Thursday, November 23, 2006
Most organic chemists need dry solvents from time to time and almost every single day you bump into someone who's looking for dry DMF, ether, acetonitrile, THF etc. When I was working in Cambridge this was never a problem. They simply have a still for every single solvent you could imagine. Above there's a picture from the still room in Cambridge. Pretty nice innit? A massive fire hazard but quite handy as long as it doesn't blow up. Now I've moved on and where I work now we've just been inspected by OH&S (Occupational Health & Safety) and they don't like stills because of the fire hazard etc. and I have to say I completely agree with them. Nice as they may be they are dangerous and more or less completely redundant. The solvents you can buy nowadays are of super high quality and do not require distilling so it's basically only distilled in an effort to dry it. Now I will concede that there are stabilisers in many solvents, in particular ethers, but it is very rarely something that will affect your chemistry. So what should you do? The perfect setup that will provide guaranteed anhydrous solvents every day consists of good quality super activated molecular sieves (MS) and a Karl Fischer (KF) apparatus. MS are expensive but you can reduce the cost significantly by buying bulk quantities. We used to get ours from Grace Davison in big drums and they were very very good. The Karl Fischer apparatus (see picture below) can be purchased from Metrohm. All you have to do when you have this set up is add some 3 or 4 Å MS (depending on the solvent) to your solvent close the flask tightly and when you come back the next day take a small quantity out with a syringe and needle and squirt it into you KF apparatus. The display will now show you how many ppm's of water there was in the volume you just added. Everyone doing anhydrous chemistry should have this set up. It's safe, you are always confident about whether your solvent is dry or not and it requires close to no maintenance. Unfortunately, molecular sieves will not dry everybody's favourite solvent THF so you have to hold on to one still. Also you cannot add anything with acidic protons to a KF apparatus successfully. It will think it's all water. So for example acetone or methanol wouldn't work. Regarding MS I believe that you should never attempt to dry them yourself and never recycle them unless they are going into exactly the same solvent (just bin them when they are dead). Many chemists think they are saving money when they reactivate MS. However, I seriously doubt that is the case with the amount of energy not to mention time required to do so. So in other words buy good ones and bin them when they stop working. If you aren't sure whether the MS you've got are any good put a couple in the palm of your hand and add one drop of water. If they get really really hot they a very good and if they only warm up a little bit they are rubbish. If you are in a situation where you can't get good MS I guess you will have to dry them. Apparently one way of doing this is to throw them in a microwave and nuke them on max power until they start glowing. At this point you have to stop immediately unless you want the entire microwave to melt and transfer the MS to a desiccator that you stick on a high vacuum pump (I haven't tried this myself so no guarantees). If you don't feel like burning your department to the ground there is always the good old vacuum oven that most departments have. Just heat them under vacuum for a couple of days and then stick them in a desiccator attached to a high vacuum line. Drying solvents overnight using good 4 Å MS should get acetonitrile, DMF, DMSO, dichloromethane, toluene, ether, 1,2-dichloroethane, chloroform and hexane down to a water content of 10 ppm or less. If you check your freshly distilled THF on the KF it should be around 15 ppm. So for successful living convince your boss or department to get one of these babies and shut down all those damn stills for good. D!