A NEW SEMIMICRO TECHNIQUE IN THE TEACHING OF PREPARATIVE CHEMISTRY JOSEPH M. CONNOLLY and GRAHAM OLDHAM Loughborough College, Loughborough, Leicestershire, England T H E R E are a number of disadvantages associated with the teaching of preparative chemistry on the normal macro scale, particularly where liquid products are being prepared. These may be summarized as folloa7s: (1) A considerahle time is required for setting up the PYREX INSERT apparatus, carrying out the experiment, and purifying the product. (2) The apDISTILLATION TUBES paratus is frequently bulky 22.2Y". ',I, and is also expensive if \ 51: " D 7:OW FINGER" ground joints have to be em- PYREX SIMPLE OR VACl W N I>ENSER 1 5 X I S O Y r . ployed. (3) The cost of Figure 1. component Put. of Appuatus chemicals is relatively high. (4) The use of poisonous or noxious gases and their disposal in quantity often distilled with little loss and, as the distillate and vapor do not come into contact with either the stopper or the presents difficulties. In order to overcome these difficulties it is ob- rubber sleeve on the lower side arm, the apparatus is viously desirable to carry out the experiment on a equivalent to one of all-glass construction. Further, small scale. If, for this purpose, apparatus of conven- since the distillate is permanently jacketed with vapor, tional design is used a number of defects become ap- little or no oxidation can occur and no drying tube is parent. In any process employing distillation the losses necessarv for the dist,illation of liauids attacked by inherent in the method are, firstly, the amount of liquid water vapor. A modified distillation tube (F), fitted with a small required to wet the surface of the condenser and collecting vessel, and secondly the amount of liquid which is, column filled with glass helixes or other suitable packa t any instant, condensing on the upper surface of the ing, is used for ordinary or vacuum fractionation. A distilling flask. Where small amounts are being used cup (C), provided with a small orifice, enables either exthese losses may constitute quite a high percentage of tractions or steam distillation to be carried out. Below are given a number of examples of the use of the yield. Further, if the liquid being distilled is corrosive towards cork or rubber, an all-glass apparatus the apparatus in its various forms for some standard purifications, preparations, and identifications. must be used. These disadvantages have largely been overcome in PREPARATIONS apparatus designed by the authors and described else- Direct Distillation where.' Briefly, the apparatus consists of a "cold Acetyl Chl~ride.~Acetic acid (1 ml.) and a slight exfinger" type of condenser ( A ) to the bottom of which is cess of benzoyl chloride are placed in the distillation attached by means of glass hooks a small collecting cup tube (Figure 2) and warmed gently. The acetyl chlo(B). The whole is fitted into a distillation tube (E) pro- ride distils and collects in the cup. A cork with a hole vided with two side arms as shown in Figure 1. The lower side armmay be used to hold a thermometer, a de- of suitable diameter bored in it forms a convenient means livery tube for the introduction of a gaseous reagent, or of removing and holding the cup. Chrornyl Chloride. A finely ground mixture of potasan air leak tube when vacuum distillation is being carsium chromate (2 g.) and potassium chloride (1.5 g.), ried out. The upper side arm is used for the application together with concentrated sulfuric acid (2. ml), are of vacuum or as an exit for gaseous by-products. placed in the distillation tube and gently heated. In this apparatus small quantities of liquid can be ~ h r o m ychloride l distils and is collected in the cup.
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BROFN,H. C., J . Am. Chem. Sac., 60,1325(1938).
mixture the ,acetone is continuously recycled over For this experiment the normal cup B is replaced by the perforated cup C which is half filled with solid barium hydroxide. Acetone (3 ml.) is placed in the distillation tube and nentlv boiled. the vaDor condensing on the cold finger a n i dripping throigh the cup. The boiling is continued for 20 hours when the cup is replaced by one of the normal design. The thermometer is now fitted and the fraction boiling between 160' and 165'C. collected.
. baryta.
PURIFICATIONS
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2.
