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Author: Subject: Aluminium metal + base as a dehydrating agent?
garphield
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[*] posted on 26-9-2020 at 21:24
Aluminium metal + base as a dehydrating agent?


Wiki says reaction is NaOH + Al + 3 H2O -> NaAl(OH)4 + 3/2 H2. As this converts water into hydroxide + hydrogen, there is no way for the equilibrium to go backwards, so any water should be permanently eliminated. Obviously wont work for acids but for alcohols, amines, ect it might be good. Might try this with some ethanol or ethylene glycol tomorrow.

One thing I am worried about is if it might react with alcohols in the same way as it would with water. I don't think it would be able to dehydrate them to ethers or alkenes because that requires an acid catalyst iirc but I don't know for sure. If it just converts it into an alkoxide that should be fine because it will be easy to hydrolyze. Don't think reductions would be an issue with most things because if that worked people would just use this instead of aluminium amalgam.
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Maurice VD 37
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[*] posted on 27-9-2020 at 11:15


The trouble is that NaOH reacts with aluminum even without water. The equation is : 6 NaOH + 2 Al --> 2 Na3AlO3 + 3 H2
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YoctoByte
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[*] posted on 27-9-2020 at 23:22


But wouldn't Na3AlO3 irreversibly react with water to form NaAl(OH)4 and NaOH? Because then it would still work.

If your solvent can withstand strong bases you could also use calcium oxide which might work even better.
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h0lx
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[*] posted on 1-10-2020 at 19:36


You could just make an aluminium amalgam, that reacts with water without a base
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TriiodideFrog
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[*] posted on 2-10-2020 at 00:24


Why don't you try adding HCl to Zinc? This reaction produces zinc chloride, a hygroscopic compound and hydrogen gas.
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njl
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[*] posted on 2-10-2020 at 06:09


Lots of things CAN be used as dehydrating agents, but its just not worth it most of the time. You can store 7 moles of water per mol of anhydrous magnesium sulfate (which is cheap and everywhere), so why bother?
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TriiodideFrog
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[*] posted on 2-10-2020 at 18:13


True. If you want an interesting way to get magnesium sulfate, you can use heat Epsom salt ( Magnesium sulfate heptahydrate )
to remove the water. Surprisingly, Epsom salt is about 53% water! After the water is removed, you will get magnesium sulfate. Be warned, the flask you use for the removal of water will probably crack.
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YoctoByte
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[*] posted on 3-10-2020 at 23:52


If you don't want to waste a good flask you can also dry the salt in a glass oven dish. Since MgSO4 is non-toxic you can do this in your oven. Slowly increase the temperature and don't make the layer too thick (1 cm max) because otherwise you end up with a solid mass which is hard to break up.
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TriiodideFrog
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[*] posted on 4-10-2020 at 04:32


Ok. I will try the experiment again with your suggestion. Thank you!
:)
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Tsjerk
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[*] posted on 4-10-2020 at 09:48


Throw the Epsom salt in a microwave resistant container of sorts, as long as the opening is not smaller than the container itself, and cover it with something like a plate.

Put in a microwave at high for 5 minutes. You will be left with a bone dry brick of MgSO4. As soon as the water is gone the salt doesn't absorb microwaves anymore, so you don't have to worry about overheating.

I use these ceramic cups you serve dessert in and put a plate normally used for sandwiches on top.

Just don't use a flask, that is just dumb.
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Pumukli
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[*] posted on 5-10-2020 at 09:55


Perhaps the magnetron of the microwave oven requires a minimum load (say 50 ml water at least) in the cavity (the oven) to work properly. Running the oven without this minimum load stresses the magnetron (wave relfections) and may be lead to its untimely death. If it was a real concern or not I don't know.
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symboom
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[*] posted on 5-10-2020 at 13:07


Magnesium sulfate heptahydrate has a lot of water contained in it here is nilered extracting the water from it.
https://m.youtube.com/watch?v=8GVSuKkuLzY




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garphield
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[*] posted on 9-10-2020 at 19:58


the reason i asked is because i am working on a synthesis of ethylenediamine from cheap materials and en forms an azeotrope and is basically impossible to get completely anhydrous - only thing i could find online is distilling over sodium metal. i thought this might work and if it does it would be much better than an alkali metal because its much cheaper. if dehydrating agents that didnt chemically destroy the water worked i think those people would have used them instead, and you can't use sulfur chloride, P2O5, thionyl chloride ect because those produce acids which immediately form salts with en. although maybe en might be a strong enough base to react with Al and water on its own, en aluminate might be an interesting compound but you would still probably use NaOH so you don't lose en as salts. a post with the details of the synth and yields will probably be up tomorrow or sunday if you are interested, although after that i will probably do some more stuff to see if the yield can be improved and if it can be used to make ethanolamine, 1,2-aminopropane, 1,2,3-aminopropane, ect.
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