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[*] posted on 29-7-2020 at 13:07
Separation of nitrotoluene isomers


In the mono nitration of toluene, the ortho and para product are mainly formed. How can you separate the two? Some threads mentioned steam distillation being able to remove the ortho product. Others said to attempt fractional crystallization.

Are there any other, possibly better, methods of separating the two from each other?
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[*] posted on 29-7-2020 at 17:55


Apparently just cooling the mix below -12C works just fine in precipitating the para product.
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[*] posted on 1-8-2020 at 00:18


Actually, cooling to -20C is best according to what I found.
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[*] posted on 15-8-2020 at 03:11


Well, just to give an update in case anyone cares,
unless your vacuum pump can handle the vapors, I wouldn't recommend it.

Gravity filtration doesn't work. I used freezer temp salt water which failed. I tried again using compressed air duster. Failed. I managed to get around 500mg. With a lab freezer to make larger crystals and a buchner funnel, don't even try it. It won't work.
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[*] posted on 15-8-2020 at 09:05


Freeze to crystallize out most of the para isomer (if it doesn't crystallize, try leaving it in for longer, because it really should. If it still doesn't crystallize, perhaps your toluene isn't fully nitrated. Try distilling and see if toluene comes over at 111C).
After vacuum filtering (I love fridge pumps, they can take a punch and are cheap), save your liquid ortho isomer, then you could wash the p-nitrotoluene crystals on the filter with a tiny amount of ice-cold light pet ether to wash off remaining o-nitrotoluene.

To remove the last bit of p-nitrotoluene (BP=238) from the o-nitrotoluene (BP=222C), slowly(!) fractionally distill over the o-nitrotoluene, leave the last bit in the boiling flask which will contain a mix of the isomers.

Nitration of toluene will also produce 5% meta-nitrotoluene, which has a melting point of 59C and a boiling point of 232C. I don't know if it will crystallize out in the freezer with the p-nitrotoluene, or if it stays dissolved in the o-nitrotoluene. In the latter case, it will also stay in the boiling flask during distillation if you stop in time. If it ends up with the p-nitrotoluene you could distill this and discard the first fraction to purify p-nitrotoluene.

Depending on what you intend to do with the product(s), separation might be easier to achieve downstream. For example, several methods exist for separating toluidines. However, I have good experience with producing relatively pure o and p nitrotoluene.
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[*] posted on 15-8-2020 at 18:46


In my experience, sticking it outside in a Minnesota winter night (–25 °C) will freeze the p-isomer out from a mixture.



As below, so above.
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[*] posted on 16-2-2021 at 13:39


I plan on doing this in the very near future. I'm trying to see how low I can get with ice/salt, and I can't seem to get lower than -18.7 °C (just 1.4 °C shy, lol) >_< I'd prefer not using my freezer that I use for food.

Anyone able to get to the right temperature for separation using salt/ice? I know that you should be able to get to -20 °C, but I can't get there.

Edit: Nevermind, it looks like it was my multimeter (OWON B35T+), which seems to be a little questionable at negative temps. Ironically my cheap crappy thermometer said it got to -19.9 °C, which is probably good enough.

[Edited on 16-2-2021 by SuperOxide]
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[*] posted on 17-2-2021 at 04:45


Hi SuperOxide, I did this recently, not yet fully finished.
My suggestions:

1.
during the nitration, always add nitration acid into the toluene, never vice versa. Adding toluene into nitration acid would produce much more dinitrotoluenes.

2.
distill the nitrotoluenes mixture under vacuum, in my case 25-30 g of dinitrotoluenes stayed as a remainder in distillation flask for 1 mol of nitrotoluenes (no matter I performed nitration at 0-5 C, there were always dinitrotoluenes residuum) - collect distillate distilling in range not more than 25-30 C (e.g. I collected at T 70-100 C under 2-stage rotary oil vacuum pump, at the end the rate of distillation decreased 5-10 times which is very remarkable together with significant temperature increasing - my pump is powerful enough to distill dinitrotoluenes, but that would clog my condenser if using cold water circulation)

