The Use of Semimicro Technic in Elementary Organic Chemistry-II' NICHOLAS D. CHERONIS, PETER G. ARVAN, and HERMAN TEIFELD Chicago City Colleges, Chicago, Illinois
A PREVIOUS paper2 one of the authors described INseveral small pieces of apparatus and instructions for their use in semimicro work in the elementary organic chemistry laboratory courses. The progress in the develoument of s i m ~ l au~aratus e and ~roceduresfor semimicro work in the organic laboratory was reported in several communications' and published in book form.4 In the present paper a summary of the various types of apparatus is given together with a discussion of their application in teaching. The operations commonly performed in the organic laboratory involve distillations, crystallizations, filtrations, and separations of immiscible liquids. The semimicro equipment for distillations consists of distilling tubes or flasks having a capacity from 10 to 25 ml. The distilling tubes have been more successfully used in semimicro work than ord'mary distilling flasks of 10 to 25 ml. with straight side am^ because they are easier to clean and because they permit boiling with less danger of contaminating the distillate by a spray from the bumping liquid. The distilling tubes are made from ordinary six- and eight-inch Pyrex test tubes. The side arm is about 80 mm. in length and is curved slightly upward before it is bent downward for about a length of 10 mm., forming an end which can be connected to a delivery tube. A Claisen type of distilling tube, useful in distillations under reduced pressure, is made from an eight-inch F'y~extube. The fractionating column originally described gave rise to difficulties due to accumulation of liquid a t the top of the column, and therefore it has been modified. It has a length of 180 to 200 mm., and a diameter of 8 to 9 mm. a t the lower end and 20 to 22 mm. a t the upper end for the insertion of a cork holding the thermometer. A suiral of nichrome or iron wire about -130 mm. in length is placed inside the column to offer surface for condensation. The boiling vessel is an eightinch tube fitted with a cork which holds the column. The semimicro condenser is of the "cold finger" type. It consists of a glass tube about 150 mm. long and 8 to 10 mm. wide, sealed a t one end. Through the opening is inserted an inner tube 160 mm. in length and
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.A
4 mm. in width. The inner tube is held in position by means of a small piece of rubber tubing. The condenser is attached to the water lime by means of 5-mm. rubber tubing. Figures 1 and 2 show photographs of s e t u ~ for s simple and fractional semimicro distillations.
FIGURE1.-DISTILLATION BY MEANSOF EIGHT-INCH DISIILLINO TUBE
AN
For vacuum distillation an eight-inch tube is modified like a Claisen flask by sealing a side neck of about 18 mm. in diameter. The arm is sealed about 80 mm. from the mouth of the tube. The side arm has a length of about 140 mm. and is bent downward abont 85 mm. from the side neck. An eight-inch tube with a microcondenser serves as a receiver. The volume of -lianid -- ~ can be distilled by such an arrangement is 5 to 1 Presented before the Division of Chemical ~ & , ~ of ~ the t i ~which ~ American Chemical Society, 103rd meeting, Memphis, Tennes- 15 ml. Steam distillations are performed by using see, April 21, 1942. either a six-inch or an eight-inch setup. In the former, CHERONIS, J. C m x . Evuc., 16, 28 (1939). Qivivision of Chemical Education, A.c.s.. 10Znd meeting, as described in an earlier paper2,a 250-ml. Erlenmeyer Atlantic City, New Jersey, 1941; 103rd meeting, Memphis, flask is employed as a steam generator; with the eightTennessee, and 104th meeting, Buffalo, New York, 1942. 4 cHERoNIs, ,,Semimino and Mano Organic Chemistry,,, inch tube, a metal bath 90 mm. in diameter and 70 mm. Thomas Y.Crowell Company, New y a k , 1942. in depth is used. The top of the bath has four openings: 431 ~
~
.~---
two holes have a diameter of 20 mm.. one 26 mm., and the fourth 35 mm. The eight-inch tube which contains the liquid to be steam distilled is fitted into the large opening by means of a cork or rubber stopper and reaches almost the bottom of the tube. Into one of the small openings a glass tube is fitted as a steam inlet and through the other a long glass tube to serve as a water and pressure gage. The remaining opening of the metal bath is closed by a solid stopper. It has been found that students have less difficulties with the eightinch tube distillation setup than with the six-inch tube. Figures 3 and 4 show photographs of both setups. For semimicro crystallizations, beakers of 25-, 50-, loo-, 150-, and 250-ml. capacity have been found convenient. When the amount of substance to be crystallized is 100 to 200 mg. and the volume of solvent is small, test tubes may be used. Stirring rods of 4-mm. tubing are preferable to rods made of 6-mm. glass since in the use of the latter there is danger of tipping over the small beakers. Several types of filtration apparatus have been tried. The use of centrifuge tubes for two years did not prove entirely satisfactory with beginners. Similarly a filtering tube adapted after Wexlers proved quite useful with well-trained advanced students, but was not good for beginners. In this type of filter tube a mat of asbestos fiber or paper pulp is formed above a small layer of glass beads. It is necessary for each student to have two or three of these filter tubes and to clean them WEXLER. J. CHEM.EDUC., 18,167 (1941).
