V. Horak and DeLanson R. Crist Georgetown University Washington. D.C. 20007
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Small Scale Organic Techniques Filtration and crystallization
In this country a gradual trend toward small scale experimentation in the organic chemistry laboratory was further Although an emended by E'. Schneider' and N. Cher~,nis.~ undergraduate organic lahoratory was developed based on these principles? small scale experimentation in this area has never been adopted as a general concept. T o abandon completely the classical macro scale techniques (10 g or larger quantities) in favor of semimicro methods (preparations with less than a gram or derivatizations with less than 100 mg) has seemed neither entirely logical nor popular. Introducine certain semimicro techniaues and some small scale egperiments, on the other hand;is well justified in the undereraduate proeram. One of the most important . reasons is that considerable time (and sometimes expense) can he saved so that more non-routine experiments can be accomplished in already over-crowded semesters. Another reason is pedagogical in nature, since experimentation with small quantities requires neat and quantitative work, while in macro scale experiments students tend to hecome careless and disregard quantitative aspects. Finally the introduction of small scale preparations is a logical result of the increased use of modern physicochemical methods which require only minute samples for identification and structural determination. For these reasons the popularity of small scale experimentation is increasing. Unfortunately, however, one still finds derivatizations carried out with gram quantities. These procedures must now he regarded as absolute anachronisms and should unauestionahlv he eliminated from laborarory textbooks. Small scale exoerimentation csnnut he done sucressfullv just by reducing the quantities given for a macro scale experiment, simultaneously with the size of reaction vessels, size of ground joints, and general miniaturization of the equipment. Semimicro experimentation can he fully explored only when specific methods and techniques are used, which particularly match this scale and have been developed for these reasons. In the present article small scale filtration techniques are discussed as an example of a semimicro method applied to crystallizations of small quantities of materials. These techniques were adopted with success in the undergraduate laboratory program a t Georgetown University. The only special equipment necessary for experiments is a capillary filter stick (see the fig.). I t is formed from a thick-wall glass capillary of about 2 mm i.d. which is bent twice at right angles. The two parallel arms are of different lengths, the longer one ending with a large opening of about 10-15 mm length. The short arm is equipped with a rubber stopper it1 to match the reeular laboratorv test tuhe (16 X 166 i m j : This filter stick i5 used in the ekperiment below for two overations: (1) hot filtration of the solution to remove insbluble impurities before crystallization, and (2) separation of crystals from the mother liquor after the crystallization process has been completed. The hot filtration is done in a special way which makes this operation particularly easy, safe, and fast4 Instead of utilizing water aspirator suction, a gentle spontaneous suction is produced by a vacuum resulting from condensation of hot vapors of a hoiling liquid in a closed system. Specificallv.. the filter stick is connected hv means of a rubber stopper to a test tube containing a small volume of the solvent used in crystallization (e.g., ethanol) and the open end ~
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Journal of Chemical Education
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cooled
Steps in me finer stick memad lor hot fillration and crystallization: (aJactivation of suction test tube: (bl finration of the hot solution: (c) removal of mother liquor and collection 01 wstals: (d) drying and transferring of crystals to SarnDle container. is packed with cotton. The test tuhe is heated until the boiling solvent starts dripping from the cotton packing. In this process all the air previously present in the test tube is replaced by vapors of the liquid. Simultaneously, the filter stick is warmed by the hot vapors passing through its entire length as required to prevent crystallization of the hot solution in the caoillarv durinr! filtration. If the long stem of the filter stickis thLn immersed into the solution-to be filtered (hot or cold). a slieht decrease in temperature of the solvent vapors spktaneously pulls the sdution through the filter. Deliberate cooling of the suction test tuhe facilitates the filtration process which is usually completed in a few seconds. Volumes of less than 1-25 ml (or even larger by using Erlenmeyer flasks of adequate size) can be handled by this technique. The hot filtration method outlined above is superior in all respects to other methods used for the same purpose. In particular, the condensed vapor suction technique cannot he substituted with a conventional water aspirator. Here, loss of vapors exhausted by a vacuum line causes excessive evaporation of the hot solvent a t the outlet of the filter stick, and resultant cooling may cause precipitation of material which blocks the capillary. Crystals can be separated from the mother liquor by in-
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Sehneider, F., "Qualitative Organic Microanalysis," John Wiley & Sons, N.Y., 1946. Cheronis. N. D.. "Micro and Semimicro Methods." in Weissberger, A,, (Editor) "Technique of Organic Chemistry," Vol. VI, Interscience Publ., Inc., N.Y., 1954. Cheronis, N. D., "Semimiera Experimental Organic Chemis-, try," Hadrian Press, N.Y., 1960. Horak, V., Chem. Listy, 54,123 (1960)
verse filtration using the same filter stick which is now fitted to a test tube equipped with a side arm. Suction can he applied hy a water aspirator (or house vacuum) which is suitable for this operation since the solvent is cold. T h e enlarged end of the longer stem is packed with a small quantit y of cotton leaving a large portion of the cavity empty for collecting crystals. The main portion of the solution above the crystals is removed first. Finally, after most of the crystals are collected in the filter stick, fresh cold solvent is added to the test tuhe to help trap the rest of the crystals as well as wash the product free of mother liquor. The crystals are dried in a stream of air hy continuing the suction. t d w cwstals mav loosen and fall Since a t this ~ o i n the from the filter stick, it shouid hd held ove; the desired sam~ l container. e After drying, the cwstals can he released by gentle tapping. ~ e ~ e n d i nong the quality of the crystals, use of a spatula may be required for quantitative recovery. Contamination of the sample with fibers can he avoided by not scraping the cotton plug a t any time. Procedure Dissolve 50 mg of benzanilide in the necessary amount (about 5 ml) of hot carbon tetrachloride. Routine use of heating mantles and reflux condensers is recommended for such purposes. Assem. ble the filtration apparatus from a test tube (1fiO X I6 mmJ containinc ahout 0.6-1.U rnl uf carhon tetrachluride and the frlter s t ~ k ~~,
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equipped with a rubber stopper (see the fig., part a). Pack the enlarged end of the filter stick lightly with cotton. Heat the test tube in a bath of boiling water while placing the long stem of the filter stick in another test tube. After the distilling carbon tetrachloride has dripped from the cott6n plug for -15 s, insert the filter stick in the hot solution of benzanilide. Spontaneous filtration begins shortly and is completed within a few seconds. Before suction stops, quickly wash the walls of the test tube with a small volume of carbon tetrachloride, and finish emptying the test tuhe, which can be accomplished quantitatively by a "vacuum cleaner" effect. Disconnect the filter stick and set the solution aside to cool to room temperature. While waiting for crystallization to be complete, remove the cotton from the filter stick and clean it by washing with ethanol and acetone. Pack the open end of the long stem with a small portion of cotton and connect the filter stick to the suction test tube (see the fig., part c ) with its side arm attached to the vacuum line. Use this device to remove the mother liquor from the crystallized solution. Draw off the upper layer of solution first. By lowering the filter stick continue the suction until crvstals are collected in the filter stick. Wash the walls of the test tuhe with cold carbon tetrarhluride and collect the cryatnls as quantitatively as p~asible.Repeat the waqhing 2 or 3 rimes wrrh minute volumes of cold advent. Clamp the apparatus so that the stem with crystals extends into a small test tuhe and continue with suction for another 10-15 min. Tap the stem to shake crystals into the test tube and use a small soatula to comolete the transfer. Comolete the drvine " " of the sample by applying vacuum for another 15 min and take a melting point. The pure and dry benzanilide melts at 163%.
Volume 52, Number 10, October 1975 / 665
