charles
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Collect solvent vapour
when i have to evaporate the solvent from an apolar solution without warm it i use to connect my vacuum sistem (a freeze vacuum pump, a CaCl2 tube
connected with a oil flask) to the flask where is the solvent. now my problem is that using low boiling point solvent (pet. ether, dcm, thf...) i
can't able to catch solvent's vapours in the receiving flask because at my vacuum (58 torr) solvents evaporate at room temp so they go to
the oil trap then into the pump (that is going to ruin). the oil trap can take only few mls of solvent because at 58 torr it starts to boil even in
the oil.
i can't think about a method to stop the solvent cold vapour before the oil trap! i really need to stop it because is dangerous and ruins the
pump and also because i want to conserve the solvent and distillate...
tnx
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Tsjerk
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You can use a cold trap, like a salt/water bad, to trap your solvents in.
At university I one's saw a cold trap using liquid nitrogen.
If you have a lot of time and a old fridge, you could build the cooler of the fridge in a water(salt) or oil bad to cool it.
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garage chemist
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Special vacuum pumps that use no oil are normally used for this. The solvent vapor comes out at the air outlet of the pump and readily condenses at
atmospheric pressure.
Those pumps are nearly always membrane pumps with the membranes being made of a resistant plastic.
In order to condense the solvent in vacuum (when you don't want to buy the special pump I mentioned), you will have to use a cold trap cooled by
liquid nitrogen. Everything other is not cold enough, even dry ice/acetone (-80°C) cannot condense low- boiling solvents in vacuum!
[Edited on 21-9-2005 by garage chemist]
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neutrino
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If you're only going for 58 torr, you might want to look into an aspirator. They're cheap and should work well for this application.
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charles
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58 torr is enough for my vacuum needs but i can't buy a pump teflon coated where solvent can pass trought and condense OUT of it.
so to stop solvents vapor i need liquid nitrogen... or use a water aspirator, but with this device is not possible to collect solvents.
using a cold water/ice trap is not perfect since apolar solvents will form an upper layer on cold water and @58 torr will boil even @ 0°C (and water
will boil @ 40°C)
i can find cheap N2 cans but i can't imagine how use it to froze the collecting flask... just putting it in a closed box with the tip of N2 tube
(from can) gassing in the box? it's enough leave the can valve open and wait until N2 evap from liquid and cool the flask? the gas going out from
the tip of a tube from the can it's really going to cool? or the temp falling will be only @ the valve? i have to keep the internal pressure of
the can till the end of the tube? i have tu put a valve at the end of the tube or i can use just the can valve?
tnx...
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bio2
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........... because i want to conserve the solvent and distillate..............
Raise the pressure to coincide with a temperature you are able to condense when removing the reaction solvents.
Then lower the pressure to collect your high boilers.
This is the way it is normally done, besides a liquid N2 Dewar besides being expensive to operate is a huge pain in the ass. Considering the cost you
might as well discard the solvents as it would be cheaper if not ecofriendly.
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garage chemist
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That would be the most logical slution: just distill off the solvent under normal pressure!
Is your compound really THAT heat sensitive?
It would help us if you told us which component has to be freed of which solvent.
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charles
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when i use THF and LiAlH4 i would avoid heat (heat-sensitive amine compound) and dont' want to allow going THF in the pump since it's very
dangerous and ruin all my vacuum sistem, and also thf vapour are not a good thing in my room.
it's possible that nobody have to condense DCM or THF vapours @ 58torr without having a rotovapor or liquid N2?
how do you evap off solvents without boiling it? can i use liquid nitrogen without buy a dewar?
teflon coated pump are too expensive for me...
tnx guys
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garage chemist
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I refuse to believe that your amine decomposes at 66°C.
Maybe the instructions advise the use of vacuum but surely this is just a precaution since it could "maybe" decompose a few percent of the
compound.
In reality, vacuum isn't necessary at all, I'm sure.
Distill off the THF on a 80°C water bath and you should be fine.
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