thoroughly after each use, an operation which the student a t first omits. The apparatus found most convenient for semimicro filtrations consists of an ordinary glass Bunsen funnel, a perforated porcelain disc, and an eight-inch tube with a side arm. For ordinary filtrations which do not necessitate suction the glass funnel is used directly, as, for example, in filtering hot solutions. The same funnel is converted to a micro Biichner funnel by fitting into the funnel a 20-mm. perforated porcelain disc with beveled edges. The filter paper used is slightly larger than the disc. The funnel is fitted into the opening of the eight-inch tube having a side arm and suction is applied as usual. The separatory tube previously described was slightly modified. Separatory tubes are made of both six- and eight-inch tubes. It was found advisable for each student to have stoppers fitted with the tubing and clamp so that they could be changed from one test tube to another. The long glass tubing is 4 mm. in diameter and 180 mm. in length, with one opening reduced to a fifth of the original size. For an eizht-inch tube a No. 4 rubber s t o ~ ~iseused. r and the airlnlet tubing is 230 mm. long. i.'hrough the
other hole a short piece (50 mm.) of glass tubing is inserted so that it reaches the surface of the stopper but does not protrude through it. Two pieces of rubber tubing 45 to 50 mm. long and 3 to 4 mm. in diameter are fitted to the two glass outlets. Extraction with ether is accomplished in any test tube. The solvent is added and the proper separatory stopper is inserted and held down by the thumb. The saew clamp is closed and the tube is shaken; while the stopper is held down the clamp is cautiously opened to release any pressure, closed, and the tube is inverted for separation. The separatory tube may be used as a dropping funnel. Figure 5 shows a photograph of the separatory tube. All the semimicro apparatus is commercially a ~ a i l a b l e . ~ USE OF SEMIMICRO TECHNICS IN TEACHING
I t should be stated a t the outset that most appraisals of teaching methods suffer from a disproportionate amount of opinion and too little experimental evidence. It is a common procedure for authors to claim that a particular selection of subject matter and arrangements gives superior results over traditional methods. The experimental evidence usually offeredfor such opinions is not of the same type or rigor as is offered for chemical behavior. To a certain extent this is inherent in all educational experimentation. The subjective factors in the results obtained cannot be removed even if the same teacher instructs two groups of students. Further, a teacher may obtain certain results with a teaching technic which cannot be duplicated by other teachers with similar groups of students. There are some factors which cannot be easily defined and duplicated, and the chief of these is the ability of certain teachers to impart enthusiasm into students. The same student may react psychologically to a particular teacher so as to exert an unusual amount of effort to master subject matter or technics, which he will not do in the same way with a different teacher. Therefore the question as to whether the semimicro can be used in place of the traditional macro technic in teaching elementary organic chemistry amounts to asking whether any teacher can use these methods and obtain the same results as with the traditional ones. This, of course, immediately involves a detailed statement as to what a teacher is aiming to accomplish by the laboratory work in organic chemistry. Since to the authors' knowledge there is no unanimity of opinion on the specific objectives of the laboratory course in organic chemistry, the approach followed in the appraisal of the semimicro technics was to try them first on small groups of students after they had been initially introduced to the macro methods for about five weeks. Later, a group of about 40 students was divided into two matched groups; one group used the semimicro technic exclusively while the second group used the macro in most of the experiments, and the semimicro in a few which could not be easily adapted to macro methods.
