Distillation

You see that distillation setup in fig. 1? Well, when one puts something into the reaction flask and heats it, low boiling stuff like organic solvents (acetone, ether, DCM etc.) vaporize and then condense inside the condenser. They then drip down into the receiving flask. Next (usually) comes water, then finally (as far as drug chemistry is concerned) comes high boiling drug oils. Distillation is the most preferred and reliable way to purify substances from one another.

There are three basic forms of distillation: simple, vacuum and fractional distillation, which can be an extension of the first two. Simple distillation is just plain old distillation with no vacuum and is used for purification of low boiling mixes or for removing solvents such as ether. For vacuum distillation a thick walled or reinforced hose is connected from a vacuum pump to the nipple (tee hee!) of the vacuum adaptor. Application of a vacuum greatly reduces the temperature of boiling and is a must for most of the distillation in this book. Fractional distillation is merely vacuum distillation except an added glassware component such as a small Vigreux column or a Claisen adaptor packed with shards of glass is placed between the reaction flask and the 3-way joint of a normal setup. Either of these additions create a gauntlet of extra surface area elements that distilling liquids must pass through in order to condense. This affords a greater degree of separation between mixes of oils that have similar boiling points. However, it has been Strike's observation that most of the bulk separation in this book using distillation can be achieved with regular old vacuum distillation.

If the chemist has done the appropriate thing and bought a heating stirplate then a magnetic stir bar is always stirring in the reaction flask. If the chemist only has a hotplate then the chemist is going to have to add some boiling stones to the flask. Boiling stones can be either little chips of teflon, little wadded up balls of teflon tape or little pieces of a shattered porcelain counter top tile. All of these things have lots of microscopic pockets of air in them that help to facilitate the start of boiling. Many of the solutions in this book, especially organic solvent solutions, need these boiling stones in them if they are going to be heated. If there are no boiling stones, then a phenomenon called superheating may occur whereby the solution will start to get hotter than its actual boiling point temperature. Eventually it will burst into boiling but a lot of it will have gone sploosh! into the condenser. Not good.

Another thing or two to remember when distilling is to wrap aluminum foil around the reaction flask. This will help stop heat loss so that things will distill quicker and at lower temperatures. Sometimes, if one is going to distill a solution that is just solvent and product, all that pure solvent that comes over first is perfectly reusable and should be saved for future extractions.

The last thing about distillation that needs mentioning is the swapping out of collection flasks. When distilling under vacuum it will be necessary to remove the flask that has collected all the solvent or crap and replace it with a clean one to collect all the product that will be coming over shortly. This swapping of flasks needs to be done quickly as soon as the vacuum hose is disconnected. By quick Strike does not mean lightning fast. Just quick enough so that the hot solution in the reaction flask does not get too much hotter from the heat source it is sitting on, because if it does, and the vacuum is reapplied, it may pull a little form of that superheating stunt and go sploosh! into the condenser.

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