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Author: Subject: Anhydrous Conditions
gfinch
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[*] posted on 17-5-2021 at 13:48
Anhydrous Conditions


I am planning on running a reaction which has yields very dependent on the degree to which the reaction can be kept anhydrous. Does anyone have tips for keeping all water out of a reaction? Reaction starts with addition by addition funnel and is then switched to an extended reflux.



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Antigua
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[*] posted on 17-5-2021 at 13:56


1. Oven dry and then flame dry the glassware. You'd be surprised how much water can cling to the glass and remain unseen.
2. Use drying tubes filled with anhydrous calcium chloride on top of your condenser and on top of your addition funnel (in a way that allows air to get it so as not to create subpressure).
3. In a perfect setting do your best to backfill any glassware with dry nitrogen or argon instead of humid lab air.
4. Use dry solvents and reagents (molecular sieves are actually better at trapping water than even sodium-benzophenone according to a PPM measurement).
Honestly just watch good Grignard guides and you'll see what anhydrous conditions are.
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monolithic
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[*] posted on 17-5-2021 at 14:14


Quote: Originally posted by gfinch  
I am planning on running a reaction which has yields very dependent on the degree to which the reaction can be kept anhydrous. Does anyone have tips for keeping all water out of a reaction? Reaction starts with addition by addition funnel and is then switched to an extended reflux.


I've never done truly inert atmosphere but what usually works for me is: reflux condenser --> intermediate round bottom flask acting as a suck back trap with something like https://www.ebay.com/itm/121702120904?hash=item1c5602fdc8:g:... --> test tube filled with mineral oil. Load your reactants, purge with inert gas (bubbles coming out of the mineral oil) and then heat to reflux.

[Edited on 5-17-2021 by monolithic]
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Fyndium
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[*] posted on 17-5-2021 at 23:13


Do really that much water remain after oven treatment that flame drying is necessary? Also, many gas combustions produce water and carbon dioxide, hence it actually appears counter-productive to me.

Just a question out of curiosity.
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xdragon
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[*] posted on 18-5-2021 at 07:28


For absolutely dry conditions, after oven drying, pull hard vacuum, thoroughly dry with heatgun, purge with absolutely dry inert gas, repeat 3x. Schlenk line makes things easier here.

It would definitely help to know which reaction you are explicitly referring to. There is a huge bandwith with "dry" conditions. As you seem to be looking for AlCl3: a lot of Friedel-Craft acylations are quite robust and usually, oven drying, dried solvents and a drying tube should be enough for decent yields. Especially if your AlCl3 is fresh.
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zed
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[*] posted on 18-5-2021 at 08:27


You have included NO information on what you are doing. What reaction?

And.... Yes. It matters!
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