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Semimicro apparatus for organic chemistry (Wilkens-Anderson Co., Chicago.)
The laboratory course in which the experiment was tried is a one-semester course consisting of 96 actual hours of individual laboratory practice. Table 1 shows the content of the course; Table 2, the equipment issued to each group of students, and Table 3 shows the performance of each group. Table 3 requires considerable explanation of the methods employed in each experiment, particularly by the group using the semimicro technics. TABLE 1 CONTBNT OW LABOPATORY COURSE IN E L B U B ~ I R Y OEOINIC CBBIIISTRY (One Semester, 96 Hour.)
Pvoccdww 1. Purification by cry.tdliration. 2. Purification by di~tillation(simple and fractional distillation), 3.
Determination of melting points.
4. Identification of element.. T C IT~~ b Erpnimcnlr e
Prefiororiont
Reparation of methane prmaration of acetylene ~r0i.rti.s of hydr&.%rbms Pro~crtiesof monohalides properties of hydrory mmpound. properties of aminer Properties of aldehydes and ketones Fropecties of earborylicadda Properties of a w l chlorides and amides 10. Preparation of diaronlum ralts and their reactions 11. Carbohydrates 12. proteins
10. Cyelopentpnotpne 11. Benroic acid 12: Ethyl acetate -
identiiication of the elements commonly present in organic compounds. The mixtures commonly employed for crystallization with macro procedures are: (a) acetanilide, 90 g.; sand, 7 g.; benzoic acid or naphthalene, 3 g.; (b) benzoic add, 90 g.; sand, 7 g.; salicylic acid, 3 g. The student is given 5 to 10 g. of this mixture. Usually no difficulty is experienced in obtaining the pure compound after two cryst@lizations. If the semimicro technic is used and the student is given 0.5 to 1 g. of one of these mixtures, considerable difficulty is experienced in obtaining a pure product; at least, one-half of the students have about 0.2 g. left after the first crystallization. This is to be expected, since all the procedures are new, and it is expecting too much that a beginner start with 0.5 to 1 g. of an impure compound, carry out two or three crystallizations, and still have a sufficient amount to show for his labors. The difficulty can be remedied by adding only 0.1 to 0.2 per cent of benzoic acid or salicylic acid as an impurity. Under those conditions it is possible to obtain 0.2 to 0.6 g. of the crystallized pure compound in one crystallization.
* Wutz-Fittig method. t
optional.
Pr,ar.tio"r ( I 5 ,t"dentn) Number of students with more than 12 pmparationr
2 Grignard method. TABLE
7
2 1
1 2 I 1 1 2 1
1 1 1
4 12 1 2 1
2 4
1 I
1 1 1
6 1 3
Beakers (25 t o 400 ml.) Condenrm Cylinder, grad.. 10 ml. Chldum chloride tube Erlenmeyer flasks. 25 ml. Erlenmeyer flaak, 126 ml. Evaporating dish, 30 mi. Fractionating eo1vmn Funnels, 50 mm. Metal water bath Mortar and pestle Porcelain disc, 20 mm. Tjermometer, 360' Test tubes, 8 in. Teat tubes, 6 in. Tert tube, 8 in. with side arm Watch dasse. Burner Clamps. Hoffm~n,screw Clamps. buret Set cork borers Soatula. Monel blade Test tube rack Tert tube holder Pair goggles Feet rubber tubing, 5 mm. P w t rubber tubing. 3 mm. Feet rubber tubing, 8 mm.
