find the book on NT's, are you sure it's not in the "reference" section where a password is required, or did I just pass right by it.

Regarding cooling and separation. Seems to be what NI was referring to as well. I don't have a centrifuge on hand here, can I cool and recrystalize as suggested by NI. I think instead of fractional melting, Ill just accept some loss to the separation of the crystalized P-Isomer then re-distill to obtain the pure O-Isomer.

Any glaring issues with that?
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[*] posted on 16-5-2008 at 08:14


You can probably use a glass filter, filter with suction and press the filter cake to get most of the liquid out.

Go to the library, press Ctrl+F and enter "nitrotoluenes", there you have it. The book is mostly about TNT, but the mono- and dinitrotoluenes are covered as well.




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[*] posted on 16-5-2008 at 12:51


Quote:
Originally posted by garage chemist
Cooling to -4 - -10°C and centrifuging gives a liquid of nearly pure o-NT, the solid is the para and meta NT.


Yet Urbanski says (p. 276 of vol. 1) "When the mixture has been cooled to about -12°C, p- nitrotoluene crystallizes out, while the o- and m- isomers remain liquid. Further cooling results in the crystallization of o- nitrotoluene, while the m- isomer remains liquid, forming an eutectic with the o- isomer."

Cumming: "The o-compound may be separated from the p-compound by fractional distillation, using a column. 40% of the mixture is distilled off, consisting chiefly of o-compound. The p-compound is obtained from the residue by cooling at 0°.

If the mixture is cooled to about -20° the p-compound crystallises out."

Things not in question:
The Vigreux is not a very good column.
Fractional distillation in vacuum is a good idea for avoiding explosions.
But first, wash with dilute NaOH and then water until neutral or better steam distillation. This gets rid of the colored material.

Bretherick's: "Solutions of 4-nitrotoluene in 93% sulfuric acid decompose very violently if heated to 160C"

"Explosions have occurred during fractional distillation to separate mixed nitrotoluene isomers when excessive heating was employed, or when the materials were held at more moderate temperatures but for long periods. The factors involved were identified by a series of systematic thermal stability tests on various mixtures, some including dinitrotoluenes. It was established that decomposition would not occur on the large scale if still temperatures were limited to 190C."
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[*] posted on 18-5-2008 at 02:08


Besides the above references, the freezing method is also described in detail in Organikum (at least in the old edition I have). Mind however, that this will only work if there is no toluene present or else nothing will crystallize out no matter how long you keep it in the freezer. This means, a vacuum distillation to remove the unreacted toluene from the crude mixture from the nitration is advised.



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[*] posted on 19-5-2008 at 06:22


I was just going to ask about cooling in the presence of unreacted Toluene, as it seemed to me, the isomers (even in a solid state) would remain liquid in the toluene.

This brings up my other concern then. I was very careful during the addition of the toluene to maintain temps of 30C or lower, however, for the sake of discussion lets assume there is some presence of di-nitro in the mix (probably a safe assumption). I can smell the unreacted toluene and I was planning to distill this off. Is this safe, or do I need to do this in-vaccuo? I know you specifically said "Vacuum distillation" Nico, but I was just curious.

one other note: I have neutralized the unreacted acid with NaOH and washed with distilled H2O until clear. So that step was taken care of.

[Edited on 19-5-2008 by RxnJackson]

[Edited on 19-5-2008 by RxnJackson]
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[*] posted on 19-5-2008 at 15:26


Unreacted toluene is good if there was a slight excess but you shouldn't be able to smell it over the nitrotoluene. Adding toluene to the acid is not a good way to make the mononitrotoluenes.
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[*] posted on 19-5-2008 at 15:37


Yes, the mononitration of toluene always uses reverse addition: the nitrating acid is slowly added to toluene.
Doing the opposite increases the likeliness of having dinitrotoluenes in it, even if the temperature stayed below 30°C.

But the removal of unreacted toluene can be done at atmospheric pressure- the temperatures required are below the boiling points of the nitrotoluenes. I've done it this way as well. I even once did the fractional distillation of the nitrotoluenes at atmospheric pressure.




