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] |
