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maxidastier
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Even crude TsOH, containing some o- and m- isomers, generally is OK for use as a catalyst. |
1. I've made TsOH by heating 75ml of 96% H2SO4 and 200ml toluene, removing water with a dean stark trap.
I used conc. hydrochloric acid to let the para isomer crystallize, vacuum filtered it and dried it.
2. I've distilled 345g of water-free Ethylene Glycole from anti-freeze agent.
3. Then I added 4,3% (14,8g) of the TsOH as catalyst to the flask, added a short Vigreux column and distilled at the temperature given in the pdf,
110-130°C.
4. I added some NaOH to the distillate and fillled it into a seperating funnel. The next morning the liquid was brown (from polymerised acetaldehyde)
and a layer of polymerised acetaldehyde was at the bottom. That would have been ok, but there was not one upper layer, but two.
There should have only been one containing the crude Dioxane.
5. I decided to follow the instructions, separated the acetaldeyde layer, filled the rest into a round-bottom flask together with a few grams of NaOH
and distilled again.
But there doesn't seem to be any dioxane, because the steam temperature was 82°C and not 101°C as it should have been for dioxane.
I thought it might be the extra layer formed in the seperatory funnel and decided to continue. However, only at the very end of the distillation the
temperature went up to around 100°C, but there was only a small residue left, which began to get black, fuming and stinking.
Can you tell me what I've made wrong. Is it true, that
| Quote: |
Even crude TsOH, containing some o- and m- isomers, generally is OK for use as a catalyst. |
Why didn't I get dioxane then or did I?
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Nicodem
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Quote: Originally posted by maxidastier  | | 4. I added some NaOH to the distillate and fillled it into a seperating funnel. The next morning the liquid was brown (from polymerised acetaldehyde)
and a layer of polymerised acetaldehyde was at the bottom. That would have been ok, but there was not one upper layer, but two.
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Sounds like you did not use enough NaOH to thoroughly dry the product. Dioxane and water are freely miscible, but dioxane with NaOH(aq) should not be.
Still, due to the high higroscopicity of dioxane, water will still efficiently partition between both phases (as will dioxane in the NaOH(aq)). This
means the upper layer will not be dry dioxane (assuming it is mainly dioxane at all).
| Quote: | 5. I decided to follow the instructions, separated the acetaldeyde layer, filled the rest into a round-bottom flask together with a few grams of NaOH
and distilled again.
But there doesn't seem to be any dioxane, because the steam temperature was 82°C and not 101°C as it should have been for dioxane.
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If you used no distillation column, or the column you used is not efficient, you will not be able to separate the initial azeotrope dioxane/water
fraction from the pure dioxane, thus it is quite normal that you got a wide boiling point fraction. Without fractionation, you distill all the
intermediate compositions from the azeotrope onward (thus the bp follows the changing composition). This is no evidence that your synthesis gave no
dioxane. It only means that, if it does, your work up and equipment are not up to the task and should be further optimized.
| Quote: |
Can you tell me what I've made wrong. Is it true, that
| Quote: |
Even crude TsOH, containing some o- and m- isomers, generally is OK for use as a catalyst. |
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Yes, it should be true.
| Quote: | | Why didn't I get dioxane then or did I? |
You'll only be sure once you prove it, but you need evidence, at least a sharp boiling point, unless you have some spectroscopic or chromatographic
mean of analysis. Which again brings up the isolation issue. I would say you are probably only having troubles in properly drying the presumed dioxane
layer (the top one, of course). Try drying your final product over NaOH pellets, this time making sure the NaOH does not completely dissolve (the
lower layer must become saturated!). Separate the upper layer and distill again, preferably using at least a simple distillation column.
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blogfast25
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I'm guessing you probably can't really overdo it with the alkali, right? A bit like 'salting', really...
[Edited on 8-8-2011 by blogfast25]
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maxidastier
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Actually, I used quite a lot NaOH and not everything dissolved...
Furthermore the azaeotrope b.p. with water should be 87°C, not 82°C.
Why did the residue become black, smoking and stinking? Is it because the concentration of the NaOH increased?
I'm going to dry the Dioxane with some molecular sieve 3A.
After that distill again and test the b.p.
It's really frustrating, since this is my second attempt now working properly.
Edit: The mid layer would be water you say? Then it would be quite a lot water!
You should get 100ml water during the synthesis.
I've got at least 50ml polymerised Acetaledhyde.
So I should have left roughly 200g pure Dioxane. That would be 100% yield.