1 7 1 1 1 3 1
2 1 1 1
1 2 1
2 1
1 1
1 1 3 12 8
1 1 1 1 2
3 1
1 1 9 I/,
Adapter Beaker. (150-1000 ml.) Condenser. 400 mm. Condenser tube, 500 mm. Cylinder, grad.. 100 ml. Distilling flasks. 50 t o 250 Calcium chloride tube Erlenmeyer flask.. 50 ml. Erlenmeyer nark. 125 mi. Erlenmever Bark. 260 mi. Evaporating dish, 30 ml. Filtering flask. 500 mi. Flasks, 200 m1. Flask, 500 ml. Funnels. 65 mm. Bilchner fuond. 100 mm. Mortar and nestle Pail Separatory funnel. 250 mm. Thermometer, 360' Test tuben, 8 in. Test tubes. 6 in. Tert tubes. 4 in. spstuia, porcelain Test tube holder T & tube rack Burner Clamps, buret clam.,. universal set &rk borers Pair goggle. Cork ring Feet rubber tubing, 6 mm. Foot rubber tubing. 5 mm.
d
As shown in Table 1, the introductory experiments include the purification of an impure compound by crystallization, practice in simple and fractional distillation, determination of the melting points of the crystallized substance as well as of an unknown, and the usual
Cydohexenc n-Butyl chloride Irr-Butyl chloride r-Butyl bromide Methyl iodide Aniline Ethyldimethyl carbinol Acetaldehyde Cyc1opentanone Bcnroie acid Ethyl acetate
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* SM tM
Semimiera technic. Macro technic.
In the experiment of purification by distillation, both groups used a mixture of equal parts of methanol and water. The group using the macro technic employed 100 ml. of the mixture for the fractional distillation, while the group using the semimicro employed 20 ml. of the mixture. These amounts were found necessary for beginners. Each student was required to plot the results obtained as shown in Figure 6, which is hased upon the data of one of the better students. The difficulties encountered were about the same in both groups. The time required for the semimicro fractionation is less than that required by the macro, varying from one-third to one-half. I t should be remarked that practically all students require help in order to plot the curves from the data obtained. Reference to Tables 1 and 2 shows that the test tube experiments are the same for both groups; therefore, comparison can be made only in the preparations.
I t will be noted that n-decane is used to illustrate the preparation of a paraffin hydrocarbon by the Wurtz method. The use of this type of preparation using the macro method is found in very few manuals.' The reason for the absence of many experiments of this type is the danger in handling 10 to 15 g. of sodium metal. The danger is minimized by employing the semimicro method, in which the amount of sodium utilized is 2.5 g. Further, fair yields are obtained in the preparation of n-hexane, n-octane, and n-decane. Table 4 lists the yields obtained using the semimicro method. It will be noted that the student determines the refractive index of the product. A Fishers refractometer has been found very useful for students. It is inexpensive and easy to operate, giving direct
readings to the second decimal, the third being estimated. The determination of refractive indices with this refractometer offers a convenient method in most cases for checking the purity of students' preparations. In some cases the impurity may have a refractive index very close to that of the product. This is the case in n-decane, for which nD = 1.4108, while n-amyl chloride nD = 1.4119. Therefore, the checking of refractive index is valid only if the student has refluxed'the crude decane with 0.5 g. of sodium, which reacts with any unchanged amyl chloride. The preparation of cyclohexene is preferred to the traditional preparation of ethylene as an illustration of formation of olefins by the dehydration of alcohols. Table 5 lists the student yields of cyclohexene from 5 ml. of cyclohexanol. It will be noted that the variation of yields is between 13 and 72 per cent, but that one-half of the students obtained above 50 per cent of theory. The yields obtained by the group working with the macro method were on the whole higher. Few yields were below 20 per cent, while the average was 59 per cent. This is true in practically all preparations; the yields by the macro methods are consistently higher. This is to be expected if the unavoidable losses due to handling are considered. If the yield according to theory is 25 g. and according to literature 20 g., and the student loses in addition 5 g. through inexperience in technic, he has lost 20 per cent through this variable. If in the semimicro method the yield according to theory is 5 g. and according to literature 4 g., and the student loses 2 g. through inexperience the loss is 40 per cent. The same trends are shown in Table 6, which lists the student yields in the semimicro preparation of n-butyl bromide using 5 g. of l-butanol. In the macro method the amounts used were five times the amounts used in the semimicro.
