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Magpie
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Preparation of “Absolute” Ethanol (<100 ppm water)
Absolute ethanol (<100 ppm water) is often a requirement for a successful synthesis in organic chemistry. Every mole of water can result in a loss
of a mole of product.
This procedure is based on that found in Vogel (ref 1). The use of 3A mole sieves is added to bring the water content down from ~1% to <0.01%, Ref
2).
‘
A. Reagents
325 ml of Everclear (190 proof ethanol, or “rectified spirit”)
107g of clean garden slaked lime (Ca(OH)2)
30g of 3A mole sieves
NaOH for glassware cleaning soln
silicone oil for oil bath (DOT 5 brake fluid works well)
CaCl2 desiccant for guard tube
HNO3 washed fiberglass wool
B. Apparatus
1 liter 2 or 3 neck RBF
condenser
vacuum adapter
3-way distillation adapter
500 ml alcohol storage bottle with ground glass fitting that fits the vacuum adapter
large ceramic crucible
muffle furnace
thermometer
hot plate
strong overhead stirrer
oil bath
insulating blanket and/or aluminum foil
guard tube
powder funnel
C. Procedure
Clean all glassware with an aqueous ethanol solution saturated with NaOH. Then dry for 2 hours at 130°C. This will produce sparkling glassware.
Here's the recipe for the cleaner: https://www.inrf.uci.edu/wordpress/wp-content/uploads/sop-we...
a. Dehydrating the Ca(OH)2
1. Weigh out 107g of slaked lime into a large ceramic crucible. Place crucible in a muffle furnace and set for 800°C.
2. After 3 hours remove the crucible and place in a desiccator to cool. Be very careful when removing lid from desiccator. Remove it slowly.
b. Reflux the Everclear
1. Set up an oil bath on a hotplate. Affix a 2- or 3-neck 1 liter RBF in the oil bath.
2. Add 325 mL of Everclear to the RBF. Add the CaO to the RBF using a powder funnel.
3. Install a reflux condenser in a side neck.
3. Install an overhead stirrer in the vertical neck of the RBF and begin stirring.
4. Set the oil bath at 100°C. Heat sufficiently to bring the slurry to reflux. The slurry will thicken considerably as the CaO takes on water.
5. After 6 hours at reflux turn off heat. Remove bath but leave the stirrer in the RBF. Leave condenser running until slurry is cool. Leave the
guard tube in place in the RBF. Let sit overnight.
c. Activate the 3A Mole Sieves
Note: 3A mole sieves are rated at 22 wt% H2O absorption. Assuming 1% H20 left in the ethanol this would be 2.63g H2O. Assuming, conservatively, 15
wt% water absorption by the sieves, wt sieves required = 17.5g. Use 30g.
1. Set the muffle furnace to 300°C.
2. Place 30g of 3A mole sieves in a crucible, place in furnace, and heat for 1 hour..
3. Remove from furnace when T = 200°C and place in a desiccator to cool.
4. Pour the cooled mole sieves into the 500mL ethanol receiver and cap.
d. Distillation of the Ethanol
1. Setup for simple distillation using the silicone oil bath (100°C) and overhead stirrer.
2. Place HNO3 washed fiberglass wool in the horizontal leg of the distillation adapter per Vogel. This is to remove any entrained CaO.
3. Stir and distill the ethanol into the receiver with the mole sieves. Use an insulation jacket if needed.
D. Discussion
Using a strong, reliable overhead stirrer is critical for this procedure.
Although I have used this procedure at least 3 times I never did write down the yield of absolute ethanol.
E. References
1. A Textbook of Practical Organic Chemistry, 3rd ed, A.I.Vogel, 1959, page 166, “Dehydration of Rectified Spirit by Quicklime.”
2. JOC article: “Drying of Organic Solvents: Quantitative Evaluation of the Efficiency of Several Desiccants,” Williams and Lawton, University
of Johannesburg, August 12, 2010.
Questions, comments, and suggestions are welcomed.
