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Al Koholic
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Drying ethanol via ester hydrolysis.
When possible, it seems to be the case that using dry solvents for various procedures is generally more favorable that using water containing
solvents.
Producing dry ethanol for the synthesis of sodium ethoxide for example, using sodium to dry diethyl ether or THF for use in grignard, etc, etc. There
is undoubtably an important place for dry solvents in the laboratory.
Certainly some solvents are purified with more ease than others. If I'm not mistaken, methanol, acetone, toluene, etc don't have tendencies
to form azeotropes with water making them easy to purifiy by distillation. For this reason I figured the most important solvent this topic should
focus on will be ethanol, an omnipresent substance in the lab and one of the most difficult to *dry* without some more exotic methods. Who here is
ready to perform an azeotropic distillation with benzene to get pure ethanol? I'm not...
Anyway, getting to the point now, I've been thinking about using esters to perform the drying. What if one were to mix with 95% ethanol, some
ethyl acetate for example. Say enough to suck up all the water on hydrolysis. Could you simply reflux this solution, hydrolysing the ethyl acetate
into ethanol and acetic acid, drying the solution out in the process? A simple protected (drying tube) distillation could then be carried out to
isolate the ethanol in a pure state no?
[Edited on 18-10-2004 by Al Koholic]
[Edited on 19-10-2004 by Al Koholic]
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JohnWW
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You mention methanol, acetone, toluene, etc., as not forming azeotropes with water. Actually, water forms two-phase minimum-boiling-point binary
azeotropes with toluene and benzene - see Perry chapter 13, and the International Critical Tables.
As for your proposal "What if one were to mix with 95% ethanol, some ethyl acetate for example. Say enough to suck up all the water on
hydrolysis. Could you simply reflux this solution, hydrolysing the ethyl acetate into ethanol and acetic acid, drying the solution out in the process?
": Ethanol also forms a minimum-boiling-point azeotrope with ethyl acetate, with 46% ethanol and boiling at 71.8ºC. Besides, ethyl acetate does
not hydrolyse so easily merely in the presence of water (present as the impurity) as you suggest, except with added strong alkali or acid, which could
cause complications due to solubility in either of the products and formation of an alkali metal acetate. I also note that a ternary azeotropic
mixture is formed by water, ethanol, and ethyl acetate together, with 12.4% ethanol, 60.1% ethyl acetate, 27.5% water, boiling point 70.3ºC.
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mick
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I can not see it working.
If the ethyl acetate picks up water then it will go to ethanol and acetic acid and acetic acid could catalysis the esterification. You could end up
in a circle.
If you could extact the acetic acid?
mick
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neutrino
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Vogel's 3rd had a thorough discussion about this very thing. I would post the relevant pages, but I'm away from my normal computer right
now. What I remember is that standard anhydrous salts can be used to break azeotropes and dehydrate pretty thoroughly (esp
CaSO<sub>4</sub> . Magnesium metal can be added to alcohols to dry them
very thoroughly, as it forms Mg(OH)<sub>2</sub> and H<sub>2</sub>. There was something about using esters in there, but I
can’t remember it.
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BromicAcid
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There is always that great method using NaOH that Organikum mentioned in the thread he started on this subject.
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Magpie
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Molecular sieves have been noted in my organic lab text as drying agents for solvents. But I have no idea whether they would work for ethanol, or how
expensive they are.
The single most important condition for a successful synthesis is good mixing - Nicodem
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neutrino
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They are usually pretty cheap and should work for anything (I think). As promised, here's the pertinent text from Vogel about dehydration.
Attachment: Drying.rtf (15kB)
This file has been downloaded 3006 times
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Al Koholic
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Excellent text...thanks for the attachment there neutrino.
Mentioning ethyl acetate in the opening post was just a suggestion for the overall concept here. So it seems with higher molecular weight esters and
some Na, one can get very dry ethanol.
Anyone think that dried KOH or NaOH could substitute for the Na metal in this scheme? Since I imagine the sodium is there just to provide the basic
conditions and alkali metal ions I would imagine NaOH might just work as well.
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Organikum
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You didnt read the post Bromic linked to, did you Al Koholic? It answers your questions completely.
Toluene as Xylene as Benzene as dried by simple boiling/distilling 10% of the volume away, they form heterogenous azeotropes with water. The resulting
aromatic solvent is dry enough for virtually all purposes.