3.
I tried to freeze this distilled mononitrotolenes mixture and found that -18 C in freezer was too low, the whole content solidified.
Then I tried -5 C (outside during winter) which was good and I got 31,4 g of 4-nitrotoluene crystals from 2 mols of the mixture (from cca 250 g) - I separated crystals by vacuum filtration. I washed crystals with 10 ml of freezing cold isohexanes on sintered glass at the end.
Then the filtrate on cooling down to -10 C yielded again some crop of crystals but much less than the first crystallization, I separated these crystals by decantation and then let the filtrate to freeze to -15 C which again gave some crystals, the remaining liquid poured out = decantation. Temperature -15 C revealed as good temperature unlike -18 C in freezer (complete solidification). Although m.p. of pure 2-nitrotoluene is -10,4 C, in my case it did not solidify at -15 C due to m.p. depression caused by impurities, but at -18 C it solidified completely even with impurities.
By shaking the flask with decanted crystals I forced them to adhere to one side of the RBF, I stoppered the flask and laid it to the opposite wall (so the crystals stayed up) and let it to lay on flask side for few days outside lab in winter temperatures in such a position where crystals up - some amount of liquid collected at the bottom and dry and purer crystals stayed stuck on the opposite wall, they did not fall down into the collected liquid. Slight temperature oscillations (outside during winter) caused to melt some portion of the crystals but that helped to improve their purity at the cost of decreased yield, I let them in the flask always for 1 week and removed liquid from the bottom every day, all this liquid was kept together and added to the step No. 4.
This crystalline 4-nitrotoluene still melts at 30-40 C and should be recrystallized from methanol (not yet done). Lit. m.p. 51,63 C.

4.
I will vacuum distill the liquid through column packed with Raschig rings (not yet done) which will yield pure liquid 2-nitrotoluene as distillate and 4-nitrotoluene as residuum. Then crystallize the residuum from methanol to obtain more 4-nitrotoluene.

5. I played with recrystallization of dinitrotoluenes using 95% denatured ethanol and got more than 5 fractions but I must investigate it further (using 150 ml of hot ethanol for 15 g of dinitrotoluenes, cooling down to 20 C yielded 10 g of crystals, then cooling down mother liquor to 0 C gave another crop of less pure crystals and final cooling down the filtrate to -18C gave another crystals. Then a distillation of the used ethanolic mother liquor showed that there was almost no distillation residuum so almost everything crystallized out at -18 C. So very likely cooling down 10% solution of dinitrotoluenes in 95% ethanol is optimal temperature where 2,4 dinitrotoluene crystallizes out and other isomers stay in mother liquor. Cooling down to -18 C crystallizes out almost everything (coz almost no residuum when distilling this mother liquor) thus producing mixture of all isomers which is undesirelable, I would like to obtain only 2,4 dinitrotoluene.

[Edited on 17-2-2021 by Fery]




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[*] posted on 17-2-2021 at 08:46


Quote: Originally posted by Fery  
Hi SuperOxide, I did this recently, not yet fully finished.
My suggestions:

1.
during the nitration, always add nitration acid into the toluene, never vice versa. Adding toluene into nitration acid would produce much more dinitrotoluenes.

2.
distill the nitrotoluenes mixture under vacuum, in my case 25-30 g of dinitrotoluenes stayed as a remainder in distillation flask for 1 mol of nitrotoluenes (no matter I performed nitration at 0-5 C, there were always dinitrotoluenes residuum) - collect distillate distilling in range not more than 25-30 C (e.g. I collected at T 70-100 C under 2-stage rotary oil vacuum pump, at the end the rate of distillation decreased 5-10 times which is very remarkable together with significant temperature increasing - my pump is powerful enough to distill dinitrotoluenes, but that would clog my condenser if using cold water circulation)

3.
I tried to freeze this distilled mononitrotolenes mixture and found that -18 C in freezer was too low, the whole content solidified.
Then I tried -5 C (outside during winter) which was good and I got 31,4 g of 4-nitrotoluene crystals from 2 mols of the mixture (from cca 250 g) - I separated crystals by vacuum filtration. I washed crystals with 10 ml of freezing cold isohexanes on sintered glass at the end.
Then the filtrate on cooling down to -10 C yielded again some crop of crystals but much less than the first crystallization, I separated these crystals by decantation and then let the filtrate to freeze to -15 C which again gave some crystals, the remaining liquid poured out = decantation. Temperature -15 C revealed as good temperature unlike -18 C in freezer (complete solidification). Although m.p. of pure 2-nitrotoluene is -10,4 C, in my case it did not solidify at -15 C due to m.p. depression caused by impurities, but at -18 C it solidified completely even with impurities.
By shaking the flask with decanted crystals I forced them to adhere to one side of the RBF, I stoppered the flask and laid it to the opposite wall (so the crystals stayed up) and let it to lay on flask side for few days outside lab in winter temperatures in such a position where crystals up - some amount of liquid collected at the bottom and dry and purer crystals stayed stuck on the opposite wall, they did not fall down into the collected liquid. Slight temperature oscillations (outside during winter) caused to melt some portion of the crystals but that helped to improve their purity at the cost of decreased yield, I let them in the flask always for 1 week and removed liquid from the bottom every day, all this liquid was kept together and added to the step No. 4.
This crystalline 4-nitrotoluene still melts at 30-40 C and should be recrystallized from methanol (not yet done). Lit. m.p. 51,63 C.

4.
I will vacuum distill the liquid through column packed with Raschig rings (not yet done) which will yield pure liquid 2-nitrotoluene as distillate and 4-nitrotoluene as residuum. Then crystallize the residuum from methanol to obtain more 4-nitrotoluene.

