so i was wondering what would be an apparatus to perform a reflux under vacuum. i posted a picture of the only thing i could think without buying
additional glassware. so forgive the picture i attached, i know it's a riot but hey it gets the point across
panziandi - 28-4-2009 at 15:48
You'll be hard pushed to find a round bottom flask that has a male cone to fit in the top, a stopper would suit better!
Personally I would opt for a T-piece or a cone-to-tubing connector (both very cheap) and place them above the condenser. Although having not used it
to reflux under vacuum I have used it to remove gaseous by-products from a reaction whilst refluxing.
What are you planning on doing out of curiosity?DJF90 - 28-4-2009 at 15:57
Panziandi has a good point, and the same point I would have made myself. A stopper would be just fine. Panache - 28-4-2009 at 17:03
i thought a round bottom with a male cone was a stopper. True, they are generally smaller if constructed from a rb (like 10ml), but i have several
sizes up to 500ml. I always assumed they were used to create headspace in a system. Another variant is the test tube with male cone stopper.
This then leads me to ask what are these 500ml round bottoms i have with b34 male cones designed for, if not stoppers for systems where some head
space is needed. they are factory blown (schott) so there must be a specific use for them.turd - 29-4-2009 at 12:39
This then leads me to ask what are these 500ml round bottoms i have with b34 male cones designed for, if not stoppers for systems where some head
space is needed. they are factory blown (schott) so there must be a specific use for them.
Well, the people using Schlenk techniques often use flasks with male cones. c.f. Schlenk tubes/flasks. Tubes are preferred when using Ar for
the higher headspace.
My guess is that OP wants to isomerize some allylbenzene to the corresponding propenylbenzene with solid KOH. This can also be done without vacuum,
but apparently the vacuum helps in temperature control and its supposed to remove water from the KOH(?).EmmisonJ - 30-4-2009 at 14:11
the vacuum is to reflux at a lower temperature to hopefully help against polymerization. it theoretically should help remove water from the KOH prior
to isomerization as well.
vacuum just seems like a good idea when you're in need of a reflux when your lowest bp compound is still 200+C to keep side-reactions to a minimum
[Edited on 30-4-2009 by EmmisonJ]turd - 1-5-2009 at 07:56
the vacuum is to reflux at a lower temperature to hopefully help against polymerization. it theoretically should help remove water from the KOH prior
to isomerization as well.
This is a quite cumbersome way of temperature control, since maintaining a given vacuum is harder than maintaining a given temperature. So I think you
got it backwards: the point is to remove water and maybe oxygen(?), with reflux at lower temperature being a nice side effect.
Quote:
vacuum just seems like a good idea when you're in need of a reflux when your lowest bp compound is still 200+C to keep side-reactions to a minimum
I don't see why you would need a reflux. Reflux is usually only performed in order to work at an easily reproducible temperature. Indeed, I've done
this reaction in a closed system below reflux temperature with good yields. Though I applied vacuum until reaching the reaction temperature. Running
it under constant vacuum may increase the yields somewhat, but I think not by much.