Diethylether is easily dried by NaOH or better KOH pellets. This takes some time, a week at least, but gives ether dry enough for any Grignard.
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Al Koholic
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No no, I did read it...as it was formed in fact. Perhaps I should have put some more consideration into the title of the topic as it seems to be
generating some confusion. This was intended to focus on drying with esters but was open to any interesting side notes.
[Edited on 19-10-2004 by Al Koholic]
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mick
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Is the solvent dry?
This is how I try to check stuff I use.
A quick empirical test.
I add 1 ml or so of the solvent to bit of calcium hydride, calcium carbide should do, with a quick shake.
If nothing happens the solvent is very dry.
If it is slightly cloudy and gradually clears then it is dry for most stuff.
If is is slightly cloudy and does not clear then it is OK for gram scale reactions.
If it bubbles then it is wet.
mick
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unionised
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A fine method, save only that this thread is about alcohol.
CaH2 + 2 EtOH --> Ca(OEt)2 +H2
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mick
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It is only a quick small scale empirical test, the assumption being that CaH2 reacts faster with water than the average solvent and is not too
violent. If it is cloudy on addition at room temp then I would assume there is water. If the CaH2 settles and starts reacting with the solvent that is
the end of the test.
mick
The idea is that as powder not lumps, the hydrogen forms on the surface rapidly with water and the rate of settling is somehow related to the water
content of the solvent. I have not tried it with calcium carbide but it could work.
mick
[Edited on 23-10-2004 by mick]
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unionised
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It's still off-topic and I wouldn't try it with halogenated solvents.
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mick
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I checked out ethanol (about 5 ml) and calcium hydride (only a bit). I could not easily tell the difference between absolute ethanol and stuff that
had been over molecular sieve. Both went cloudy and most of the calcium hydride settled out but the cloudiness did not clear as fast as I thought.
Added a drop of water and instant bubbles. Thanks, just thought, the calcium hydride is crap, contains too much hydroxide, so it is going to be
cloudy.
mick
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Nicodem
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Does anyone know if CaC2 reacts with absolute Ethanol?
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Why would it not? Generally speaking, any base with pKa (of the conjugated acid) above 15 will react with ethanol. Given that the pKa of acetylene is
24 you can bet CaC2 reacts with water and ethanol. Just as you can be even surer that calcium hydride reacts with both and any other
protic solvent as well since the estimated pKa of its conjugated “acid”, H2, is of about 40!
What is all this about drying ethanol?
What is wrong with the azeotropic drying with toluene. The boiling point of the ternary azeotrope of water-toluene-ethanol is slightly below the
boiling point of dry ethanol so it can be used to dry it. Someone could even find out that this is one of the methods used industrially by searching
the net before asking: http://mccoy123.com/qvf-ethanol.htm
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unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
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garage chemist
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I removed my posting from here and posted it somewhere else since I saw that it was the wrong topic for this. I'm sorry that I did this so
late...
The problem with the azeotropical drying is that you need a column. I don't have one.
But the reaction of CaC2 with Ethanol should produce Calcium Ethoxide, which should in turn help to dehydrate the ethanol.
I'll see how this turns out.
To remove the acetylene, I'll reflux the ethanol while bubbling air through it. This should get rid of any dissolved gas.
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Nicodem
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Why would you need a column for the azeotropical drying?
All you have to do is calculate how much toluene you need to dry a certain amount of 96% ethanol (check the composition of the ternary azeotrope), add
the calculated amount of toluene but with some excess and distill off some more than the theoretic amount off the azeotrope. What remains in the flask
is almost absolute ethanol contaminated with a minimum amount of toluene.
If you filter of the precipitated Ca(OH)2 after leting CaC2 dissolve in 96% ethanol you will be left with a solution of acetylene and some Ca(OEt)2 in
dry ethanol. That's true, but you can't get rid of the acetylene the way you proposed. Pasing (dry) air trough the refluxing solution will,
besides being very hazardous, take away also the ethanol vapors. Strange enough, but ethanols partial pressure at its boiling point is 1atm, meaning
that the air will take it away.
I would rather not heat a mixure of air, acetylene and ethanol gas, though.
Edit: I realized where you got the idea that a column is needed. From the link I provided above . That is a design for a continuous flow proces where the tolune is being recycled. I would say that if one uses pure toluene in at least
some excess no column is needed.