5. I played with recrystallization of dinitrotoluenes using 95% denatured ethanol and got more than 5 fractions but I must investigate it further (using 150 ml of hot ethanol for 15 g of dinitrotoluenes, cooling down to 20 C yielded 10 g of crystals, then cooling down mother liquor to 0 C gave another crop of less pure crystals and final cooling down the filtrate to -18C gave another crystals. Then a distillation of the used ethanolic mother liquor showed that there was almost no distillation residuum so almost everything crystallized out at -18 C. So very likely cooling down 10% solution of dinitrotoluenes in 95% ethanol is optimal temperature where 2,4 dinitrotoluene crystallizes out and other isomers stay in mother liquor. Cooling down to -18 C crystallizes out almost everything (coz almost no residuum when distilling this mother liquor) thus producing mixture of all isomers which is undesirelable, I would like to obtain only 2,4 dinitrotoluene.

[Edited on 17-2-2021 by Fery]


Thanks! However I plan on making the mono-nitrotoluene, not the di-nitrotoluene. I think that changes things up a bit (except for your first point of adding the nitration mix to the toluene).
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[*] posted on 17-2-2021 at 09:47


Yes I understand, I wanted to produce only mononitrotoluenes like you... unfortunatelly even when nitrating at 0-5 C I got cca 10% of unwanted dinitrotoluenes... standard nitration procedure runs even at much higher temperatures, I just tried to go as cold as possible and still dinitrotoluenes produced (30 g at temp from standard procedures and 25 g at temp 0-5 C).



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[*] posted on 18-2-2021 at 20:23


I plan on making some mononitrotoluene soon, and I'm particularly interested in the ortho isomer, which it looks like the most accessible way for me to separate it from the other isomers is by freezing it (as mentioned above and in other threads).

The 2-nitrotoluene page on Wikipedia says depending on which form it crystalizes in, the ortho isomer can melt/freeze at either −9.27 °C (for the α form) or −3.17 °C (for β). And the Pubchem page says the same thing.
Now I would think that this means I would want to chill it down to a temp just above that (eg: -8.27 °C), so that the ortho is a liquid, but the meta and para are solids (thus you would just pour it off).

However it seems like most recommend bring it to even lower than that. How doesn't this just freeze all three isomers? Even in one of Chemplayers old o-nitrotoluene synth videos, they mention bringing it down to -12 °C, and it still not freezing at all. I'm a little confused as to how this can happen if it should be a solid at -12 °C.

(I'm really hoping this question isn't as stupid I have a feeling it is, lol)

Edit: Is the -12 °C temp meant to separate all the mononitrotoluene isomers from the toluene itself? (which has a mp of -95°C). If so, that makes sense. But I don't think that's what yall are referring to (esp since ChemPlayer said they brought their o-nitrotoluene to -12 °C and it was still a liquid).

[Edited on 19-2-2021 by SuperOxide]
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[*] posted on 19-2-2021 at 05:55


Superoxide, by my experience from 250 g of mononitrotoluenes (necessary to vacuum distill the crude product of nitration to separate a little of dinitroderivates) by crystallization at -15 C I got cca 50 g of 4-nitrotoluene crystals which means that resulting liquid mixture still contains a lot of 4-nitrotoluene together with 2-nitrotoluene (and cca 4% of 3-nitrotoluene). So freezing is good to obtain part of 4-nitrotoluene but not enough for obtaining pure 2-nitrotoluene. For pure 2-nitrotoluene you need vacuum distillation on column which does require not only efficient column with good packing (Raschig rings) and vacuum pump (my 2-stage with theoretical max. vacuum 0,3 Pa decreased boiling point of the 2 mononitrotoluenes to 70-80 C) but it does require distillation head to set reflux ratio (so 10 drops return back into the distillation column and 1 drop passes further into receiver). I mean this: https://www.sciencemadness.org/whisper/viewthread.php?tid=71...
The theoretical melting points are correct but practically you should freeze the mixture more but not too much more :)
In my experiment -15 C was OK but -18 C was too much. Maybe when using mixture contaminated with unsepared dinitrotoluene the mixture could withstand freezing to -18 C due to the more contaminants the mixture could be even more impure so the melting point even more depressed.
But your goal is 2-nitrotoluene and there you have the only way vacuum distillation on efficient column (plus other equipment necessary to perform it).




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[*] posted on 19-2-2021 at 08:14


Yeah, could be contamination. Depending on the actual procedure followed there could still be sulfate, nitrate, solvents, or side products that are depressing the freezing point.
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[*] posted on 19-2-2021 at 09:31


Quote: Originally posted by Fery  
For pure 2-nitrotoluene you need vacuum distillation on column which does require not only efficient column with good packing (Raschig rings) and vacuum pump (my 2-stage with theoretical max. vacuum 0,3 Pa decreased boiling point of the 2 mononitrotoluenes to 70-80 C) but it does require distillation head to set reflux ratio (so 10 drops return back into the distillation column and 1 drop passes further into receiver). I mean this: https://www.sciencemadness.org/whisper/viewthread.php?tid=71...