Using TsOH as catalyst they got 85% yield= 210g Dioxane.
Now these are optimum conditions, so let's say I would get 75% yield which would be 184 g = 180 ml Dioxane.
After distilling and so on I've left about 150ml of a mixture of water and dioxane. Now, if you get 100ml water during the synthesis, there would be
left 50ml Dioxane, a 21% yield.
That's fucking annoying 
And I used a vacuum-jacketed Vigreux column with effective length of 200ml.
If crude TsOH really isn't the problem what else?
[Edited on 9-8-2011 by maxidastier]
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blogfast25
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2 mol ethylene glycol === > 1 mol dioxane + 2 mol water!
A vacuum jacketed vigreux would provide insulation, not reflux. It would act like a well insulated steam pipe, unless you had a refluxing valve at the
top...
[Edited on 9-8-2011 by blogfast25]
I meant that the synthesis inevitably generates more water than dioxane.
Explain what you mean by a 'jacketed vigreux'? Unless you have some coolant running through the jacket (even air would be fine) the jacket acts like
an insulator, that's not really what you want from a vigreux...
[Edited on 9-8-2011 by blogfast25]
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maxidastier
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Sry, but what are we talking about right now?
About the synthesis or removing water or...?
As for the synthesis the pdf recommends using a short vigreux. Or do you mean I need a column head?
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blogfast25
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Synth.: inevitably you get lots of water as a byproduct.
Vigruex: for any fractionation to occur there has to be some refluxing going on. Liquid needs to be dripping down the column and back into the
reboiler flask, otherwise you only have a one distillation separation. With a Vigruex reflux is provided by natural heat losses of the column and
resulting condensation of rising vapours. Is this the case in your set up?
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maxidastier
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I've got a vacuum jacketed vigreux column. That's what German-English google translates.
The vacuum jacket provides better fractioning and prevents too much heat loss, but there still is heat loss for refluxing!
I also know that the synth produces lots of water, but as I said in my previous post, the yield of pure Dioxane should be around 80% with p-TsOH. I've
only got 35% yield, because after the second distillation there were 160mls of a Dioxane-water-mixture. After drying with molecular sieve I've left
roughly 90ml!
From 347g dry Ethylen Glycole I should have got around 200ml Dioxane (84% yield)
I'd be very happy if someone could give it a try with p-TsOh.
Also I'd like to know how the other isomeres could harm the formation of dioxane.
[Edited on 9-8-2011 by maxidastier]
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blogfast25
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| Quote: Originally posted by maxidastier | I've got a vacuum jacketed vigreux column. That's what German-English google translates.
[Edited on 9-8-2011 by maxidastier] |
That's probably a poor translation because there's no vacuum involved. I understand German fairly well: what was the German term? In English 'jacketed
Vigreux' would probably be what you refer to.
A small but steady airflow through the mantle would probably boost fractionation: the more reflux, the more theoretical plates the instrument
delivers...
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maxidastier
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Well, I can't let air flow through the mantle, because the mantle is evacuated-->vacuum mantle (german: Vakuummantel) -->better isolation since
no air not much transport of heat
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blogfast25
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Hmmm, in my opinion the vacuum mantle is a bit of an unnecessary option and is best used with a refluxer valve at the still head to control reflux
ratio. That allows to carry out fractionations with accurately defined reflux ratio R = L/D. For most purposes a naked or very lightly insulated
vigreux is more than adequate and provides good reflux (and it's a lot cheaper)...
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maxidastier
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I always thought an vacuum mantle would be better fro fractionating. Maybe it is, but only in combinations with a still head, which I don't have.
Nevertheless, that can't be the problem. I still have only 35% yield instead of 85%....
So, what could be the problem? Can the other isomeres of TsOH have any influence?
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Nicodem
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| Quote: Originally posted by blogfast25 | | For most purposes a naked or very lightly insulated vigreux is more than adequate and provides good reflux (and it's a lot cheaper)...
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If the distillation column is not isolated, its resolution power drops sharply. The more isolated it is, less heat loss, less temperature/composition
gradient deterioration, the better the efficiency. It appears to me that you are unaware on how the distillation columns work. Their internal reflux
is gradient driven (the less volatile composition goes downstream as the liquid phase, the more volatile goes upstream as the gas phase). Obviously,
any heat loss deteriorates the equilibrium, hence all better (expensive) distillation columns, especially those filled with Raschig rings, are always
isolated by a vacuum jacket and sometimes even by a reflective layer to reduce radiation heat loss. This topic has been discussed ad nauseam a couple
of years ago. You can find the thread if you UTFSE.
