Sciencemadness Discussion Board

hydrodistillation of xylene?

EmmisonJ - 12-6-2009 at 14:41

background info:
the purpose of this question is ultimately to purify a propylbenzene after it has been isomerized via EtOH and KOH. the amount would be way too small to distill unless a microscale apparatus is acquired, it's only 4ml. the crude propylbenzene could be extracted from the post-isomerization mess with xylene and wash it with dh2o. xylene was chosen due to it's high bp. then add fresh dh2o back to the xylene and hydro-distill the dh2o to remove the volatile propylbenzene from the xylene, leaving all the post-isomerization impurities solvated in the xylene. however i'm curious as to what effect the presence of this water will have on the xylene's bp? can xylene be hydro or steam distilled? if so then it sounds like this idea/method of purification isn't as clever as i thought it was.

[Edited on 12-6-2009 by EmmisonJ]

Sedit - 12-6-2009 at 14:52

I know toluene manages to come over when steam distilling so where as im not 100% I would think that xylene would follow also. Tar and polymerization will more then likely be left behind after a steam distillation so it should suffice in purifying it to an extent.

EmmisonJ - 12-6-2009 at 14:59

thanks, if i was any decent at equations i wouldn't even have to ask but i'm definitely no good with equations of this nature yet. still a little too green.

i'm curious what the presence of an equal amount of water to xylene in a distillation setup would lower xylene's bp to, that would help determine how much xylene may come over in relation to water and the propylbenzene

[Edited on 12-6-2009 by EmmisonJ]

not_important - 12-6-2009 at 15:09

p-xylene and water form an azeotrope boiling at 94.5 C, 60% xylene and 40% water; the other 2 xylenes form similar azeotropes.

I should add that a third substance that does not interact with water significantly different than xylene, say n-butyl-benzene, will co-distill along with the water and xylene in proportion roughly equal the the ratios of its and xylene's boiling points. When co-distilling with a non-interacting carrier (water for simple hydrocarbons) the mix boils when the sum of vapour pressures (water+A+B...) equals the ambient pressure. Using tables of vapour pressures the approximate expected boiling point and azeotrope composition can be predicted. This becomes more difficult when the compound interacts more or less strongly with water, the mono-nitro phenols being a good example, although predicting roughly which will distill better can be done.




[Edited on 12-6-2009 by not_important]

Nicodem - 12-6-2009 at 15:23

There is no isomerisation of propylbenzene (assuming you mean n-propylbenzene) that can be base catalysed. Utmost it can be isomerized to cumene (isopropylbenzene) but that can only be acid catalysed. Obviously you are confusing something.

PS1: What is "dh2o"?
PS2: I'm moving this to Beginnings section due to the absence of any references.

DJF90 - 12-6-2009 at 17:20

I think by dH2O he means distilled water...

EmmisonJ - 12-6-2009 at 18:25

Quote: Originally posted by DJF90  
I think by dH2O he means distilled water...


you got it, sorry it's definitely a lazyman's acronym.

Sedit - 12-6-2009 at 19:07

I think the reference to propylbenzene was ment to mean propenylbenzene if im not mistaken. Suggesting a double bond between the number 1 and 2 carbons as opposed to allylbenzene whose Pi bond is terminal. Is this correct EmmisonJ?

EmmisonJ - 13-6-2009 at 04:33

yeah that's correct sedit, i'm curious which will be the first to be hydro-distilled by the water, the propenylbenzene or xylene. seems this is the only real way of purifying it that i've been able to come up with because the xylene will basically dilute the propenylbenzene allowing for smaller mechanical losses during distillation, hopefully

not_important - 13-6-2009 at 05:31

Co-distillation, which steam distillation is a subset of, is not very good at separating similar substances even if their boiling points are somewhat dissimilar. That's why steam distillation of essential oils from plants gives such complex mixtures, two compounds with boiling points 50 C apart will steam distill simultaneously (although in a ratio reflecting their BPs).

Mechanical losses in co-distillation frequently are low, because the added substance (steam/water) 'scrubs' the walls of the distilling apparatus. For small amounts of product you'll need to extract the distillate and wash the collecting flask, commonly with ether. Also, that's why some steam distilling operations return much of the condensed water to the still pot, it keeps the total volume of water down making workup easier.

gsd - 13-6-2009 at 06:54

Quote: Originally posted by not_important  
p-xylene and water form an azeotrope boiling at 94.5 C, 60% xylene and 40% water; the other 2 xylenes form similar azeotropes.


