Irving Allan Kaye
Brooklyn College of the City University of New York Brooklyn, New York 11210
Purificcltion of LOW-Melting Compounds
The purification of organic chemicals melting in the region 28-70°C by recrystallization is frequently difficult to achieve due to the tendency of such lowmelting compounds to precipitate from solution as oils (la). Sometimes this difficulty may be overcome by employing sufficient solvent so that the solute precipitates well below its melting point, a procedure which usually leaves a considerable amount of the solute in solution. An alternant procedure, which frequently overcomes this "oiling-out" difficulty and involves much less solubility loss, involves the low-temperature (-78°C) crystallization of the product from a solvent inwhich the compound shows appreciable solubility a t room temperature. Solvents,snch as acetone, methanol,pentane, ether, and equimolecular mixtures of carbon tetrachloride and chloroform @a), which do not crystallize at dry-ice temperature and which do not become viscous at this temperature (2b), are particularly useful. The description of the procedure which follows involves no expensive or exceptional equipment and is particularly useful in research as well as in graduate and advanced undergraduate organic chemical laboratory courses [where such compounds as decalin-l,&dione (5), (-)isopinocampheol (4) and 7-trichloromethyl-8-chloroA1-p-menthene (5) have been prepared by our students and crystallized from pentane (in the first two instances) or acetone]. The compound to be purified is dissolved in sufficient solvent so that when the resulting solution is chilled to Dry Ice temperature, a filterable slurry, rather than a solid cake, is formed. A foamed-plastic container' is an inexpensive vessel, which may be substituted for a Dewar flask, for chilling the flask containing the solution in powdered solid carbon dioxide. During the chilling period (frequently 15-30 min, but longer in some instances) a stoppered flask containing the pure solvent, a glass dropper (wrapped completely in aluminum foil), and a Buchner-type filter funnel are all chilled in powdered Dry Ice. A conventional filtration assembly (6, 7) may be used, but one which is particularly useful, especially where the filtrate is to be concentrated for the recovery of additional product, consists of a flask for collecting the filtrate, a rubber filtration adapter,=and a Biichuer-type filter funnel surrounded by a foamedplastic jacket1 containing powdered Dry Ice. To minimize moisture condensation in the filter funnel, the upper opening of the jacket (above the funnel) should be sealed with aluminum foil a t all times except when the funnel is being filled. After filtration and washing with the Dry Ice-chilled solvent, dispensed with the prechilled glass dropper, the jacketed funnel is removed from the filtration adapter when liquid no longer is 696
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expressed from the funnel. The apparatus is then reassembled without the Dry Ice jacket and left connected to the vacuum source until some time after the funnel has reached room temperature. If a low-boiling solvent was used, the product may be sufficiently free of solvent at this point so that its melting point may be determined; with less-volatile solvents air-drying at room temperature for several hours may be necessary. A melting point apparatus, more reliable than the silicone fluid-filled Hershberg apparatus, (lb) for determining the melting points of compounds melting below 60-70°C may be readily assembled from components found in most chemical laboratories. It 1 These are available (especially during summer months) in neighborhood shops as inexpensive ice buckets and are similar in size and shape to the much more expensive polyvinyl chloride foam ice buckets sold by Fisher Scientific Co. (catalog number 11-676). The disposable foamed-plastic containers in which fuming nitric and other acids are shipped may also be employed and may he converted into Dry-Ice containing jacket.; for Biichner-type filter funnels. This is readily accomplished by drilling, with a cork borer, a hole in the bottom of a foamed-plastic container just large enough to make