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blogfast25
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| Quote: Originally posted by Nicodem | [It appears to me that you are unaware on how the distillation columns work.
[snip]
You can find the thread if you UTFSE. |
No, I can assure you I know perfectly well how distillation works and it seems we’re suffering from the same confusion that occurred on the thread
you mention (in which I took part).
I worked for months on end with a 10 m (about 15 cm ID) semi-industrial fractionation column for the rectification of technical methanol to UV-grade
methanol (very pure, UV transparent and almost completely water free). It was vacuum jacketed and glass ring packed and fitted with a condenser and
flow splitting timed reflux valve at the top, to provide accurately controlled reflux.
Most of us haven’t got the luxury of the latter device and do rely on natural cooling to get some condensate flowing downwards and get some
fractionation going. I use it all the time with a home made acetone still, packed with good grade steel wool. It refluxes alright and acetone comes
over at about 1 C above theor. BP, until near the end. No insulation worth mentioning...
A Vigreux column, perfectly insulated and without a condenser/reflux valve at the top acts as a steam pipe and provides a single
stage distillation, no more.
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maxidastier
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I understand what you mean, but did this lead to my bad 35% yield?
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blogfast25
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No, I don't think that's the cause. Maybe you should try once with straight conc. H2SO4? I've been meaning to do this synth for ages but haven't got
round to it yet...
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sternman318
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Is this the thread?http://www.sciencemadness.org/talk/viewthread.php?tid=12409
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maxidastier
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What is the better way to precipitate the p-TsOH: Boiling and cooling in conc. HCl or bubbling HCl gas through it? Do the two methods differ in
consumption of HCl?
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blogfast25
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If your gas absorption is 100 % (with HCl that shouldn't be a problem) then there should be no significant difference between both approaches.
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maxidastier
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So, it's easier to dissolve the crude TsOH in conc. HCl, boil and cool down?
Because bubbling HCl gas through the solution is more time consuming.
And the p-TsOH will precipitate in both ways in the same amount or do I get a better yield when I bubble HCl through it?
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blogfast25
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As far as I know that solubility is a state function and only depends on actual, final HCl concentration (and of course temperature), not on how
you arrive at that concentration.
Recrystallisations from concentrated HCl are common with a few substances, of the top of my head: hydrated AlCl3, ZrOCl2.8H2O, (NH4)2SnCl6.
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maxidastier
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Ok, thanks for your reply. But can I reach a higher concentration of HCl by bubbling it through water? I've only 30-33% HCl aq. Maybe you could reach
37% with the bubbling method? Or do you reach less then 30%?
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blogfast25
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37 % is more or less the theoretical limit that can be achieved and it's slightly higher than the 30 - 33 % you've altready got. 37 % is made by
absorbing HCl in water, to saturation point but for the hobbyist that's not so easy to achieve: 37 % means just over 1 mol of HCl gas absorbed into
100 ml of solution! If you've got a decent sized HCl generator that's maybe not too difficult (ahem!) And saturating a small amout of solution is of
course easier.
Why don't we stay down to earth and you try and precipiatate that stuff from 30 - 33 %, huh? If failed, you have plan B!
By the way: there's no need to bubble the HCl gas through the water: it's so soluble that the 'inverted funnel method' (which prevents suck back)
works just as well...
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maxidastier
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Sry, but I didn't now the "inverted funnel method". My literature tells me to bubble through water....
Could you explain this method, please?
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blogfast25
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HCl is so extremely soluble in water that bubbling isn't even really possible from what I've seen with my own eyes. As the gas flows through the tube,
when it hits the water it kind of forms one stationary bubble, with the gas being constantly absorbed at the interface between bubble and solution.
The heavier than water solution then tends to sink to the bottom.
In the inverted funnel method, an inverted funnel (spout pointing upwards) is connected to the HCl source and the circumference of the top of the
funnel (now bottom) is hovered a few mm above the solution level (by adequately clamping the spout). The gas flows onto the solution surface and is
immediately absorbed. It's always best to stir the solution constantly. Inverted funnel avoids suck back.
But suckback can also be prevented with the 'bubbling method' by having an empty vessel between the HCl source and the bubbler: any suck back then
just sucks some of the solution into the empty vessel.
Clear?
[Edited on 24-8-2011 by blogfast25]
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