Assembly for Simple Distillation
Dry Distillation
Acetone. Calcium acetate (4 g.) is dried in a small evaporating dish and then intimately mixed with copper powder (2 g.). The mixture is transferred to the distillation tube and heated until the acetone distils. The product is purified in the normal way. Reaction with a Gaseoua Reagent
Sulfur Monochloride. The apparatus is set up as shown in Figure 3. Sulfur (4 g.) is placed in the distillation tuhe and heated in an oil bath at 130°C. until molten. Heating is interrupted and chlorine (prepared by dropping concentrated hydrochloric acid on to solid potassium permanganate) is passed through a d e livery tube inserted in the lower side arm. Excess chlorine is absorbed in sodium hydroxide solution as shown. After five minutes the passage of chlorine is stopped and the excess chlorine blown out. The product is distilled, the sulfur monochloride collecting in the cup as a pale yellow liquid. Formic A d . The normal preparation of formic acid from glycerol and oxalic acid results in the formation of a dilute solution which is then converted into lead formate. This is carried out on a small scale using conventional apparatus so that a yield of approximately 5 g. is obtained. This is transferred to the distillation tuhe and heated. Hydrogen sulfide, washed through wat,er and dried over calcium chloride, is passed in and the formic acid collected in the cup. To remove hydrogen sulfide the formic acid is redistilled over a little lead formate. Catalysis with Immediate Removal of Pmduct
Diacetone Alcohol. The reactionis an aldol condensation carried out with acetone, using baryta as a catalyst. The reaction is reversible and as there is only a small percentage of diacetone alcohol in the equilibrium
In addition to the more obvious purifications involving straight distillation the apparatus may also be used in the last stage of a small-scale preparation involving vacuum distillation. In this case the upper side arm is connected to a vacuum pump via a manometer and the lower side arm is fitted with an air-leak tube. The apparatus could therefore be used, for example, in the purification of phenol or phenyl hydrazine. If a substance is to he isolated by steam distillation, the modified cup C is used. This has a small orifice in the bottom and is provided with glass hooks which are longer than those on the normal cup B. A small disc of filter paper, 3.5 cm. in diameter, is folded as shown in Figure 4 so that a cone angle of approximately 40' is ohtained. The paper is wetted and inserted into the cup so that a close fit around the upper edge is obtained. Approximately 1 ml. of the liquid to be steam-distilled is placed in a 25 X 150-mm. test tube, together with an equal volume of water. A few tiny pieces of porcelain are added and the cold finger attached. The liquid is boiled gently, the distillate collecting in the filter paper. The excess water drips back into the distillation tube, the amount of water admixed with the distillate thus being a minimum. The ratio of water to substance must be kept low for compounds whose solubility is appreciable, e. g., aniline. If the substance is a solid a portion of it may collect on the condenser whence it may be removed. By removal of the cup the apparatus may be used for
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D is used. This is a reasonably close fitinside the distillation tube and has a small glass hook attached to facilitate its withdrawal. The cup C may be replaced by one of the normal type if the solvent is to be removed by distillation. IDENTIFICATIONS
OPEN CONE HERE
purifications involving sublimation or vacuum sublimation. EXTRACTIONS
For the extraction of solids the modified cup C is fitted with a filter-paper cone or small Soxhlet-type thimble and the extraction carried out in the normal way. If it is necessary to conserve the extract for crystallization or evaporation and weighing, a small insert
It will be seen that the apparatus is also of value in the identification of organic compounds and it has been found useful in many cases where conventional apparatus has not proved very satisfactory. The method previously given for the preparation of acetyl chloride may, for example, be used in the identification of acids yielding volatile acid chlorides. Aralkyl ethers may be identified in the following manner; The ether (2 ml.), together with hydriodic acid s.g. 1.7, (6 ml.), is placed in the distillation tube and the cold finger condenser without the cup is inserted. The mixture is boiled gently on an oil bath for 45 minutes and then cooled to room temperature. The cold finger is removed, cleaned, the cup attached, and the whole replaced. The alkyl iodide is very slowly distilled off, collected in the cup, and subsequently identified by normal methods. Other uses of the apparatus in qualitative organic analysis will readily present themselves to the reader. CONCLUSION
It will be seen that the apparatus described may be used to carry out a wide range of operations on the semimicro scale. It is inexpensive, easy to make, and is therefore quite suitable for individual student use. If the apparatus is being used for demonstration purposes, a large lens of the television type will enable the class to view the experiment with ease.