Well that's definitely out of reach for me. I only have a hand pump vacuum thingy (though I am told that that can be used for vacuum distillation, so I may give it a go), and I don't have a fractionating column, and certainly don't have that fancy piece of amazing looking glassware you linked to. lol.

Quote: Originally posted by Fery  
The theoretical melting points are correct but practically you should freeze the mixture more but not too much more :)
In my experiment -15 C was OK but -18 C was too much. Maybe when using mixture contaminated with unsepared dinitrotoluene the mixture could withstand freezing to -18 C due to the more contaminants the mixture could be even more impure so the melting point even more depressed.
But your goal is 2-nitrotoluene and there you have the only way vacuum distillation on efficient column (plus other equipment necessary to perform it).

Quote: Originally posted by njl  
Yeah, could be contamination. Depending on the actual procedure followed there could still be sulfate, nitrate, solvents, or side products that are depressing the freezing point.

This is still a little confusing to me though because I've heard multiple times that if you bring it down to below -12 °C and it doesn't freeze, then that's a sign of purity (again, in the video Chemplayer uploaded (I linked to the specific part in the video as well) as an example). Could Chemplayer be implying that it's supersaturated or something? And that's why it's still liquid below the point at which it should be freezing?
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[*] posted on 19-2-2021 at 11:58


Mine one yielded by cooling down to -5 C quite a lot of 4-nitrotoluene crystals. But I vacuum distilled the mixture prior freezing which was not done in your mentioned experiment. I vacuum filtered these crystals and washed with 10 ml of isohexanes mixture with the same temperature. From cca 250 g of mixture I got 31,4 g of 4-nitrotoluene, but it still melts at 30-40 C and requires recrystallization from methanol (not yet done). The filtrate was then further cooled to -10 C and gave some crystals again, they are still in a flask and I did not weigh them yet. Mother liquor was now not separated by filtration but by pouring out of flask, shaking flask strongly so crystals adhered to one side of RBF, the flask stoppered and laid in such a position on flask side that crystals stuck in upper part so liquid flew down slowly and liquid was removed every day by simple pouring out. Then the same repeated by further cooling down to -15 C and similar yield of 4-nitrotoluene crystals obtained. I approximate the crop at -10 C to be 5-10 grams and -15 C again similar amount 5-10 grams but I did not yet weigh them, they are adhering to flask side and I will sometimes recrystallize them from methanol. But they are almost without liquid contamination, almost all liquid drained down from them during 1 week at outside winter temperature (no need to mess sinter funnel, no need to wash with isohexanes, but much slower and you need winter temperature).



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[*] posted on 21-2-2021 at 08:08


I'll chime in!

This was the first nitration I did, and it was a bit of a challenge because I was on a bit of a schedule and had reserved too little time for the nitration. Stirring could also have been better; there will be two layers and those should ideally be kept emulsified at all times. I was using a rather small stir bar at 800 rpm, IIRC. The layers were separate save for the puny vortex in the middle. I'm certain that this slowed the reaction down considerably.

Anyway, during workup the layers were behaving unexpectedly and, when I went to chill down the product which looked perfectly fine, no crystals would form and the mixture stayed liquid all the way to -28 or so. There had to still be a fair amount of toluene present.

To get rid of the toluene, I ended up steam distilling the mixture until the non-polar droplets would sink to the bottom. I would then check the aroma of fresh distillate for hints of toluene, just in case. After this I believe I was left with a yield of around 35%. Thankfully I usually up the scale wherever I can, so this was still just enough for what I had in mind. However, I hydrogenated the o-nitrotoluene and had the yield been higher I could have properly distilled the resulting toluidine. Though even that would greatly benefit from reduced pressure and a short path distillation apparatus, which I did not have.

"Freeze distillation" was used to crudely remove the para-isomer which was filtered out of the ortho-isomer in three portions and washed with petroleum ether, then stored away. As a minor product, the meta-isomer was completely disregarded. The mixture did indeed get somewhat colder than 11 degrees without completely freezing. It is almost certainly caused by impurities, though for me this was of no apparent consequence. However, it could be that the crude work up here had something to do with the difficulty I experienced while purifying my end product, methaqualone.

SuperOxide, the handheld vacuum pump is great for quick vacuum filtrations and okay for quickly stripping the last few milliliters of solvent at the end of a distillation. However, it will quickly get destroyed by solvent vapors, so you won't get very many full distillations out of it. The gasket is likely the first thing to go, which would mean one has to be pumping constantly.


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