Sciencemadness Discussion Board

Will MeOH in DCM retain H2O?

Acuyo - 31-10-2016 at 11:32

In other words, if one were to distill some DCM, but methanol came over as well - quite a bit really as after over 10 washes I could still detect a heavy alcohol smell (I let it run up to about 42C w/out a column, apparently that was a mistake) -- If I used this for an extraction that contained water, would the alcohol in the DCM grab some of that water?

I used some of this bad run, impossible-to-wash DCM for an organic extraction, and it freezes almost as thick as ice at about -5c.

I have what I hope is a much better run of DCM this time, but before I proceed I though it'd be good to have an idea of what might be happening here.

Do DCM and MeOH form an azeotrope?

One more: does Na pyrosulfate pull water as well as alcohol from DCM, or just alcohol?

ALSO, sorry, you can call me question boy today: I tried to dry some of the first DCM run with MGSO4 -- it dissolved; I'm using this DCM to attempt to extract some specific aromatics -- is it possible there's been a reaction between the magnesium sulfate and any organic aromatic, I believe there are probably at least a half-dozen compounds in there.

Many Thanks

DraconicAcid - 31-10-2016 at 14:18

If your magnesium sulphate dissolved in the stuff, then I'd assume that there's more water and methanol in it than DCM. Have you measured the density?

Either use a less soluble drying agent, or keep adding it until a saturated aqueous solution separates from it.

Maroboduus - 31-10-2016 at 18:00

Water has a strong tendency to pull the methanol out of most DCM/methanol solutions. Same with DCM/Acetone.

It's hard to understand how you could wash a methanol/DCM solution 10 times and has much Methanol left in there. And methanol's odor is usually masked pretty well by the smell of DCM.

It's also strange that you'd get much methanol coming over at 42C.

What are the actual contents of what you're distilling? if it's just methanol/DCM you ought to have gotten an azeotrope at 38C
that's 7% methanol.

Acuyo - 31-10-2016 at 18:43

Quote: Originally posted by Maroboduus  
Water has a strong tendency to pull the methanol out of most DCM/methanol solutions. Same with DCM/Acetone.

It's hard to understand how you could wash a methanol/DCM solution 10 times and has much Methanol left in there. And methanol's odor is usually masked pretty well by the smell of DCM.

It's also strange that you'd get much methanol coming over at 42C.

What are the actual contents of what you're distilling? if it's just methanol/DCM you ought to have gotten an azeotrope at 38C
that's 7% methanol.


My second run seems to make a bit more sense.

I used a random brand of stripper for the first one and didn't read the ingredients closely. The second can states only meoh and dcm; and I used a Vigreaux.

So far this second run seems better, I'm not losing nearly as much during the washes and after adding in some sodium pyrosulfate, it seems to be clearing up nicely.

The meoh coming over at 42 is mysterious, maybe it came over as an azeotrope with something else? (acetone?)

edit: I just re-read the original source: acetone, methanol, dcm, toluene and xylene.. Guess I know which brand I won't be using again..

So is MgSO4 normally ok for drying DCM?

Meltonium - 1-11-2016 at 07:06

Quote: Originally posted by Acuyo  


So is MgSO4 normally ok for drying DCM?


I'd say so, yes. It isn't really okay to dry alcohols because it will dissolve (unless you're just going to distill the alcohol again, then it's fine). However, if you have methanol in there with the DCM, the MgSO4 will partially dissolve in it. Alternatively, you can use anhydrous potassium or sodium carbonate to dry your DCM. It works really well.

On another note, you can tell what you're collecting when you're distilling by looking at the condenser. If the stuff condensing is beading up a lot, then it is most definitely DCM. If the stuff condensing is more of a smooth sheen, then it's probably methanol.

[Edited on 1-11-2016 by Meltonium]

Maroboduus - 1-11-2016 at 08:30

Calcium chloride absorbs water and forms complexes with alcohols, at least the low weight ones.
It is lower capacity than some other salts, but very complete.

You might use this after conventional drying with Mg or Na sulphate to get the last bits of water and residual alcohol out.

Acuyo - 2-11-2016 at 08:40

Thanks, I appreciate the answers.

The aqueous solution I'm extracting from is mostly water, NaCl, plants fats & oils, and some other things; so I'm not too concerned about water in the solvent.

My target compound is very soluble in alcohol though, so I really need it all out of there unless it came back into the DCM after extraction, as the next step is distillation.

If the aqueous solution is saturated with NaCl, would it have a tendency to push the methanol back into the DCM?

Obviously not an ideal situation, I just wonder if the theory would work?