Frank 8. Mallory Bryn Mawr College Bryn Mawr, Pennsylvania
A Convenient Laboratory Sublimation Apparatus
M a n y organic chemists rely almost exclusively on recrystallization as a method of purifying solids even though this technique has several possible drawbacks. Undesirably large amounts of material may he lost in mother liquors; considerable labor, skill and dexterity is often required; sometimes long trialand-error processes are needed to find suitable solvents or solvent mixtures; low-melting solids tend to oil out; and it is difficult to maintain inert atmospheres for the recrystallization of compounds which are sensitive to moisture or air oxidation. All of these objections are avoided by the use of reduced-pressure sublimation. However, this is not to say that sublimation is without same unique limitations: some compounds may undergo thermal decomposition or polymerization during the relatively long heating periods that are required; compounds with molecular weights greater than about 300 as well as some compounds with melting points below about 60" may sublime a t rates which are inconveniently slow. Each of the two methods has its place. Sublimation is an excellent method for the initial purification of crude solid products from synthetic reactions. This method will leave behind relatively nonvolatile materials of high molecular weights as well as various inorganic substances which typically are present, and generally will not result in the loss of appreciable amounts of the desired products. By contrast, recrystallization a t this stage often involves significant sacrifices in yields in order to get rid of such kinds of impurities. Previously-existing types of apparatus for reduced pressure sublimation,' especially the 'lcold-finger" types, suffer from one or more of the following faults: it is difficult to disassemble the apparatus to remove the sublimate without inadvertently dislodging some of the crystals and causing them to fall back into the residue: the capacities are limited to one gram or less; the use of rubber stoppers precludes obtaining a high vacuum and thus makes many sublimations impractically slow; and standard taper joints of size 45/50 and larger will almost invariably freeze if held together at reduced pressures for long periods, whatever type lubricant is used (even Teflon sleeves). WIBERG,K. B., "Laboratory Technique in Organic Chemistry." McGraw-Hill Book Co.., Inc.., New York. 1960. DD. 115. II~'JAECKEL, R.,in HOUBEN-WETL, " ~ e t h o d e kder &&ische Chemie," 4th ed., Georg Thieme Verlsg, Stuttgart, 1958, vol. 1, part 1, p. 937; VOGEL, A. I., "A Text-Book of Practical Organic Chemistry," 3rd ed., Longmans, Green and Co., Ltd., London, 1956, p. 156, p. 1108; TIPSON,R. S., in "Technique of Organic Chemistry," Interscience Publishers, Inc., New York, 1951, vol. 4,pp. 613-641.
The simple apparatus%hown in the figure has been found to provide the means for clean, easy, and successful reduced-pressure sublimations. The material to be sublimed is freed from solvents and placed in the bottom piece of the apparatus (the cup). An alternative procedure which is particularly effective for crude, sticky material is to wash the solid into the cup with ether and to remove the ether under a stream of air or nitrogen with gentle heating. The upper piece of the apparatus (the condenser) is connected to a pump capable of producing a vacuum of 0.05 mm or less. The water jacket is connected, the joint is lubricated and assembled, and the apparatus is evacuated. The apparatus is clamped in a vertical position in an oil bath with the oil level almost to the top of the socket joint. The bath is maintained a t a nearly constant temperature, generally about 5-30' below the melting point of the compound being sublimed, by using a hot plate controlled by a variable transformer. The sublimate adheres firmly to the inner wall of the condenser in a region about 10-50 mm above T the oil level in the bath. To remove the snblimate the apparatus is disassembled, the grease iswiped off the ball joint, and the condenser is inverted over a suitable container. The sublimate is dislodged from the walls with a spatula and falls out through the 9-mm outlet 1 that previously served as the T - 5 0 0 9 .ock.I pump connection. The last traces may be rinsed out with a solvent,. Many different solids have been sublimed at reasonable rates under these conditions: m-phenylenediamine (1 g/hr), p-dimethylaminobenzaldehyde (2 g/hr), 3-chlorophenanthrene (0.2 g/hr), benzalaniliue (1 g/hr), dihydroqninaldine dimer (CPOH1?NI) (4 g/hr) and 4-methylstilbene (2 g/hr). Other examples include o-nitroaniline, 2,4-dinitmaniline, p-benzoquinone, p-dibromobenzene, and 1,4-dihydroxyanthraquinone. The apparatus in the figure is designed for 1-20 g of sublimate. Vor less material a smaller apparatus built around a 28/15 spherical joint may be used. A large apparatus sf similar design (but without a water jacket) using a 102/75 spherical joint has proved successful for sublimations involving up to 150 g.
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Such an apparatus is now mailable as Catalog No. K-85500 from the Kontes Glass Co., Vineland, Neu Jersey. Volume
39, Number 5, May 1962
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