Aloesci
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Cold and Minty
I'm experimenting with making some menthol crystals from peppermint oil I've distilled.
Industrially, they freeze the oil colder than around -22.2°C where the menthol crystallises out.
My first idea to reach this temperature was an salt and ice bath but apparently those only get to about -21°C
Then I thought of using butane from a canister to cool it down but butane boils at -1°C so that wouldn't work either.
What could be some easily obtainable and effective options for cooling below -22.2°C? Or even a lot colder?
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Antigua
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CaCl2 + ice gets lower than -35 if I recall correctly. Just toss some CaCl2 with ice and add a bit of water to slush it up and put in your freezer. Or
buy dry ice.
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Fyndium
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About 30% CaCl2 can go even as low as -54C, at least in theory. In practice, I've got currently a polyurethane box of 20 liters filled with CaCl2
solution sitting in a chest freezer at -35C and it's fully liquid and clear. I use it as cold trap coolant primarily, and for specific reactions,
because dry ice is exceedingly expensive.
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mackolol
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Keep in mind, that the CaCl2 has to bee in form of hydrate to create the -35C solution with ice.
Also, you can use dry ice + acetone solution as mentioned if you have dry ice on hand
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Fery
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After crystallization, you'll obtain an oil depleted of menthol, but there is still a lot of menthol present from which it could be separated by
vacuum distillation using powerful column.
https://www.sciencemadness.org/whisper/viewthread.php?tid=15...
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unionised
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Looking at that paper I wonder if reduction of the peppermint oil with borohydride before fractionation would increase the menthol yield.
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Aloesci
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Thanks Antigua, Fyndium and mackolol for the information about CaCl2! I have a bunch of it from some dehumidifying products, so I will test around
with some different amounts with ice and see just how cold I can get!
I work in a lab for blood transfusions and we use dry ice to keep blood plasma frozen. They might let me take some away.
Also I've read that peppermint oil is about 36% menthol and 24.5% menthone so a reduction could perhaps significantly improve menthol yield? I don't
have the chemicals to try but it could be a fun idea.
For now I'm enjoying how minty it smells over here 
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Fery
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reduction of ketone to alcohol group using NaBH4 won't be stereoselective... I wonder whether there is a difference between the scent of the 2
stereoisomeric products
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UC235
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Menthol is a solid at room temperature. A decent freezer should be about -20C. The yield might be a bit lower but probably not by much. I'm not sure
where you're getting -22.2C from. Commercial menthol is typically produced from mentha arvensis oil, which is largely menthol whereas peppermint
contains a lot of menthone.
I should add that actual essential oil composition varies wildly depending on weather, plant parts distilled, growth stage, exact cultivar, and soil
chemistry.
[Edited on 3-3-2021 by UC235]
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zed
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Ummm. I obtained some material reputed to be Borneol/Isoborneol. So, even though I have a pint of Peppermint Oil in the cupboard, I will be using
it for the purpose I originally bought it for. Spraying around my foundation, in an effort to keep ANTS out of my house.
Now, Menthol it turns out, is a top catalyst for producing Sodium by Nurdrage's Method. But, fer repelling insects, I'm thinking Menthone might be
the best choice. I was using a semi-natural lotion called "Off" a few years ago. Full of Menthone. When you put this stuff on, bugs leave you
alone. Seriously; it really repels them. Deet, can't compete.
So, my quest has sort have been the opposite of yours. How can I convert so-so Menthol, into uber repellant, Menthone? Maybe Perborate?
Oh yeah. Back to Borneol/Isoborneol. A dealer on Ebay is selling it. I'm assuming it's the real stuff. Time will tell. One thing I can say..."It
is not Camphor!" Camphor I know. It has kind of a sharp, penetrating odor.
This Borneol, has more of a musty, earthy smell. Not Camphor! https://www.ebay.com/itm/BORNEOL-Isoborneol-Crystals-250-g-F...
[Edited on 22-3-2021 by zed]
[Edited on 22-3-2021 by zed]
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Fery
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zed you can oxidize menthol using aqueous environment with dichromate+H2SO4, or in acetone environment using CrO3 (Jones reagent) or menthol in ether
and oxidizing agent aqueous dichromate + H2SO4
http://www.orgsyn.org/demo.aspx?prep=cv1p0340
https://sci-hub.st/https://doi.org/10.1021/jo00802a005
https://en.wikipedia.org/wiki/Menthone
If you wish I can find you the CrO3/acetone method, we had it as a practical preparatory problem in chemistry olympiad.
[Edited on 22-3-2021 by Fery]
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Aloesci
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If anyone is curious i managed to extract some very crude menthol from some mint, and i made a video about it. I'm a (very) amateur chemist so the
video is not really meant for anything documenting my own playing, but feel free to check it out
https://www.youtube.com/watch?v=e9UlMjoeukc
I don't think my extraction extracted all the components of the oil so i was left with a waxy solid and i didn't carry out any freezing like i
described.
How could i recrystallize my product seeing as it melts between 36–38 °C?
Any help would be appreciated.
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zed
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Menthone to Menthol? Here, I would try to use Aluminum Isopropoxide and Isopropyl Alcohol, as the reducing agents. Seems like it might be the right
thing.
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