The USE of SEMI-MICRO TECHNIC in ELEMENTARY ORGANIC CHEMISTRY NICHOLAS D. CHERONIS Chicago City Colleges, Chicago. Illinois
T
HE trend in laboratory practice for the teaching of chemistry in the past few years has been toward reduction in size of apparatus and in amounts of materials used by the student. This is commonly called the semimicro method. The application of this method to qualitative analysis is already quite general in this
ment is introduced. This is found to be of definite advantage, as i t speeds up the work of the student tremendously. The preparation of amounts of from 25-150 grams often requires setting up of apparatus which takes up the major part of the laboratory period. This is necessary in some preparations to acquaint the student with methods and technics used in the organic laboratory. A good number of the experiments, however, can be performed with small amounts without sacrificing an; of the objectives of the macro method; as a matter of fact, i t is a reasonable claim that the addition of the semimicro technic is desirable aside from the fact that i t enables the student to do more work in the time available. Several small pieces of apparatus used in this course are described below, and detailed instructions as to their set-up and use are given for work commonly met with in. the beginning courses of organic chemistry. Also, some typical preparations are described, as performed by means of this small scale equipment. I.
MICRO-CONDENSER
A piece of glass tubing, 20 cm. long and 4 or 5 mm. in diameter, is bent according to the diagram shown in Figure 1. The lower end is fitted loosely into a 10-mm. country. In the development of an introductory course hole of a cork that fits a Pyrex tube 18 X 150 mm. so in organic chemistry, devised to show both general that it can be raised up and down as desired. Each group properties and differences in reactivity,' i t be- end of the opening can be filed to make the U-tube fit came necessary to adapt about half of the work to well. A thin slice of cork a t point A insures against small quantities. It was found that the solution of the undue tension. Each end a t BB is fitted with 80-90 problem in obtaining semimicro apparatus for work in cm. piece of rubber tubing (diameter, 5 mm.). If the elementary organic chemistry did not lie in diminishing fitting is loose, then a short piece (5 mm.) of 3-mm. the size of the condenser or of the Biichner funnel; rubber hose is inserted a t each end of the U-tube. One but in the adaptation of improvised and modified com- end of the hose is connected with the water outlet and mon small pieces of apparatus. A good part of the the other is placed in the sink or trough. equipment can be made and assembled by the student, A better type of micro-condenser is shown in Figure or can be bought a t considerablyless expense than small 2. It is an adaptation of the "cold finger" condenser condensers, distilling flasks, or funnels. type. The jacket is 150 mm. long and 7 mm. in diameIn this course, the student is first introduced through ter, with the side arm 25 mm. from the top. The the usual macro apparatus to methods of purification inner tube is 170 mm. long X 5 mm. in diameter with of organic compounds and the determination of con- a bend a t the top. It fits in the jacket through a stants. Near the middle of the course, the microequip- short piece of 3-mm. rubber tubing. This condenser is N, D,, ,,Reactivity experiments CasRoms, an introduc. more adaptable and less easily broken. It can be bought commercially. tory course in organic chemistry,"J. CAEM.EDUC., 14,480 (1937). FIGURE 1
FIGURE 2
28
11.
DISTILLATION TUBES
Unless the liquid to be distilled is over 10 cc., a distilling tube is used. Figures 3, 4, 5, and 6 show the various types of tubes. Figure 3 is an ordinary 18 X
usually gives better results. For substances which attack the nichrome wire, glass wool, loosely packed (strip), is to be used. N. ARRANGEMENTS FOR REFLUXING AND DISTILLATION
Two set-ups for heating under r d u x are shown in Figures 9 and 10. 1. Heeting under Rejlu3c.-Two very small porcelain chips or boiling stones are placed in a Pyrex tube, 18
150-mm. Pyrex test-tube, while the tube in Figure 4 has the same diameter, hut is 115 mm. long. It is made from the 150-mm. tube by cutting and then firepolishing the edge. The distillation tube shown in Figure 5 has a side arm with an inner seal to prevent
X 150 mm.; then the mixture to be refluxed is poured in. The cork bearing the condenser is fitted to this tube, and the height is so adjusted that the end of the condenser is above the boiling liquid. To avoid a closed system a small opening is bored or cut in the cork. The
liquid discoloring from the cork or rubber going into the receiver. Figure 6 is an ordinary 150-mm. tube with a side arm. If the amount of liquid to be distilled is above 10 cc., then a small distilling flask is used. However, a larger tube can be used, thus eliminating the more expensive apparatus.