[Edited on 24-2-2018 by Magpie]
[Edited on 24-2-2018 by Magpie]
[Edited on 26-2-2018 by Magpie]
[Edited on 26-2-2018 by Magpie]
[Edited on 26-2-2018 by Magpie]
[Edited on 26-2-2018 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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happyfooddance
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I am confused. Are you using slaked lime or CaO?
Very detailed write-up, otherwise!
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Magpie
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What you buy at the garden center is slaked lime, Ca(OH)2. By heating it in the muffle furnace at 800°C it is converted to quicklime, CaO.
The single most important condition for a successful synthesis is good mixing - Nicodem
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happyfooddance
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My bad, I was reading too quickly through your post!
Why do you believe that a stirrer is critical? Dry boiling stones work for me.
[Edited on 2-24-2018 by happyfooddance]
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Sulaiman
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In C,a,1 where did the aqueous ethanol come from and what is its concentration ?
..........................................................................
In the writeup you set muffle furnace to 300 C but in a reply you mention 800 C
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"D. Discussion
Using a strong, reliable overhead stirrer is critical for this procedure"
"Using" -> "The use of" ?
[Edited on 24-2-2018 by Sulaiman]
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aga
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Is there some method to determine the Absolute absence of water that i've failed to encounter ?
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Magpie
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The CaO/Ca(OH)2 slurry becomes very stiff. The CaO must come in intimate contact with the ethanol to take its water.
The single most important condition for a successful synthesis is good mixing - Nicodem
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Magpie
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This is a good question. There is a recipe. I need to look it up, again.
I keep this solution in a used alcohol can for reuse. It works really well when heated. It eats a very thin glass layer off the glassware.
The 300C ifs for the mole sieves activation, the 800C is for converting Ca(OH)2 to CaO.
Quote: Originally posted by Sulaiman  |
"D. Discussion
Using a strong, reliable overhead stirrer is critical for this procedure"
"Using" -> "The use of" ?
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I don't understand this comment. Please explain.
[Edited on 24-2-2018 by Magpie]
Edit: Fixed broken quote
[Edited on 2-26-2018 by Texium (zts16)]
The single most important condition for a successful synthesis is good mixing - Nicodem
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Magpie
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| Quote: Originally posted by aga | | Is there some method to determine the Absolute absence of water that i've failed to encounter ? |
Yes, use of the Karl Fischer apparatus. http://www.cscscientific.com/moisture/karl-fischer
[Edited on 24-2-2018 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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aga
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I hate to be the Devil's advocate here, but did you perform a Karl Fischer titration on the resulting ethanol ?
Following procedures is great, but the older they are, the less likely that the result is actually 100% what you want.
Not being able to test the resulting material is something that seriously holds me back from doing many things.
I'm still trying to extract vicine from fava beans after a year or more.
Even when i get pure white crystals using a published paper, i have no way to know for certain what they are - might be just a sugar.
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Magpie
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No, I do not and never will, have Karl Fisher capability. I say my ethanol has <100ppm water because I followed the Vogel and Journal of Organic
Chemistry procedures. Then I use my alcohol and get an excellent yield. This is good enough for me.
The single most important condition for a successful synthesis is good mixing - Nicodem
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aga
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Well, that makes it all OK, obviously.
A Chemist with your experience knows best, clearly.
Testing things is a waste of time - just follow something written years ago and it'll all be fine.
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Magpie
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| Quote: Originally posted by aga |
Testing things is a waste of time - just follow something written years ago and it'll all be fine. |
No, that's not the point. It worked as advertised. For every mole of water in your product you lose a mole of product. If your product has a MW of
180 you will lose 180g of product for every mole (18g) of water in your reaction vessel. If your ethanol is not dry you will get a lousy yield.
KF apparatus were $16,743 in 2000. If you would buy me one or rent time on one I will use it. It won't get me a better yield.