[Edited on 24-8-2005 by Nicodem]
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unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
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garage chemist
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The air stream through the refluxing ethanol will NOT take away the ethanol, since the ethanol vapor- air mixture has to pass through the reflux
condenser where the ethanol will condense. But the Acetylene won't condense there, no matter how much of it the air contains. This is the
important thing here.
And the mixture won't get hotter than about 78°C, and there will be complete abscence of ignition sources. I wouldn't have any safety
concerns about it.
The "Organikum 15. Auflage" says that a column is needed when one has to separate liquids whose boiling points are closer together than
80°C. This statement is where I got the idea that a column is needed.
The separating capability of a simple distillation is greatly overestimated very often.
In the organic laboratory, a column is used in almost every distillation. It's a fixed component of every distillation setup.
[Edited on 24-8-2005 by garage chemist]
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Nicodem
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Sorry, you are right about the refluxing ethanol.
I don't know how efficient this way of removing acetylene is though. Some ethanol (the partial pressure at the condenser temperature) would
inevitably pass trough and if you would need huge amounts of air you might be left with considerably less than you expected. Also drying the air would
be almost as annoying as using a more practical way to dry ethanol. How are you going to test the presence of acetylene in ethanol or outgoing air?
Not with a lighter I hope.
You underestimate the explosivity of acetylene. It can explode even without an ignition source. Not likely but it is possible. It is not predictable
like methane or other hydrocarbons.
Are not the vapors over a boiling mixture that can form an (lower boiling) azeotrope, going to have the composition of the azeotrope until one of the
components is over (and the azeotrope composition breaks up)?
At least that is what I thought before, but perhaps I confused this with the theory of steam distillation.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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garage chemist
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Some preparation I remember (preparation of polyethylene without pressure, using a very special catalyst) calls for oxygen- free petroleum ether. By
"oxygen- free", there was meant "free from dissolved gaseous oxygen".
It was accomplished by refluxing the pet. ether while slowly bubbling dried nitrogen through it.
Apparently this removes all of the dissolved atmospheric oxygen, if it's enough for this very delicate synthesis.
So the acetylene should be removed completely by the air stream.
To test for presence of acetylene, one could go after the rather strong phosphine smell of it (due to phosphide-containing carbide).
I'll remove the acetylene after distillation of the nearly absolute ethanol from the residual carbide/calcium hydroxide/ethoxide.
Otherwise traces of moisture in the used air would produe acetylene again, as would the slow reaction of the ethanol with the carbide.
My synthesis doesn't call for absolute ethanol btw, only 99% ethanol is needed (use of really anhydrous ethanol actually lowers the yield).
I understand that one will not be able to obtain absolute ethanol with CaC2, not even by dissolving lots of metallic sodium in 99% ethanol (the
equilibrium hydroxide + ethanol <---> ethoxide + water lies too far on the right side for this).
REAL ABSOLUTE ETHANOL can only be prepared by ester hydrolysis of, for example, ethyl formate with sodium ethoxide. Go to Lambdasyn for a detailed
procedure (or ask Mephisto, Lambdasyn is in german).
Another useful ester for this is diethyl phthalate, again this gets hydrolyzed by sodium ethoxide (procedure is in Organikum).
Those reactions bind even the slightest traces of water.
The ester must be easy to remove from the absolute ethanol by distillation, since a considerable portion of it will remain undecomposed.
Ethyl formate boils low enough to be easy to separate with a column, diethyl phthalate boils high enough for this.
To test for absoluteness, a drop of a solution of aluminium triethylate in benzene is added to the ethanol. If the ethanol contains more than 0,005%
water, a voluminous white precipitate will occur. This test is incredibly sensitive. Any access of undried air will result in a positive reaction,
especially since ethanol of more than 96% concentration is very hygroscopic.
The -undoubtably inferior, but much cheaper- method of azeotropic water removal can only work with an efficient column. Azeotropes of compounds behave
no different from the vapors of pure compounds, that means you need a column if the boiling points are closer together than 80°C.
[Edited on 24-8-2005 by garage chemist]
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unionised
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Moonshine stills seem to work quite adequately without a column to speak of and that's certainly not a boiling point difference of 80 degrees.
Also, it's possible to get pure alcohol by distillation without adding drying agents or third solvents. Anyone care to guess how?
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neutrino
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Salt?
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unionised
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Which bit of "without adding drying agents" is giving you trouble?
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neutrino
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So, 'no drying agents' means 'no chemicals whatsoever'?
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