THE USE OF UNKNOWNS IN TEACHING ORGANIC CHEMISTRY

LEONE OYSTER, RIPON COLLEGE RIPON. WISCONSIN

A method for insreasing the inkrest and oalue of the preliminary laboralory work i n organic chemistry by the use of unknowns to be identified by their melting points, or boiling points, i s sugges&d. Unknowns are suggested for the pualitatiue avalysis of organic compounds, and an unknown alcohol may be identified by making the methyl ester of 3,5dinitrobenzoic acid. Compounds suitable for u n k n m s are giuen.

The usual laboratory work for the student beginning organic chemistry seems designed to kill all interest in the course before it is well started. In the first few weeks the emphasis is decidedly on technic, and it will be an exceptional student, indeed, who can get much thrill out of the calibration of a thermometer, the determination of boiling points, melting points, and the qualitative tests for carbon, hydrogen, nitrogen, halogens, sulfur, and phosphorus. I always feel obliged to explain that organic chemistry is not all like this, and if the students will be patient for a few weeks while they are acquiring the fundamental and necessary technic, very soon they will be making interesting preparations.

The treadmill monotony of these operations may be broken to some ex- tent by the use of unknowns. The only reference to the use of unknowns in the laboratory manuals listed at the end of this article is given by Williams and Brewster (1). The authors recommend that after making a melting-point determination with a known$ubstance, the student obtain an unknown from the instructor, the unknown substance being one of eight substances whose melting points are given in the experiment, ranging from 115 to 168 degrees Centigrade. After the determination of the melting point, the student is to mix some of the unknown substance with some of the compound thought to be identical with it, and determine the melting of the mixture.

The same procedure mav be applied to the determination of boiling points. After calibrating the thermometer by noting the boiling points of water, ethylene dihromide, and nitrobenzene, and making corrections for barometric preszure, and the exposed thread of mercury, as given by H. Fisher (2), the student is given an unknown liquid which is one of the compounds listed with their boiling points on the bulletin board. After the determination cf the boiling point, and making the corrections, he identifies his liquid. If an air condenser is to be used in the determination of the boiling point, this is plainly indicated on the label of the unknown. The following compounds are suggested for unknowns: ethyl alcohol, n- propyl alcohol, isopropyl alcohol, toluene, isoamyl alcohol, ethylene di- bromide, chlorobenzene, bromobenzene, aniline, nitrobenzene, and quino- line.

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After the qualitative analysis of a known sample for carbon, hydrogen, nitrogen, halogens, sulfur, and phosphorus, according to the directions given in (3) , (4 ) , (5) , (6), (7), (a), (9), ( l o ) , or ( l l ) , an unknownsolidisgiven to the student. The unknown sample may be a compound containing any combination of two or more of the above elements. Some compounds use- ful as unknowns are: acetanilide, bromoaniline, benzoic acid, m-bromo- benzoic acid, casein, and thiocarbanilide. In this unknown the student makes no effort to identify the compound given him, but reports the ele- ments found, and is graded on this.

In the laboratory manuals of Fisher (12) and Jones (13j a method is given for identifying methyl alcohol by making the methyl ester of 3,5- dinitrobenzoic acid, and determining its melting point. Fisher gives the melting points of the ethyl, propyl, and isopropyl esters.. An unknown alcohol (any of the above) is given to the student, who colverts it to the ester of 3,5-dinitrobenzoic acid, purifies it by recrystallization, and after drying it, determines its melting point, and from this identifies the alcohol.

The objection may be made that doing unknowns requires time, but the extra practice which the student gets in doing the unknown after doing the known amply compensates for the small amount of time consumed.

The advantages of the system are given below. 1. I t gives an immediate practical value to the experiment that the

beginner often misses. Without the unknown in view, he does the technic experiments because he is required to, not because he understands why they are necessary, or how they are applied to a practical problem.

2. It improves technic, and lessens careless m%nipulation. If a careless student has to determine the melting and boiling points of certain known compounds only, he does the experiment hastily, heedlessly, and notes down the correct results regardless of whether they were the results actually obtained. Using mixtures for the determination of melting points, as suggested by Adkins and McElvain (14) and Adams and Johnson (15), does not entirely prevent this as it is easy enough for the careless student to compare notes with a conscientious neighbor as to the melting of the differ- ent mixtures. To be successful in determining the unknown, careful technic is needed, and the student cannot be sure he will be successful with it, unless he has first succeeded with the known.

3. I t gives the instructor a good standard for grading. The success of a student in doing an unknown is a definite, and fair means of measuring his technical ability, and his understanding of the experiment.

Literature Cited ( I ) WILLIAMS AND BRBW~TER, "Laboratory Manual of Organic Chemistry," 3rd ed.,

D. Van Nostrand Co., New York City, 1928, p. 6. (2) FISHER, "Laboratory Manual of Organic Chemistry," 2nd ed., John Wiley and

Sons, Inc., New York City, 1931, pp. 12-26.

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ADAMS AND JOHNSON, "Laboratory Experiments in Organic Chemistry," Mac- millan Co., New York City, 1928, pp. 73-9. ADKINS AND MCELVAIN, "Practice of Organic Chemistry," McGraw-Hill Book Co.. New York City. 1925, pp. 7-9. COHeN, "Practical Methods of Organic Chemistry," 2nd ed., Macmillan Co., New York City, 1910, pp. 1 4 . COOK, "Laboratory Experiments in Organic Chemistry," 2nd ed., P. Blakiston Co., Philadelphia, 1928, pp. 1-2. Frswsn, loc, cit.. pp. 4&52. LO-Y AND BALDWIN. "A Laboratory Book for Organic Chemistry," John Wiley and Sons, Inc., New York City, 1926, pp. 11-2. NORRIS. "Experimental Organic Chemistry," 2nd ed., McGraw-Hill Book Co., New York City, 1924, pp. 3942 . NOYES, W. A,, "Organic Chemistry for the Laboratory," 2nd ed., Chemical Publishing Co., Easton, Pa., 1911, pp. 26M. WILLIAMS AND BREWSTER, LDC. <it., PP. &lo.

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FrsHen, 2oc. cit . , pp. &lo. JONES, "Laboratow Outline of Organic Chemistry," rev. ed., Century Co., ~ e w ~ o r k City, 1921, pp. 1&20. ADXINS AND MCELVAIN, LOG. d . , Pp. 17-8. ADAMS AND JOHNSON, 106. cit., pp. 13, 17, 19.