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[*] posted on 20-5-2008 at 06:02


Just as a note, I can smell the unreacted Toluene, but just barely, there is obviously a much stronger odor of the nitro-toluene which I am farmiliar with.

I also did the addition in reverse, added the Toluene to the nitrating mixture as I have seen workups and papers on both. The Nitro-Toluene Compound Paper that exists somewhere on this forum offered reasons for doing it this way, as well as a number of other refs I have seen. I can dig them up if people are interested or quote them distinctly. My general understanding though, was that there is more likelyhood of creating di nitro compounds if the acids are added to the toluene as it initially creates concentrations of nitrating agent which can locally increase the conditions necessary to create di nitro compounds as the reaction is two suspended layers.

Though I think I could understand how this could be the case for the reverse as well (the addition of the toluene to the nitrating agent).

G.C. Would 150C be an appropriate distillation temp for the removal of the toluene?
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[*] posted on 20-5-2008 at 15:18


Quote:
Originally posted by RxnJackson
added the Toluene to the nitrating mixture as I have seen workups and papers on both. The Nitro-Toluene Compound Paper that exists somewhere on this forum offered reasons for doing it this way, as well as a number of other refs I have seen. I can dig them up if people are interested or quote them distinctly

... the reaction is two suspended layers.


Yeah go ahead and quote them. They probably mention some sort of mixing of the layers as well. Try vigorous stirring.




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[*] posted on 21-5-2008 at 07:32


"There is an important feature of nitrating toluene to nitrotoluenes, namely
the very low solubility of toluene and nitrotoluenes in nitrating mixtures. Therefore
the nitration proceeds in a two phase system and the rate of nitration depends
greatly on dispersion which in turn depends on keeping the two phases efficiently
stirred. Attention has previously been drawn to the importance of stirring on the
yield of nitrotoluene (Fig. 25, p. 153)."

"Sometimes, for example, if the nitration mixture is poor in nitric acid, tar
formation may take place readily. It may also occur if an insufficient amount of
the nitration mixture (calculated on HNO3) has been used for the nitration. For
this reason it is preferable to use mixtures richer in HNO3, and to use a distinct
excess of the nitrating mixture. On the other hand, both these factors (high content
of HNO3, and excess of nitrating mixture) favour oxidation processes in
practice; therefore, limits are applied, i.e. nitrating mixtures should not be too
rich in HNO3, and a very slight excess (e.g. 1-2%) of it should be used."

"The reaction most probably occurs on the surface dividing the two reacting
layers. The rate of nitration is therefore determined by the concentration of reactants
on the surface. This in turn is determined by the rate of diffusion of two phases."

"The order of introducing the reagents may also affect the composition of the
nitration product. For example, if the toluene is added to the acid, then the first
portions of toluene come into contact with a large excess of acid, and the nitration
process may proceed beyond mononitration resulting in a mixture of di- and mononitrotoluene.
If the amount of HNO3 used has been calculated accurately, the
last portions of toluene may be left unnitrated and the resultant product may
not be uniform. If, on the contrary, the acid is added to the toluene, an excess
of acid cannot occur, since each portion added reacts with the toluene very quickly."

All the above is from - "CHAPTER VIII
NITRO DERIVATIVES OF TOLUENE" from "CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES"


It appears from the above that I should not have made my addition in reverse (Toluene to Nitrating Mix), though I can still salvage my mono- products via steam distillation then straight distillation to remove the Toluene excess. Correct?
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[*] posted on 17-6-2008 at 12:18


Hi
I would like to get pure p-nitrotoluene.

I have nitrated some toluene this way :
30 mL toluene
+(25 mL 58% HNO3 & 25mL 96% H2SO4)

I washed the organic layer with a NaHCO3 solution till there was no bubbles left.
Then I heated the organic layer above 110°C to remove toluene and water left.
I got approx. 20 mL of a beautiful yellow oil (with rainbow reflects when I looked the bottom of the bottle)

I put it in the fridge (-18°C) then all the solution cristalized and gave a pale yellow solid with some little cristals in the matrix.
I took that solid and put it on a büchner filter, as the temperature was rising the ortho nitrotoluene melted and was filtered out, only the para remained on the filter paper when all was at the room temperature.

