The Vespiary

The Hive => Chemistry Discourse => Topic started by: Antibody2 on July 22, 2002, 11:36:00 PM

Title: how do they isolate p-xylene from its isomers
Post by: Antibody2 on July 22, 2002, 11:36:00 PM
i'm curious, with only 1 deg C difference in BPs i have a tough time imagining even fractional distillation doing an acceptable job of separating them. I realize that the MP is much higher, but again i would imagine that its ortho an meta isomers would easily dissolve any solid para-xylene.

best idea i could come up with was repeated fractional distillations using several columns in chain followed by an attempted fractional crystallization at -20C or so

surely there is a better way?
Title: Xylene Isomer Separation
Post by: carboxyl on July 23, 2002, 01:46:00 AM
This article might be helpful. I'm sorry I don't have it on hand, but I'll try and dig it up.

Berg, L., and An-I Yeh, 1987, Separation of m-Xylene from o-Xylene by Extractive Distillation, Chemical Engineering Communications, 54:149-159.

p-xylene (mp 13.3C) can be easily removed from m-xylene (mp -48C) and o-xylene (mp -25.2) via fractional crystalization.

Distilling p-xylene (mp 138.3C) from m-xylene (bp 139.1C) would be difficult to say the least.

M-xylene (bp 139.1C) could probably be fractionally distilled from o-xylene (bp 144C) with a good column.


The above post is purely fictional. Any resemblance to "real-life" is purely coincidental.
Title: Isolation of para xylene
Post by: lugh on July 23, 2002, 02:01:00 AM
A method for separating the xylenes has been developed by Arnold British Patent 585076 . The mixture of m- and p-xylenes is diluted with a relatively volatile, inert solvent having a freezing point below - 58.5°, the eutectic point of the xylenes, and cooled until a large part of the p-xylene has crystallized. The diluent is removed by distillation and the p-xylene crystallized from the remaining liquid. Suitable solvents are methanol, ethanol, 2-propanol, acetone, butanol, toluene, the pentanes, and the pentenes. Timmermans and Martin (J Chim Phys 23 747, 1926) p-xylene can be obtained pure quite easily by fractional crystallization; p-xylene was purified in this way until the freezing point was constant (13.35 +/- 0.03°)  :)
Title: any idea what kind of ratios might be expected?
Post by: Antibody2 on July 23, 2002, 02:21:00 AM
thank you both

any idea what kind of ratios might be expected? i still have a tough time beleiving there is that much precurser just sitting on hardware store shelves, someone pinch me.
Title: Ratios
Post by: lugh on July 23, 2002, 02:53:00 AM
Mixed xylenes are generally composed of 40% m xylene and 20% each of ethylbenzene, o and p xylene; o xylene and ethylbenzene are usually separated by distillation  :)
Title: precursor?
Post by: terbium on July 23, 2002, 04:35:00 AM
i still have a tough time beleiving there is that much precurser just sitting on hardware store shelves
Eh, p-xylene?
Title: Column chromatography.
Post by: tiresias3 on July 23, 2002, 05:15:00 PM
I would think the best way to separate out the isomers of xylene would be through column chromatography, not fractional distillation.
Title: xylen
Post by: hest on July 23, 2002, 05:42:00 PM
destilation with 0.5°C is not a problem in the industre but xylene is not photostabel the 3 xylens absorb light at diff. wave lenght ('collour') and isomericice. So iff you put a mixture of xylen infront a lamp lacking the 'collour' of one of the xylenes the other two will turn into the one.
(trickey description and bad gramma a' )
Title: Wow!
Post by: Antoncho on July 23, 2002, 06:20:00 PM
Awesome, Hest!

Never even thought it could bee possible.


Title: Alternatively..
Post by: tiresias3 on July 25, 2002, 01:06:00 AM
You could buy a case of mixed xylenes at Home Depot and purify out the 20% ethylbenzene.  To the Et-benzene you could then perform a formylation followed by a purification of the 4-ethyl-benzaldehyde, the major product.  To the 4-Et-benzaldehyde could then be performed a bromination followed by conversion of the 3-bromo-4-ethyl-benzaldehyde to 4-ethyl-3-hydroxy-benzaldehyde via an aborted Grignard or via the use of n-BuLi followed by hydrolysis.  Methoxylate with dimethyl sulfate or MeX (where X=Br, Cl or I) and KOH to get 4-ethyl-3-methoxy-benzaldehyde.  Sounds like a lot of trouble, doesn't it?
Title: Yes, but.....
Post by: neuromodulator on July 25, 2002, 02:38:00 AM
Don't forget to protect the aldehyde (by forming a Schiff base adduct most likely) if you choose to use n-BuLi route in the above reaction sequence.  Also, I ran across this reaction, which may turn out to be quite useful, in a new book called "Compendium of Organic Synthetic Methods Volume 10" (2002) by Michael B. Smith today.  It gave the following example reaction and reference: 

benzene --> phenol (using 1. e-, TFA/CH2Cl2, 2. triethylamine in 72% yield)

Fujimoto, K.; Toduda, Y.; Maekawa, H.; Matsubara, Y.; Mizuno, T.; _Nishiguchi. I._; "Tetrahedron Lett.," 1996, 37, 3623.

As I have not looked up the reference article yet, I don't know whether this reaction can be successfully applied to vanillin yet or not.  However, if it can, then that would be a significant improvement over using n-BuLi to hydroxylate the #5 position, I would think.