Sciencemadness Discussion Board - Separation of xylene isomers

Separation of xylene isomers

Keras - 1-4-2020 at 01:21

There is an old thread discussing this topic, but it's been left hanging, and it's more than four years old, so I'm trying to get fresh information by creating a new one. If this is inappropriate, please merge.

I'm basically interested in isolating m-xylene from a mix of isomers. This sounds like a challenge, but I have recently come across some useful info in a old organic chemistry book: "Perkin and Kipping's Organic Chemistry" (1934). The relevant snippet is this:

"m-Xylene is readily separated from the other isomerides by digesting with dilute nitric acid, which oxidises o- and p-xylene to the corresponding toluic acids, C6H4(CH3)-COOH, but does not readily attack m-xylene ; the product is rendered alkaline by the addition of potash, and the unchanged hydrocarbon purified by distillation in steam and fractionation. The isolation of o- and p-xylene depends on the following facts : (1) When crude xylene is agitated with concentrated sulphuric acid, o- and m-xylene are converted into sulphonic acids, C6H3(CH3)2-SO3H; p-xylene remains unchanged, as it is only acted on by fuming sulphuric acid. (2) The sodium salt of o-xylenesulphonic acid is less soluble in water than the sodium salt of m-xylenesulphonic acid ; it is purified by recrystallisation, and converted into o-xylene by heating with hydrochloric acid under pressure."

I thought this was interesting to share, and I wonder if anyone could test this (or has already tried). I'd do it myself, but under the current situation in Europe, I can't get access to my lab.

Thanks!

mackolol - 1-4-2020 at 03:05

Thanks for sharing, very interesting topic.
Quite long time ago i wanted to make lidocaine and i wanted to start by separation of xylene isomers. I did it with sulfuric acid, but after all I wasn't sure if my isomer is pure. Then I abandoned the project because making o-dimethylaniline (precursor do lidocaine) seems almost impossible to make in home lab (the separation of great amount of isomers forming during nitration, and then reduction is super hard)
I didn't know about the oxidation with nitric acid though.

[Edited on 1-4-2020 by mackolol]

Keras - 1-4-2020 at 04:07

Coincidentally, I'm interested in the same synthesis.

From the same book I discovered an alternative route to m-xylene. Instead of trying to separate it from a crude xylene mixture, acetone is condensed over sulphuric acid to form mesitylene (CH₃)₃C₆H₃. This product is gently oxidised by dilute nitric acid to form mesitylic acid (CH₃)₂C₆H₃(COOH) which is then decarboxylised to get 100% pure m-xylene.

As for getting 2,6-xylidine from m-xylene, it seems direct nitration is not viable, which makes sense given the directing properties of the methyl- groups. Apparently, a much better way is to use 2,6-xylenol as an intermediate, but I must delve into this.

EDIT: this is partly true. I found an article I had previously downloaded about nitration of m-xylene. The yield of 2,6-xylidine is only 25%, while the rest is 3,5-xylidine. A pathway to 2,6-xylenol uses o-cresol as a starting point.

[Edited on 1-4-2020 by Keras]

draculic acid69 - 1-4-2020 at 11:52

Don't the purified isomers rearrange back into a mix of isomers after a short period?

Keras - 1-4-2020 at 12:34

Quote: Originally posted by draculic acid69

Don't the purified isomers rearrange back into a mix of isomers after a short period?

What mechanism could cause the methyl groups to move around? Quantum effects?

It seems a lot of syntheses start from one definite isomer, so that suggests certain stability. IMHO, but I might be wrong.

DraconicAcid - 1-4-2020 at 12:39

Quote: Originally posted by draculic acid69

Don't the purified isomers rearrange back into a mix of isomers after a short period?

No.

Boffis - 2-4-2020 at 01:11

I did some experiments with the separation of m-xylene from the other but sulphonation with 85% sulphuric acid which only sulphonates the m isomer quickly and the water generated eventually causes hydrolysis of the o and p-sulphonic acids so eventually the reaction mixture contains slightly charred o and p xylene and m-xylene sulphonic acid. The aqueous phase is then separated and diluted with half its volume of water and refluxed then steam distilled to recover the m-xylene. Separating the o- and p- isomers requires very tight fractionation that is beyond most amateurs. However, once you have got rid of the m isomer it is often possible to separate the products of substitutions of the o and p isomers eg toluic acids etc

Keras - 2-4-2020 at 03:49

Quote: Originally posted by Boffis

However, once you have got rid of the m isomer it is often possible to separate the products of substitutions of the o and p isomers eg toluic acids etc

Once the m-isomer is gone, isn't it easier to get the o-isomer from the mix o/p using fractional crystallisation? The p-isomer solidifies at 13 °C — does it form an eutectic mixture with o?

Boffis - 2-4-2020 at 04:51

Quote: Originally posted by Keras

Quote: Originally posted by Boffis

However, once you have got rid of the m isomer it is often possible to separate the products of substitutions of the o and p isomers eg toluic acids etc

Once the m-isomer is gone, isn't it easier to get the o-isomer from the mix o/p using fractional crystallisation? The p-isomer solidifies at 13 °C — does it form an eutectic mixture with o?

I don't know, never tried it

Keras - 2-4-2020 at 06:13

Quote: Originally posted by Boffis

Quote: Originally posted by Keras

Quote: Originally posted by Boffis

However, once you have got rid of the m isomer it is often possible to separate the products of substitutions of the o and p isomers eg toluic acids etc

Once the m-isomer is gone, isn't it easier to get the o-isomer from the mix o/p using fractional crystallisation? The p-isomer solidifies at 13 °C — does it form an eutectic mixture with o?

I don't know, never tried it

Well, as soon as I can (when?), I'll try. After all, it's just a step away from the isolation of m-xylene. I'll report any meaningful findings.

draculic acid69 - 3-4-2020 at 04:39

Quote: Originally posted by Keras

Quote: Originally posted by draculic acid69

Don't the purified isomers rearrange back into a mix of isomers after a short period?

What mechanism could cause the methyl groups to move around? Quantum effects?

It was just something I read somewhere.

Sigmatropic - 3-4-2020 at 09:34

Quote: Originally posted by draculic acid69

Don't the purified isomers rearrange back into a mix of isomers after a short period?

This happens only under FC conditions, it is known as toluene transalkylation and is used industrially to produce benzene and other oligomethylated (is that a word) benzenes such as xylene, mesitylene, durene and penta- and hexamethylbenzene.
https://en.m.wikipedia.org/wiki/Transalkylation

Keras - 3-4-2020 at 10:49

Quote: Originally posted by Sigmatropic

Quote: Originally posted by draculic acid69

Don't the purified isomers rearrange back into a mix of isomers after a short period?

I didn't know that and looks interesting. How can you disproportionate toluene into benzene and xylene?

Sigmatropic - 3-4-2020 at 13:04

I expect something like AlCl3/anhydrous HCl or AlBr3/anhydrous HBr under pressure and high temp. Not something you can easily access. All my searches are cluttered by the large amount of zeolite research that has been done since the 60s.

If I'm feeling bored I might check Vogel tomorrow but seriously this isn't something practical, it's just very economical on scale.

The conditions of the redistribution aren't very accessible and the separation of the resulting mixture, which is the topic of this thread, is non-trivial to say the least.

Keras - 3-4-2020 at 13:32

Fair enough. Forget it. I’ll have a look for the sake of pure knowledge then I shall move on. Thanks a bunch.