m. FRACTIONATING COLUMNS Two types of fractionating columns are shown in Figures 7 and 8. The one shown in Figure 7 is an adaptation of the Vigreau type, while the column shown in Figure 8 contains two spirals (one inside the other) made of nichrome wire. The sdrals are made by winding the wire aroufid a glass rod: By using 3 and 6-mm. rods, two spirals are made and the smaller (3 mm.) is inserted into the larger. The wires a t each end are twisted together. This type of fractionating column
Row of water is adjusted. If the U-condenser is used, a piece of cotton is loosely placed a t the opening of the cork. 2. Distillation.-The sim~lestset-UDfor distillation is shown in Figure 11. The thermometer is not shown
30
JOURNAL OF
although i t be can inserted by using a two-hole stopper. It is preferable, however, to use the set-up shown in Figure 12 if a thermometer is used. The "cold finger" type of condenser shown in the set-up in Figure 11 is also more advantageous, as it permits no outside mois-
-
CHEMICAL EDUCATION
distillate without increasing the volume of the mix. ture. The water in the Erlenmeyer flask is heated, leaving the steam inlet unconnected. When the steam is g a erated, the burner is removed, the steam inlet connected, and the burner adjusted so that the contents of the distillation tube do not splash up to the outlet. V. EXTRACTION AND SEPARATION Extraction of aqueous solutions or dispersions with ether or other organic solvents is a common operation
t
t-
+
FIGURE12
FIGURE13
ture to condense inside the receiver. This condenser, however, should fit loosely so as not to have a closed system. The tube with the liquid to be distilled is fitted either with a cork or rubber stopper (1-hole) provided with a tube, A, which conducts the vapors upward. This is connected by a small piece of rubber tubing to the delivery tube, B, which reaches to the bottom of the receiver, alongside the condenser. As the liquid distills, the delivery tube is raised. A U-condenser may be used. In Figure 12, the side arm permits the insertion of a thermometer. A distillation set using the miaofractionating column is shown in Figure 13. 3. Steam Distillation.-The apparatus shown in Figure 14 is a set-up for steam distillation. The tube containing the liquid to he steam-distilled is inserted through the larger opening of the cork or rubber stopper and is partially immersed in the boiling water. The steam generator is a 250-cc. Erlenmeyer flask with a wide mouth. The cork has two openings, one for the long tube acting as a water gauge, and the other for the steam outlet. Through this latter opening a small glass tube reaches nearly to the top of the rubber stopper of the distillation tube, and is slightly bent a t an angle connecting through rubber hose (3 mm.) with the steam inlet glass tuhe. The length of this tuhe is such that i t reaches almost to the bottom of the distillation test-tube. Through the second opening of the rubber stopper. is a small tuhe connecting with the receiver. This arrangement makes it possible to steam-distil P cc. of liquid within the distillation tube, obtaining 10 cc. of
- __ _
-
> _ - .
FIGURE 14
in work with organic compounds. For all extractions of small amounts, the arrangement described below has been found satisfactory. The extraction is accomplished in the ordinary Pyrex test-tube (18 X 150 mm.). After addition of ether or other solvent, the tube is corked, shaken, and cooled in running water and then the cork is removed and the separatory rubber stopper is inserted as follows. The latter is made with a twohole rubber stopper. Cut a piece of 4-mm. glass tubing 22 cm. long and rotate each end over the flame until the opening is reduced to a fifth of the original size. Insert the tube through one of the holes so that when the rubber stopper is fitted tightly on the mouth of the tube, the end of the glass tubing just fails to touch the bottom. At the other end, place a 30-mm. piece of 3-mm. rubber tubing. Through the other hole of the rubber stopper insert a piece of glass tubing 70 mm. long, so that it
The rapid filtration of small amounts of crystals presents certain difficulties. The use of diminutive Biichner funnels and filtration flasks is expensive and clumsy. An arrangement using a filter stick2is shown in Figure 15. The filter stick is made of 5-mm. rubber tubing, one end of which is enlarged to permit the insertion of the paper filter mass. The material to be filtered is placed in a small beaker so as to permit the removal of the crystals after filtration. The tube is connected through a short piece of thick wall rubber tub'mg to a 250-cc. filter flask which is connected to the water pump. A small amount of filter mass is pressed between the fingers and placed in the lower end of the filter stick and then inserted in the beaker. As the filtration pro-
