[gvic]

I'm performing a large scale (100g) calixarene synthesis that involves refluxing diphenyl ether (bp 258C) for 4 hours. Previously group members had accomplished this using mineral a oil bath but my PI put a stop to that. I attempted to bring the solution to reflux using a heating mantle w/ sand but even at 115 volts that the unit was rated for I couldn't achieve reflux temperatures. I ended up going up to 130V which worked great for about an hour until the heating mantle died (the fiberglass base of the mantle bottomed out inside, I was shocked).

We have a bit of Silicon oil lying around but as it's expensive I'd rather not use it unless I have too. It's not rated for 260C either but it just polmerizes and breaks down forming a gel rather than combusting like mineral oil might do. I'm thinking a good alternative would be to use a sand bath heated by a hot plate. Has anyone been involved with similar high temp reactions and might have some wisdom to bestow?

[fledarmus]

I've used the hot bath with sand for small scale reactions, especially neat reactions in vials up to about 300C. I've also used heating mantles (no sand) up to 300C for round bottoms; I'm surprised this didn't work for you. The heating mantle does have to fit the round bottom. You also may need to insulate the top of your reaction vessel and the base of the reflux condenser - if you have enough open space above the liquid, you may be losing so much heat above your solution that even if the bottom makes it up to reflux temperatures, the top won't. I've found that wrapping the reactor in glass wool with an aluminum foil outer layer to hold it in place works well.

I've never tried the sand bath at 250C for a round bottom, but I don't see why it wouldn't work.

[poorstudent83]

I have done this very reactions heaps of times! I've always used a regular heating mantle without sand and have never had an issue with the refluxing temperature. I always use a Dean-Stark to drive the water from the reaction and have this wrapped in cotton wool and foil....

[azmanam]

You also may need to insulate the top of your reaction vessel and the base of the reflux condenser

this.  Use foil or glass wool (or both) to insulate the head space and bottom of reflux condenser.  The glass and all the atmosphere inside the vessel need to be at the BP for the vapor to efficiently reach the condenser.