[Edited on 24-2-2018 by Magpie]
[Edited on 26-2-2018 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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happyfooddance
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If you make a concentrated copper sulfate solution and soak some strips of paper in them, and then dry them (until white obviously), they make a
pretty good test strip for water, and will at least guarantee 99%, I am fairly sure of that.
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Magpie
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Here's the recipe for making the ethanol-water-NaOH glassware cleaner:
https://www.inrf.uci.edu/wordpress/wp-content/uploads/sop-we...
The single most important condition for a successful synthesis is good mixing - Nicodem
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Sulaiman
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| Quote: Originally posted by happyfooddance | | If you make a concentrated copper sulfate solution and soak some strips of paper in them, and then dry them (until white obviously), they make a
pretty good test strip for water, and will at least guarantee 99%, I am fairly sure of that. |
Would Cobalt Chloride be a more sensitive water indicator than Copper Sulphate ?
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S.C. Wack
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Instead of CoCl2 one might prefer something insoluble. Like KMnO4.
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Sulaiman
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Would KMnO4 not be immediately reduced by the ethanol, thus be no use as a water indicator ?
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aga
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Doesn't seem to happen at any appreciable rate even if it does.
In 100% ethanol it kinda just sits there as a crystal. Zero colour change.
With even a tiny amount of water it starts to dissolve, turning the liquid purple around the crystal.
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AvBaeyer
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Magpie,
Your procedure is interesting but seems like a lot of work to get a relatively small amount of anhydrous ethanol per unit of effort. Moreover, I doubt
many here have access to a muffle furnace. Years ago when I made an honest living, we had a project that required the formation of a water sensitive
imine. The reaction was run in commercial absolute ethanol using molecular sieves and worked fine except that it was a bit slow. When the sieves were
activated in a muffle furnace like you did, the reaction was faster but there was no effect on the yield and thus not worth the extra effort required.
Arguably, the situation is different if you are going to make, say, sodium ethoxide as a bit of water just plays havoc with yield. Again, we started
with commercial absolute ethanol for this type of reaction but distilled it from diethyl succinate and a bit of sodium. Any sodium hydroxide formed
from any water present caused the hydrolysis of the diethyl succinate to form a non-distillable sodium succinate salt. I think that this procedure is
also in Vogel. The ethanol distilled in this manner gave reproducible high yields from sodium ethoxide based chemistry.
Nonetheless, I admire your work here as it solved a problem you needed to have solved. Good job.
AvB
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happyfooddance
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In lieu of a muffle furnace, a steel can will work, with a charcoal fire (a little air to the fire helps)
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Magpie
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Thank you, AvB.
I first learned about how water can kill yield when alkylating ethyl malonate per Brewster (forum library.) Brewster tells how to prepare dry
ethanol. He starts with commercial absolute alcohol, which I've never had. (I now make an equivalent from Everclear [95% ethanol] and CaO.) Then he
adds sodium, which I didn't have at the time. Then he adds diethyl phthalate and heats for 30 minutes. Again, I did not have this reagent then.
Then he distills the ethanol into a bottle protected with a CaCl2 guard tube. The result is anhydrous alcohol. I don't remember how I dried my
alcohol at that time but clearly it was inadequate as my yield was lousy.
The single most important condition for a successful synthesis is good mixing - Nicodem
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Magpie
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Actually 650°C will likely be adequate to to dehydrate Ca(OH)2, IIRC. Look it up on Wiki.
Edit: Wiki says Ca(OH)2 dehydrates at 515°C.
[Edited on 26-2-2018 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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PirateDocBrown
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That's a really good idea!
Phlogiston manufacturer/supplier.
For all your phlogiston needs.
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PirateDocBrown
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Also, back in grad school, I used NaOH-saturated isopropanol (probably 99%, but I'm sure 91% would do) as a glassware soak.
You can't leave it in for more than a day or two as the NaOH would start to etch the glass. But it got almost everything clean, and far less
dangerously than piranha sauce.
Phlogiston manufacturer/supplier.
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