I repeated this 3 times and got approx. 2.5 g of paranitrotoluene.
I saw that I couldn't take more para isomer this way.

Then I think of performing a distillation.
BUT it's strongly advised not to heat above 108 °C (flash point or para isomer).
What do you think of ? It would require a pump of 15 torrs (mine is only 30 torrs) to reach boiling points at temperatures below 100°C

What about steam ? Is it possible to separate ortho and para only with water steam ? (then it would be safer !!)
Or under vacuum ?

What do you advise ???

Thank you for commenting !
Excuse me for my poor english !

Peter
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[*] posted on 17-6-2008 at 13:47


I believe that the base washing is not just to neutralize acid, it is to remove any cresols present, which bicarb will not do. Steam will not separate the isomers. Aspirator vacuum is fine. Most important is a good column, to get good separation.
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[*] posted on 17-6-2008 at 13:52


I think the temperature at which you performed the nitration is the important point for an eventual fractionnal distn. Any dinitrotoluene around would pose a hazard, more so than nitrotoluene. Flash point isn't very revelant unless there are any sources of sparks which hopefully there sin't in your distn setup :) Most commonly used solvents have flash points under RT, but we can distill them just fine under atm pressure.

I'm not sure of the temp you are not supposed to pass during the nitration to perform the distn safely, but i thinkit has been mentionned elsewhere in the forum before.




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[*] posted on 17-6-2008 at 23:41


Hi,
Thank you for you replies here is what Merck index gives about ortha and para nitrotoluene :
Derivative Type: o-Nitrotoluene
CAS Registry Number: 88-72-2
Additional Names: 2-Nitrotoluene
Properties: Yellowish liquid at ordinary temp. d1519 1.1622. mp -10°. bp 222°. nD20 1.5472. Almost insol in water. Sol in alcohol, benzene, petr ether.
Melting point: mp -10°
Boiling point: bp 222°
Index of refraction: nD20 1.5472
Density: d1519 1.1622

Derivative Type: p-Nitrotoluene
CAS Registry Number: 99-99-0
Additional Names: 4-Nitrotoluene
Properties: Yellowish crystals. d 1.286. mp 53-54°. bp 238°. Flash pt 106°C. Almost insol in water. Sol in alcohol, benzene, ether, chloroform, acetone.
Melting point: mp 53-54°
Boiling point: bp 238°
Flash point: Flash pt 106°C
Density: d 1.286

Indeed we shoud perform a distillation to reach a minus of 222°C
I'm not sure it was a good idea to distill at yhis temperature using a heating mantle to reach that. Because there may be decomposition and the liquid is not necessarily pure, perhaps it contains substances that could favor decomposition (Alkali ? MgSO4 , polynitrotoluene ?)

I think the best is to do it in a bath oil at approx 150°C with vacuum water trump for the ortho isomer to distill, and then keep the residue

What do you think of ?
Thank you !
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[*] posted on 18-6-2008 at 03:59


Note that you are talking about a 16 C difference in boiling point at standard pressure and ~230 C. This isas fractional distillation territory, you'll need a halfway decent column to pull it off. Also note that fractional distillation gets a little more difficult under vacuum, even the relatively low-grade 100 mmHg you're talking about.


A controlled Zinin reduction on the crude nitrotoluenes + unreacted toluene, followed by an acid wash to remove the aromatic amines, might be a way to reduce the amounts of poly-nitro-toluenes in the crude mix. Under some conditions this also appears to reduce the m-nitrotoluene at a higher rate than the o- and p- isomers. The crude mix then could be processed by the conventional route of freezing out some of the para isomer, followed by fractional distillation of most of the ortho isomer, leaving a mix of o- & p- isomers in the still pot.

A decent wash job will remove inorganics, using an excess of toluene to favour mono-nitration means that the mixed nitrotoluenes can be dried by distilling off the toluene which removes water as the azeotrope, no drying agents need be used.
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