Antibody2 (Bee without a Title)
07-22-02 23:36
No 335906
      how do they isolate p-xylene from its isomers  Bookmark   

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?
 
 
 
 
    carboxyl
(Hive Bee)
07-23-02 01:46
No 335954
      Xylene Isomer Separation  Bookmark   

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.

Peace.

The above post is purely fictional. Any resemblance to "real-life" is purely coincidental.
 
 
 
 
    lugh
(Moderator)
07-23-02 02:01
No 335960
      Isolation of para xylene  Bookmark   

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) smile
 
 
 
 
    Antibody2
(Bee without a Title)
07-23-02 02:21
No 335967
      any idea what kind of ratios might be expected?  Bookmark   

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.
 
 
 
 
    lugh
(Moderator)
07-23-02 02:53
No 335978
      Ratios  Bookmark   

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 smile
 
 
 
 
    terbium
(Old P2P Cook)
07-23-02 04:35
No 336005
      precursor?  Bookmark   

i still have a tough time beleiving there is that much precurser just sitting on hardware store shelves
Eh, p-xylene?
 
 
 
 
    tiresias3
(Stranger)
07-23-02 17:15
No 336138
      Column chromatography.  Bookmark   

I would think the best way to separate out the isomers of xylene would be through column chromatography, not fractional distillation.
 
 
 
 
    hest
(Hive Addict)
07-23-02 17:42
No 336145
      xylen  Bookmark   

destilation with 0.5C 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' )
 
 
 
 
    Antoncho
(Official Hive Translator)
07-23-02 18:20
No 336153
      Wow!  Bookmark   

Awesome, Hest!

Never even thought it could bee possible.

Amazing!


Antoncho
 
 
 
 
    tiresias3
(Stranger)
07-25-02 01:06
No 336711
      Alternatively..  Bookmark   

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?
 
 
 
 
    neuromodulator
(Hive Bee)
07-25-02 02:38
No 336732
      Yes, but.....  Bookmark   

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.