TABLE 4 SBYIYZCRO ~ B P A R A ~ OOP N DSC~RB (Student Yields from 10 g. of "-Amy1 Chloride) Sludml's
Yield i n
No..
Gram
Pn Can1 of Throryt
Rbfl.~lbb
Indrrt
* Nvmbers nrrnnged seeording to descending yield..
t Themy equal to 6.7 g. f Cheeked by instructor. IR) Reiection. -.
n-Deeane. no
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1.4108, 1.4120,
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ROBERTSON, "Laboratory Practice of Organic Chemistry." The Macmillan Company. New York. 1937, p. 243. "isher
Scientidc Company. Pittsburgh, Pennsylvania.
*Number. arranged aeeordiog to descending yields. t Theory eqval to 3.8 g. f Checked by instructor. Cydohuene, no 1.44921. (R) Rejstion.
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Methyl iodide was prepared from methanol, phosphorus, and iodine. It was performed as an optional
groups which show related properties, the preparation of one or two members from each group serves to illustrate reactions which are applicable to an entire class Sludcnl's Yield in Per Crnl Rcfrndivs of compounds. (b) To provide a systematic study No.* Grams of Theory t Indrxf of the general properties and reactions of the simpler groups of compounds. (6) To develop the simpler skills and technics necessary in the study of this particular field of experimental science. A number of other aims can be listed; for example, that tbe laboratory course will contribute to the teaching of the scientific method, aid in the development of critical thinking, enhance the student's enjoyment in learning "how things happen," etc. The latter aims, however, are regarded as by-products of any well-organized laboratory course in science. umbers arranged a m d i n g to dwcending yields. In appraising the semimicro technic according to the t yield amording to theory is 9.2 8. aims listed above, i t will become apparent that prinChecked by instructor. n-Butyl bromide. no 1.4398 (R) Rejection. ciples, facts about reactions, and methods of preparation can be illustrated as well if one prepares 1 to experiment by those members of the group using the 2 g. of an organic compound instead of 25,50, or 100 g. semimicro method, who mastered the technics rapidly Likewise the systematic study of the general or group and hence had a great deal of available time. Aniline reactions of organic compounds are as easily made was prepared from nitrobenzene. The Grignard re- with 0.5 g. as with 10 g. of substance. The developagent was used for the preparation of ethyldimethyl ment of skills aims, first, a t enabling the student to carbinol and benzoic acid. Both groups used the semi- understand the present work, and second, a t preparamicro technic. The details of all the preparations have tion for further study in the same field. There can be been p~blished.~ little doubt that the semimicro technic enables the In the preparation of ethyl acetate the students student to understand the first year's work in organic using the semimicro technic were shifted without pre- chemistry; however, there can be serious doubt as to vious notice to macro equipment. No instruction was whether the student who learns exclusively the semigiven as to the setups to be used, except what ap- micro technic will be able to set up the traditional dispeared in the manual. This limited trial was performed tillation equipment. Assuming that such a deficiency in order to ascertain if students who were trained in the will exist it cannot be considered as a serious matter semimicro methods would find the transition to macro for the student who will not follow chemistry, as, for equipment extremely difficult. The results varied example, the engineering or premedical student. from complete lack of ability to follow the directions Experience has shown that the student who is to conin the manual in setting up the macro distilling flask tinue in the study of organic chemistry beyond one and condenser to perfect performance. In general, year acquires quickly the use of macro equipment if the students who had good mastery of the semimicro he has mastered micro technics. technic asked no questions, while those who always The results obtained with the experimental groups had difficulties in the laboratory required help. It of students can be summarized as follows: (a) Good will be noted from Table 3 that a few students in the students do well with either macro or semimicro group using the semimicro technics were able to finish methods, and when given their choice usually prefer more than twelve preparations, while in the group the semimicro. (b) Poor students have greater initial working with macro methods no student was able to difficulties with semimicro than with the macro methfinish more than eight. Since the students were ods, and when given their choice they usually prefer matched, i t can be said that both groups had good the macro methods, since they obtain a few grams to students. This observation in no way places any show for their efforts. (6) Work with semimicro premium on the amount of work done. equipment demands greater care and a higher degree of precision than work with the usual macro equipAPPRAISAL OF SEMIMICRO TECHNIC ment. Although this may be considered