a snug fit with the uppermost part of the stem of the funnel. The sides of the jacket should extend about 1 cm above the top of the filter frnmel. The foamed plastic is easily cut with either a saw or knife; any rough edges are smoothed by rubbing with a cloth moistened with acetone. The rubber filtration adapter is sold by Fisher Scientific Co. (catalog number 7-702C) as a rubber coupler with bras? side-arm tube, far about 601. Although designed originally for converting a glass tube into a water-cooled jacket in an inexpensive condenser, i t is admirably suited far converting an Erlenmeyer flask (no larger than 125 ml in capacity, to minimize the danger of a n implosion while the flask is being evacuated) or round-bottom flask into s, filter flask. The adapter fits directly and snugly into a 125-ml Erlenmeyer flask (Corning No. 4980) and into a 324/40 outer joint. Smaller S joints, like 619/22, and smaller Erlenmeyer flasks may be inserted into the adapter up to the brass side-arm tube. The upper opening accommodates Bochner funnels up to size 2. By replacing the customary filter flasks in students' apparatus kits by these adapters (asis done a t Brooklyn College), a considerable saving in locker space and financial outlay, as well as a reduction in student breakage, may be achieved. One in use in our laboratories consists of a brass rod, 42 em long and 7 mm in diameter, to the bottom of which is soldered a semicircular brass disk 2.5 mm thick and 5 cm in diameter. Ten holes, 2 mm in diameter, are drilled in the disk 4.5 mm apart and 1 cm from the periphery of the disk. (Measurements are from the centers of the holes.) Another similarly constructed disk is attached to the rod 10 em above the lower one and the two are aligned pamllel. The rod, clamped to a. ring stand, is immersed in the bath until the upper disk is slightly above the surface of the water. The melting point capillaries are inserted through parallel pairs of holes and held in place by pieces of masking tape affixed to the uppermost parts of the tubes so as to form projecting double-thickness "handles"; each 'capillary may be identified by a symbol written on the surface of its "handle." We have also itsed a capillary support similar to the preceding but constructed from glass tubing and cork disks.
consists of a 1500-ml beaker containing sufficient distilled water as the heat-transfer medium to reach the spout of the beaker. The beaker is equipped with a mechanical stirrer, a thermometer, a device for supporting a number of melting point capillary tubes,3 a magnifying glass trained on the bottoms of the capillary tubes, and a lamp for illuminating the compounds in the capillary tubes. The bath temperature may be raised quickly by heating with a Bunsen or Meeker burner; a further rise in temperature may be achieved, with a control equaling or exceeding that obtainable with electric immersion heaters, by heating with a shielded semimicro burner (8). Rapid cooling of the bath, if required, may be accomplished by inserting a large test tube packed with ice. By employing a saturated aqueous solution of lithium chloride as the bath medium, melting points up to 140°C may be observed in the same apparatus (using a Bunsen flame as the heat source a t temperatures above 80-90°C).
Literature Cited
(1) WIBERG,K. B., "Laboratory Technique in Organic Chemistry," McGraw-Hill Book Co., Inc., New York, 1960; (a) p. 103; (b) p. 81. (2) MORTON, A. A,, "Laboratory Technique in Organic Chemistry," McGraw-Hill Book Co., Inc., New York, 1938; (a) p. 163; (b) p. 153. R. S., J. Org. C h n . , 28, 325 (3) KAYE,I. A., AND MATTHEWS, IIOR?!
(4) ZWEIFEL, G., and BROWN, H. C., J . Am. C h m . Sac., 86, 393 I1OC1, \'YV',.
(5) OLDROYD, D. M., FISHER,G. S., AND GOLDBLATT, L. A,, J. Am. C h m . Soc., 72,2407 (1950). (6) CUWMINS, A. B., "Filtration," chapter 7 in "Technique of A.) Organic Chemistry" (1st Ed.), (Editor,WEISBRERGER, Interscience Publishers (division of John Wiley & Sons, Inc.), New York, 1950, vol. 3, pp. 512-524. (7) KAYE,I. A,, AND BURLANT, W. J., J. CHEM.EDUC.,31, 127 ,l"C*\ ,'m,',.
(8) KAYE,I. A,, Chemist Analyst, 54, 56 (1965).
Volume 46, Number 10, October 1969
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