ceeds the beaker is tilted and then the crystals are pressed down with the filter stick so as to remove the last drops of mother liquor. A small amount of solvent is added for washing and the operation repeated. Finally, the crystals are removed by means of a long, thin spatula to a "drying disc" (see below) for air drying or to a watch glass for drying in the desiccator or over the water bath. 1. The Catrifuge.-The centrifuge is a means of easy and rapid filtration. It has found wide application in the semimicro technic for qualitative analysis. The difficulty, however, is that most organic crystals have low densities and are not easily thrown to the bottom of the tube. The Skau3 tuhes are too expensive for use by the students, therefore, the tuhe shown in Figure 16 was developed. This centrifuge tube is adaptable to the filtration of small amounts of crystals (200-500 mg.). The sizes may vary, the one shown in the diagram is 120 mm. long by 14 mm. in diameter, constructed to 2 mm. in the middle to permit the insertion of the filter mass plug. The end of the tuhe is shaped for the insertion of a rubber stopper. A small plug of paper filter mass is tapered and placed in the constricted opening a t the middle of the tube. A few drops of water are added and the mass is pressed with the flat end of a glass rod to spread i t on the opening. The tuhe is then centrifuged for fifteen seconds, when i t is ready for use. The mixture of crystals and solvent to be filtered is added to one tuhe, while an equal amount of solvent is added to a second centrifu~etube with no filter mass. The tubes are then centrifuged from one to two minutes. The stopper a t the lower opening is removed and the filtrate drained to a test-tube for future reference. A few drops of solvent are added and the tube centrifuged for one minute. This is repeated. For rapid drying, the washings are drained and the tubes are again centrifuged for from two to three minutes, thus removing the adhering solvent and traces of mother liquor. In this manner it is possible to dry the crystals in air within a few minutes. 2. Drying.-The removal of adhering solvent and traces of mother liquor by centrifuging not only makes drying easier, but also gives higher purity. Often a second crystallization is necessary due to the adhering mother liquor which contains impurities. The crystals are transferred by means of a long spatula (made of a flattened glass rod or better with a long and narrow stainless steel spatula) on a drying disc. 3. Water Bath.-A water bath designed for this type of work is shown in Figure 17. It is made of copper. It is 85 mm. in diameter and 65 mm. deep. The three small openings, 25 mm. in diameter, are for the tubes, while the larger, 45 mm., is for a watch glass or small casserole, and so forth. The same bath may be made of an ordinary empty tomato can. It is possible to r d u x three tubes by connecting the condenser in series as shown for the two in the diagram.
a MORTON, A. A,, "Laboratory technique in organic chemistry," McGraw-Hill Book Company, New York City, 1938.
Spnu,E. L.AND L. F. ROWE,Ind. Eng. Chem., Anal. Ed.. 3, 147 (1931); MORTON, A. A,, op. czt.
just reaches the other side of the hole but does not protrude through it. Provide the end of the short piece with a piece of glass tubing 35 mm. long. A piece of glass tubing 30 mm. long is inserted a t the end. A screw clamp is placed a t the middle of the two rubber tubes. In order to separate the two layers, the screw clamp is closed tightly and the tube is inverted and held so that the rubber stopper rests between two fingers of the left hand with the thumb pressing lightly downward upon the lip of the tube. The delivery tube is placed inside another tuhe, and the screw clamp is operated
-
with the right hand. When the interface of the two layers reaches near the rubber stopper, the flow is diminished. The junction of the two layers is visible in the glass tube, just above the screw clamp. With a little practice it has been found that students are able to extract with ether with very small losses. The use of small separatory funnels, of course, is superior, but it is more expensive. Further, the use of the separatory tube permits extraction directly from the reaction vessel, thus minimizing losses in transferring. VI.