an advanAs previously noted, an appraisal of the advantages tage, it may tend to discourage certain types of students and disadvantages of the semimicro technic for indi- who have tendencies to be careless and clumsy while vidual laboratory practice in elementary organic chem- handling apparatus. In the authors' opinion the istry can be discussed only in the light of the objectives teaching of mediocre and poor students is a t least as which one seeks to attain. The aims of the laboratory important a task as the teaching of the superior stuwork described are: (a) To acquaint the student with dents. (d) In most cases the time required for a prepathe methods used for the preparation of representative ration by the semimicro method is about one-half or members of the most important groups of organic com- less than the time required by the macro method. (e) pounds. Since the carbon compounds are arranged into The accident hazard is reduced greatly when the semiTABLE 6
Sswmw~cmm s ~ ~ ~ ~ on r rn-BU~YI. on Bnoraroe (Student Yields of 5 G. I-Butand)
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micro technic is used, since the amount of reagent and size of equipment have been diminished 80 to 90 per cent. (f) The use of semimicro technics permits greater variation in the laboratory. Many experiments, such as the use of the Grignard reagent for the preparation of carbinols or carboxylic acids, may be done by beginners. Also the technics permit a wide selection of experiments both for the exceptional and the average student. (g) Finally, the cost per student, for initial investment, for materials, and for breakage is smaller. The above conclusions indicate that a number of advantages may be reasonably claimed for the use of the semimicro technic in the teaching of experimental organic chemistry. A number of these advantages are very obvious in view of the wide adoption that the semimicro technic has had recently for the teaching of qualitative analysis. Notwithstanding these advantages there are two factors which may somewhat retard adoption of semimicro technics for teaching organic chemistry. The first of these factors is that poor students need greater care by the teacher, since they will encounter somewhat more difficulty than by the use of macro methods. If the expected yield is 2.5 g. and the student loses 2.0 g. by carelessness, he has very little to purify and show as evidence of his efforts. On the other hand, if the expected yield is 25 g. and the student loses 10 g. in handling, he still has an adequate amount of substance to purify. This is particularly true of liquids. This difficulty is not insurmountable. It requires greater effort on the part of the teacher to help the student and also to point out that other students working with the same amounts and directions obtain good results and thus to urge selfimprovement. It has been found that if the student is excused from part of the work and is asked to repeat the experiment until he himself is satisfied, the technics are mastered in most cases. The second obstacle to an immediate wide adoption of the semimicro technic is more formidable because
it is psychological. A number of teachers of organic chemistry feel an aversion to experiments involving test tubes. This, perhaps, can be overcome by "dressing up" the apparatus, but i t will, of course, increase the cost. Without entering into a discussion as to the causes of this feeling, it will be better to accept it on the basis of "de gustibus non disputandum." There is also a feeling among a good many students that it is more gratifying to prepare 25 g. of a substance. than 1 to 2 g. One can hardly fail to sympathize with such a feeling when the whole social surrounding is bent on the edification and acquisition of "quantities" whether these may be buildings or pieces of gold and silver. In the authors' opinion there is a definite trend toward displacement of the macro by the semimicro methods. These trends have been accelerated recently by forces from two directions. The first is the force that current research and indnstrial practice have on teaching methods. A large part of modern research in organic chemistry, particularly in vitamins, hormones, and other compounds not available in large quantities, is accomplished by using almost exclusively micro apparatus and micro quantities of substances. The second force is the impact of war. The demand for economy and conservation of materials, the scarcity of certain chemicals, priorities, and the desire of every teacher to do everything possible for the war effort have brought about a critical examination of accepted practices. The use of 2000 g. of tin by a class of 30 students a few years ago, in order to prepare aniline from nitrobenzene, did not seem a horrible waste to many people. Today all teachers agree that such a practice deserves a more drastic characterization than "wasteful." As a consequence, there is great receptivity a t this time to experimentation with new methods to displace the traditional and accepted. The authors wish to acknowledge the help of the following students who aided with parts of this work: Alford Anderson, Carl Anderson, and Richard Erhardt.
