[Babcock_Hall]

We tend to lose solvent to evaporation when we do an overnight reflux (and our reaction does not have much solvent to lose), and it occurs to me that we might be able to get a snugger fit between the flask and condenser.  A long time ago I remember using a polish of some sort to fit two ground glass pieces together more perfectly.  It was a fairly simple recipe, perhaps alumina of some kind in mineral oil, or something similar.  I thought that I got the idea from Shriver and Drezdzon's book on air-sensitive compounds, but I have been unable to locate it there.  Any help or ideas would be appreciated.

[discodermolide]

We tend to lose solvent to evaporation when we do an overnight reflux (and our reaction does not have much solvent to lose), and it occurs to me that we might be able to get a snugger fit between the flask and condenser.  A long time ago I remember using a polish of some sort to fit two ground glass pieces together more perfectly.  It was a fairly simple recipe, perhaps alumina of some kind in mineral oil, or something similar.  I thought that I got the idea from Shriver and Drezdzon's book on air-sensitive compounds, but I have been unable to locate it there.  Any help or ideas would be appreciated.

What are you using for cooling? I assume water, it may be better to use one of these low temperature cooling systems and cool at -10°C or below. I think the ground glass joint should be OK.

[408]

teflon sleeve or smear it with grease

and get a longer condenser

and do what disco said

[Babcock_Hall]

I found the passage I was thinking of:  page 163 (second edition) talks about lapping joints using successively finer polishing.

408, I was a little afraid of using grease because it might contaminate the reaction, but teflon sounds like a good idea.  Thanks.

discodermolide, Thanks, we do use water.  Do you think we are losing solvent vapor out the top of the condenser, not losing it between the flask and condenser?  I assume so, because it explains why you think we should cool with something else.  I have used ice water and a submersible pump to cool the coils of a rotary evaporator, but the ice would probably melt overnight.  I am unfamiliar with the -10 cooling system you mention (at least in the context of condensers).  Would you expand upon this?  Thanks.

[fledarmus]

It is quite possible you are losing solvent vapor out the top, especially if you have a low boiling solvent. I have a devil of a time keeping ether at reflux overnight. If you are running nitrogen through a bubbler over the top, you may lose even more due to the venturi effect venting off some of your atmosphere.

I had one supervisor that used to get her reactions up to temperature and then shut off the nitrogen line. Her argument was that once it was up to temperature, you were no longer heating a closed system, you were just maintaining the temperature of a closed system. I was never brave enough to try that option.

I believe Discodermolide is describing a refrigerated circulator. It pumps a cooled antifreeze mixture through your condensers rather than tap water. This type of system...

http://www.sigmaaldrich.com/catalog/product/aldrich/z653209?lang=en&region=US

[DrCMS]

Try a longer or more efficient multi-surface condensor or put a 2nd condensor on top.

[discodermolide]

Thank you Fledarmus, I was referring to such a system.
We used them in my lab all the time, saved water!

[curiouscat]

What's your solvent? Does it have a very high vap. pressure?

[nox]

If the reaction scale is small enough, I would just run the reaction in a 3 dram (or smaller depending on how much solution volume you have) and put on a teflon-lined cap. In my experience those vials are quite sturdily built and can hold a bit of pressure no problem. One of my colleagues used one of those vials to do an overnight reflux of dioxane at 140 degrees and the vial was unscathed the next morning. Plus it spares you the effort of cleaning since you can just dump the vial when done.

[Babcock_Hall]

What's your solvent? Does it have a very high vap. pressure?

We are doing SN2 reactions in acetonitrile.

[synthon]

I had one supervisor that used to get her reactions up to temperature and then shut off the nitrogen line. Her argument was that once it was up to temperature, you were no longer heating a closed system, you were just maintaining the temperature of a closed system. I was never brave enough to try that option.

Good to hear!  I also used to do that but gave up because anyone walking by would double-take and ask me about it skeptically.  Maybe once I get some more clout I'll revisit the practice. 

[synthon]

If the reaction scale is small enough, I would just run the reaction in a 3 dram (or smaller depending on how much solution volume you have) and put on a teflon-lined cap. In my experience those vials are quite sturdily built and can hold a bit of pressure no problem. One of my colleagues used one of those vials to do an overnight reflux of dioxane at 140 degrees and the vial was unscathed the next morning. Plus it spares you the effort of cleaning since you can just dump the vial when done.

Why not just use a thick-walled vial?  I've used a 1 dram vial to heat aqueous solutions at 70C, but the pH of the solution would vary.  I tried to keep it constant at 4, but after heating for a short period, it would have raised to 9.  I didn't observe that with any other vessel, so something about the vial must have been sensitive to the heat (and my reagents).