FlLTRATION
trated sulfuric and nitric acids. Place in a water bath The amounts used in the semimicro method seldom (Figure 17) and heat a t 60' for thirty minutes. Cool, exceed 2 to 4 g. for solids and 2 to 5 cc. for liquids. then insert the separatory stopper (Fignre 18) and draw It is found more advantageous to weigh the liquids off the bottom acid layer. A few drops of this remains wherever possible with the exception of water and com- in the test-tube. Add a mixture of 1cc. saturated solumon solutions which can be measnredwith a pipet 1cc. in tion of sodium chloride and 2 cc. of ten per cent. solution capacity and graduated to 0.1 cc. For weighing liquids of sodium carbonate. Rotate the tube and then separate the tnbe is attached by a fine copper wire to one end the lower layer. Now add 2 cc. of ether and pow off the nitrobenzene of a horn pan balance and after the tare is ascertained mixture into a dry test-tube. Add 0.2 g. calcium chloride (8-mesh) and allow to stand for thirty minutes. Transfer to distillation tube (provided with a very small porcelain chip), and arrange as shown in Fignre 12. Distil off the ether very slowly through the water bath. The latter is then removed and heat is applied directly with a microburner. When the thermometer begins to climb rapidly (at about 100°), the receiver is changed by inserting a dry VII.
WEIGHING AND MEASURING
the liquid is added through a dropper. A set of rough quantitative weights is used. The student can construct and graduate his own pipet and droppers. The latter can be made a little longer than the usual medicine droppers. The upper end is thickened so as to permit tight fitting with the rubber bulb. Two rubber bulbs may be used for several droppers by removing them each time without permitting any of the liquid to come into contact with the lower part of the bulb. The weighing of small amounts of solids is performed on pieces of paper placed on each pan of the horn balance. TYF'ICAL EXPERIMENTS ILLUSTRATING THE USE OF -SEMIMICRO APPARATUS
A . Prefiaration of Nitrobenzene.-Place in test-tube, arranged as shown in Figure 10,2 g. of benzene and add cautiously 4 cc. of a mixture of equal parts of concen-
tared test-tube to the cork holding the delivery tnbe and condenser. The temperature rises to 210°C. Toward the end the flame is moved up and down the distilling tube to insure complete removal of the condensing vapors, and also to avoid charring. Yield: 1.43 g. or forty-five per cent. of theory. The yields recorded in the laboratory manuals, using the macro method, are seventy to seventy-five per cent. B. Reduction of Nitrobenzene to Aniline.-The amount of nitrobenzene (1.43 g.) obtained in Experiment 1 is reduced with iron and dilute hydrochloric acid. To the test-tube containing the nitrobenzene add 2 cc. of 6 N hydrochloric acid and 1.5 cc. of water. Begin heating with a very small flame and add over a period of five minutes 2.2 g. of fine iron filings. (Note: the iron used for the reduction should be washed with ether or other solvent to remove the adhering grease.) The burner is removed before each addition. The boiling should be brisk, but the frothing should not be allowed to reach much above the middle of the tube, as there is danger of breaking the condenser. Continue boiling for thirty to thirty-five minutes until practically all the odor of nitrobenzene has disappeared.
Place the tube for steam distillation as shown in Figure 14, and add 0.5 g. of sodium carbonate. Connection should be open until the generation of steam is vigorous. Connect inlet of steam and distil 5 cc. of the liquid. Extract distillate with 4 cc. of ether, separate, and add to the ether four pellets of sodium hydroxide. Cool the tube in tap water as the solution of the alkali generates heat. Now pour the ether into a dry tube containing four pellets of sodium hydroxide. Cork, shake from time to time for one hour. Transfer to a distilling tube and remove fcst the ether, then distil the aniline with a free flame. Yield: 0.7 g. C. Acetylation of Aniline.-To the test-tube containing the aniline obtained in Experiment 2, add twice the theoretical amount of acetic anhydride which is about twice the amount of aniline, in this case 1.4 g. Insert a reflux condenser and boil for five minutes. Add 10 cc. of water, cork, and shake. Then cautiously, drop by drop, add 6 N sodium hydroxide until the precipitate is voluminous. Filter through the centrifuge, washing twice with water. Transfer a small amount into a filter disc and determine the melting point. If the crystals are impure, recrystallize once from water. Yield: about 0.5 g. D. Preparation of m-Dinitrobenzene.-Suspend a test-tube by a wire in one end of the horn balance and by means of a dropper add 2 g. of nitrobenzene. Place the tube in the water bath and add cautiously 5 cc. of a mixture of equal parts of fuming nitric acid and concentrated sulfuric acid. Insert reflux condenser and heat a t 90' for thirty minutes. Pour contents of tube into a 150-cc. beaker containing 50 cc. of water. Stir, allow to settle, and decant most of theliquid. Transfer to the centrifuge filter and rotate for two minutes. The crystals are washed with 3-cc. portions of water twice, then pressed and centrifuged again. The crystals are then placed on a drying disc. E. Preparation of Cyc1ohexene.