In literature this kind of system is always reported as azeotrope, but IMHO this is bit misleading. Azeotrope is a single phase mixture of 2 or more liquid components - even in case of heterogeneous azeotrope (like say n-butanol and water) each phase has both components in it.

An Important identification of azeotropic mixture is it boils at constant temperature.

Aromatic Hydrocarbons (AH) have negligible solubility in water (and vice versa) Hence the 2 phase mixture of any AH with H2O will always boil at constant temperature (and that temperature will always be less than 100 Deg C.) as long as both phases are present. To call this an azeotropic system just because it boils at constant temperature is not correct.

This is classical case of steam distillation. When 2 phase liquid mixture is heated, partial pressure exerted by each phase is equal to its own vapour pressure at that temperature. When sum of these two partial pressures equals atmospheric pressure, the mixture starts boiling.

gsd

EmmisonJ - 13-6-2009 at 12:41

Quote: Originally posted by not_important  
Co-distillation, which steam distillation is a subset of, is not very good at separating similar substances even if their boiling points are somewhat dissimilar. That's why steam distillation of essential oils from plants gives such complex mixtures, two compounds with boiling points 50 C apart will steam distill simultaneously (although in a ratio reflecting their BPs).

Mechanical losses in co-distillation frequently are low, because the added substance (steam/water) 'scrubs' the walls of the distilling apparatus. For small amounts of product you'll need to extract the distillate and wash the collecting flask, commonly with ether. Also, that's why some steam distilling operations return much of the condensed water to the still pot, it keeps the total volume of water down making workup easier.



so if the xylene and propenylbenzene co-steam distill then the impurities that are solvated in the non-polar layer would most likely come over as well i bet.

i hydro and steam distilled leaves a number of different ways in the past. one time i took fresh harvested leaves, shredded them up and distilled it. i ended up getting green EO from it, somehow the chlorophyll came over along with it. obviously that was a horrible run, but it was a learning experience. that's why i think if the non-polars both co-steam distill then the impurities may come over as well. the particulate has already been vacuum distilled out so whatever colored impurities these are will stay locked in the solvent, perhaps even through hydro or steam distillation.

crap, looks like it's back to the old drawing board.

Sedit - 13-6-2009 at 16:25

The green chlorophyll that came over when you steam distilled was more then likely caused by suspended drops of H2O and not true steam comming over. This can be done away with if one puts a piece of glass wool or something simular at the head of the still tube so that only true steam passes.

I have been steam distilling since I was a kid and know exactly what you speak of so if that is an issue in the case of chlorophyll then it should also prove useful to do the same when co-distilling this mixture. It's nothing more then a small plug of loosely packed glass wool, just enough as to not raise the pressure to any great extent.

If you vacuum distilled it and the colored impurities are still in the mix I would not hold my breath expecting them to be removed thru co-distillation.

not_important - 13-6-2009 at 17:42

Quote: Originally posted by gsd  

In literature this kind of system is always reported as azeotrope, but IMHO this is bit misleading. Azeotrope is a single phase mixture of 2 or more liquid components - even in case of heterogeneous azeotrope (like say n-butanol and water) each phase has both components in it...

Quite true, but if you want that data for common liquids, you look in azeotrope tables (which generally tell you if there are multiple phases formed, as well as the makeup and density of those phases)
Quote:
The green chlorophyll that came over when you steam distilled was more then likely caused by suspended drops of H2O and not true steam comming over. This can be done away with if one puts a piece of glass wool or something simular at the head of the still tube so that only true steam passes.

There are also anti-spray bulbs to be had, and a short Vigreux column can work well if wrapped with insulation - you're not trying to fractionate but just suppress spray carry-over.

EmmisonJ - 13-6-2009 at 18:43

Quote: Originally posted by Sedit  
If you vacuum distilled it and the colored impurities are still in the mix I would not hold my breath expecting them to be removed thru co-distillation.


jesus another type-o, i apologize it's been a long week at work followed by a lot of work on the house and my exhaustion is apparently showing with all of these type-o's. what i meant to say was vacuum filtered, not vacuum distilled. sorry.

my vigreux is rather long, will probably opt for the glass wool idea. basically don't pack it tight, just enough to discourage anything other than true steam to pass and not causing pressure build-up. acting as a filter, good idea.

EmmisonJ - 17-6-2009 at 08:14

could anyone point me in the direction where i could find some good reading to help with my understanding of vapor pressure, more specifically to understand what factors are at play when one can fractionally distill to separate liquids with different bp's as opposed to how some may be able to co-distill with each other (outside of azeotropes)? in other words to better understand the physics behind why some liquids will separate easily under distillation while others, with different bp's, will co-distill.