21st Century Stuff

You will not find this in your text book!

Put those sublimers away!


Now distill volatile solids easily in large quantities.



Contribution from the R&D Laboratory of Pure Synthetics, Inc. (corporation of New Jersey now dissolved)

 Posted: Aug 15, 1997 1:40pm Pacific Time    Latest Update: Oct, 2008


"A New Method to Purify Volatile Solids:
Entrainment-condensation Distillation"

By J. S. Paul Schwarz & Helen K. Schwarz (deceased)

(I am trying to write so that both students as well as professional chemists can understand. I ask the professional chemists to have patience, the students to ask their teachers to explain or for anyone to e-mail me his or her questions or comments!)


DISCUSSION: When we began to make the maple-flavoring chemical methyl cyclopentenolone [also called MCP, Cyclotene (Dow trade name), Apetizer (Pfizer trade name), and Ketonarome (Givaudan trade name)], we felt that a distillation or sublimation method would be superior to any crystallization technique. However, we did not know of any method to easily distill in large quantity a substance which was a solid above room temperature, and, upon investigation, there did not appear to be such things as industrial-capacity sublimers.

As implied above, MCP is such a compound with a melting point above 105 degrees C. when anhydrous and a melting point a little over 80 degrees C. as the monohydrate. At first, we recrystallized the product twice from water using activated charcoal to absorb colored impurities, and, while this method yielded adequately pure product, it was laborious, and the charcoal, of course, absorbed some of the product, as well as the impurities, lowering the yield.

S = stirrer motors V = to vacuum regulation
Distillation apparatusThe main difficulty with distilling compounds solid above room temperature is that the entire apparatus becomes a condenser. With normal still set-ups: still pot - condenser - receiver - vacuum line, the condenser must be kept above the melting point of the solid distilled. The vacuum line connecting the receiver with the vacuum source must be watched to ensure that it does not clog with solid. The apparatus on the left obviates these problems. The condenser is placed after the receiver which contains a gently refluxing solvent for the solid being distilled.

EXAMPLE & EXPLANATION: Methyl cyclopentenolone (MCP) from our synthesis was a dark-red to black solid. This was allowed to dry to the monohydrate stage, melted and poured into the still. This is stirred (Diagram: gray material in left-hand flask) and heated to distill the remainder of the water of hydration. The condenser on the receiver is simply cooled with water at room temperature or perhaps lower. When the still temperature (Diagram: thermometer immersed in gray crude material) reaches about 150 degrees C. most of the water has distilled. At this point, extra water (about 2 mls for each gram of expected product) is added to the receiver.

One starts the stirrer in the receiver and begins to apply vacuum slowly. After the water has completed distilling, the product begins to distill at about 170 degrees C at a vacuum of about 22" Hg (about 200mm Hg pressure). The water in the receiver is maintained at a very slow reflux (in other words: boiling, condensing in condenser and returned to receiver) by heat added judiciously by the heating mantle (Diagram: mantles are hemispherical and represented in the diagram by words). When we operated with a 22-l still and a 50-l receiver, we didn't apply any heat to the receiver since extra insulation was enough. The heat from the MCP vapor (170+ degrees) condensing and dissolving in the receiver water was enough to maintain the temperture close to reflux.

Here the water and water vapor are shown in reddish shades for easy visualization. The MCP vapor (colorless) is entrained (mixed) with the solvent vapor in the receiver and this mixture after condensation to solution runs back into the receiver. The contents of the receiver progressively become more concentrated in MCP, but the vapor from the refluxing solution is, of course, much more concentrated in water vapor which in turn entrains more freshly-distilled MCP, condenses once again in the condenser, the resulting solution running back into the receiver.

When the still pot temperature has reached about 200 degrees C, the heat is turned off on both mantles (if, indeed, a receiver mantle proves necessary). The vacuum source is disconnected, and, after the pressure inside has reached atmospheric, the solution in the receiver can be removed to effect crystallization. It is best in this case to stir by hand, for the mixture becomes too thick for normal mechanical stirring. After refrigeration, the crystals are filtered (centrifuging better) and washed with cold water before drying at about 50-60 degrees C.

There is one final trick to use to insure getting a colorless (white) product in the case of MCP. If you are interested, you can get this information by sending your request to the WEBMASTER (no charge!).

ADVANTAGES TO THIS METHOD:

Partial list of compounds (melting point) which have
been or possibly can be purified by this method:
3,4-Dimethyl cyclopentenolone (72)
3,5-Dimethyl cyclopentenolone (92)
3-methyl cyclohexenolone (63)
Indole (51)
Vanillin (83)
Camphor(180)
Phenylacetic acid (77)
Pyridoxine (Vitamin B6) (160)
Benzoic acid (122)
Iodine
M(to be done in a special still under
Msuper-atmospheric pressure)
Maltol
M(Done before 1994 for Pfizer
Munder a 10-yr secrecy agreement)
M

Pfizer used to be perhaps the world's largest maltol manufacturer, but they don't claim this as a product now as far as I can determine. From a search using Google, it now appears that the Chinese are the big maltol manufacturers.

Disclaimer: I give this information at no charge with no warranties expressed nor implied that this procedure will either be efficacious or be safe since I have no control over who might be trying to use the method nor over what chemicals will be employed. The user is responsible for determining the fitness and safety of the method in his or her own particular case.
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