-Arrange the distillation tube and condenser as shown in Figure 12, placing the thermometer, however, so that i t nearly touches the bottom of the tube. Place 5 cc. of cyclohexanol in the distilling tube and 0.5 cc. of concentrated sulfuric acid. A small boiling stone is added and the tube is heated with a microburner (or very small flame), a t such rate that the distillation proceeds slowly and the temperature does not rise above 130°C. When considerable decomposition begins the distillation is discontinued. To the tube containing the distillate add 1 g. of salt, rotate the tube, and then separate the aqueous layer by the arrangement shown in Figure 15. The cyclohexeue and traces of salt are retained in the original tube. Calcium chloride (8-mesh) is added, a lump a t a time. If there is considerable water in the tube a saturated solution forms a t first; the addition of one or two lumps after this point is sufficientto dry the olefin. After drymg the liquid is fractionated and the fraction which boils a t 80-82'C. is collected. The yield varies 1.4 to 1.8 g., or thirty-six to forty-seven per cent. of the theory. F. The Introduction of Carboxyl Group by Means of
the Grignard Reagent. Preparation of BenzoiG Acid.Arrange set-up as shown in Figure 10; a small hole is made in the cork which holds the condenser to permit the insertion of small calcium chloride tube. Weigh in a clean, dry 25 X 200-mm. test-tube 3.75 g. of bromobenzene; add 10 cc. of dry ether and connect for reflux as shown in the diagram. Now add 0.54 g. of magnesium turning and a crystal of iodine. If the reaction which soon ensues becomes violent, cool the tube by raising a beaker partially iilled with cold water. The reaction is over in forty-five minutes to an hour when only a small amount of magnesium remains. Remove the condenser and insert a delivery tube flared a t the lower end; the delivery tube reaches about 5-6 mm. within the ether layer. Connect the delivery tube to a carbon dioxide tank or generating flask. In either case the carbon dioxide should be dried by passing i t first through 2 U-tubes or towers iilled with calcium chloride. Place the tube containing the Grignard reagent in a beaker containing ice mixture and pass carbon dioxide for fifteen minutes. The cork is removed and a mixture of 2 g. of ice and 3 cc. of concentrated hydrochloric acid is added slowly. When the decomposition of the addition compound is complete insert the separatory stopper and remove the aqueous layer. Pour the ether layer into another test-tube which contains 5 cc. of 6 N sodium hydroxide and shake well. Separate the aqueous layer which contains the sodium benzoate and acidify cautiously with concentrated hydrochloric acid. The benzoic acid separates out immediately. Filter by centrifuging and wash twice with a few drops of water. Dry on a paper disc. Yield 0.45-0.60 g. melting a t 119-120°C. G. The Wurtz-Fittig Reaction Preparation of Ethyl Benzene.-Weigh in a dry 18 X 150-mm. test-tube 1.50 g. of ethyl bromide and 2.00 g. of bromobenzene and arrange as shown in set-up in Figure 10. Add 6 cc. of dry ether and 0.9 g. of sodium metal cut in very small pieces. The mixture is allowed to stand overnight, and then transferred into a distilling tube after a small plug of glass wool has first been placed in the mouth of the reaction tube to hold back the sodium bromide and unchanged sodium. The residue is washed twice with 2-cc. portions of ether and the washings added to the main portion. The residue is placed in the hood and the unchanged sodium decomposed by the addition of 5 cc. of denatured alcohol. The ether is distilled by heating the tube in a water bath; the bath is then removed and heating is continued through a small burner; when the temperature reaches about 100°C. the receiving tube is changed and the fraction boiling 130-134' is collected. Yield 0.32-0.37 g. (24-27 per cent. of theory). If i t is desired to perform two fractionations, twice the above quantities are used. The experiments which have been described illustrate the wide adaptability of the semimicro technic to the teaching of organic chemistry. Aside from the advantage of speeding up the work of the student and training in handling small amounts of substances, it becomes possible to illustrate through laboratory prac-
tie a number of principles which are diicult to show with the masro method. For instance, i t is not possible to provide every student with the apparatus required for an experiment involving the use of the Grignard reagent while the equipment described above with slight modifications can be applied to a large number
of experiments which require complicated apparatus. The semimicro technic can be used to advantage in the laboratory work of qualitative organic analysis. The author wishes to acknowledge the helpful suggestions of Mr. Walter Burfisher in the simplification and construction of some of the apparatus described.
