Sciencemadness Discussion Board

Separation of nitrotoluene isomers

fdnjj6 - 29-7-2020 at 13:07

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?

fdnjj6 - 29-7-2020 at 17:55

Apparently just cooling the mix below -12C works just fine in precipitating the para product.

fdnjj6 - 1-8-2020 at 00:18

Actually, cooling to -20C is best according to what I found.

fdnjj6 - 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.

Bubbles - 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.

Metacelsus - 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.