Irving Allan Kaye a n d ~ e n ~r u ~ska Brooklyn College Brooklyn, New York 11210
II I
Recrystallization and Melting Point Determination An introductory organic exercise
Ihe student in an elementary organic chemistry laboratory course is usually introduced to the technique of determining melting points by observing the melting tem~eraturesof one or more single - known compounds, mixtures of known compounds, and an unknown compound (1, 2). The latter may also be identified by a mixed melting point (2, 3). His initial experience in recrystallization is frequently that of purifying known and unknown compounds which may have been mixed with easily separated impurities. Occasionally the two techniques are linked together in a single exercise wherein an impure unknown compound is purified by recrystallization and identified by comparison of its melting point with those of a select few listed in a table such as the one shown (3). An interest-
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Ketone Derivatives
Semicarbazones
2,4-Dinitrophenylhydrazones
Parent ketone 5-methyl-3-heptanone 5-methyl-2hexanone 3-heptanone 2-pentanone 2-he~tanone 2.6-dimethvl-4' heptanoGe 2-octanone 2-undecanone 4-methyl-2pentanone 4-heptanone 3-pentanone butanone cycloheptanone cyclohexanone 2-methylcyclohexanone 4-methylcyclohexanone 2-cyclohexylcyElohexanone p-methylacetophenone cvclooentanone
Parent ketone 94-95 92.5-94 105-106 107.5-110 121-122 121-122
2-octanone 2-undecanone 4-heptanone 2-heptanone 2,6-dimethyl-4heptanone 5-methyl-2hexahone ~
"
59-60 64-64.5 70.5-71 71-73 91.5-93.5 92.5-94 d
~
122-123.5 122-123.5 131.5-133.5
anone 2-methylcyclohex136-137.5 anone 2-pentanone 143-145 132-135 cyclopentanone 145-146 138.5-139.5 cycloheptanone 147-148 138.5-140.5 3-methylcyclo147-149 hexanone 161-163 165-166.5 &pentanone 155-156 187.5-189 2-ethylcyclohex155-157.5 anone 188-190 cyclohexanone 159.5-161
201.5-203 203.5-204.5
a Melting points are corrected and are given for derivatives which were purified by one recrystallization. b This melting point was observed in a capillary evacuated to about 15 mm. In an open melting point capillary the compound melted a t 198-200.5OC. For a description of the procedure for sealing asubstance in a n evacuated mp capillary, see p. 41 of reference (4).
ing variation, described recently in THIS JOURNAL, attempts to develop skills in these areas in the course of a simple preparation ( 5 ) . The experiment, whose presentation follows, is essentially an exercise in recrystallization and identification, by melting point and mixed melting point determination, of an unknown Betone which has been transformed into either its 2,4-dinitrophenylhydrazone or semicarbazone derivative. I t has been our experience, as well as that of others (5, 6 ) , that a student is more motivated to perform his experiment well when he is challenged with the solution of a problem than when he is assigned an exercise in synthesis and/or technique alone. For this reason the idea of requiring the student to identifv an unknown substance is retained in the experiment. The derivatives selected for identification have been culled from a great number which were synthesized in'the course of another project. They were choseri because they are easily prepared from inexpensive ketones; are readily recrystallized from water, methanol, or a mixture of the two; and melt, in the (convenient) range of 60-200°C, a t a temperature near that of at least one other derivative included in the experiment; thus, a mixed melting point is required for their near-positive (3) identification. The experiment is intended for the early part of the course and may be used as the first exercise in synthesis and for illustrating the concept of a derivative in organic chemistry. he chemistry of the reaction need not be stressed a t this time if, as is likely, the subject has not yet been discussed in the lecture section of the course. The experiment is easily performed, requires no expensive reagents nor elaborate equipment, is not hazardous, and gives good results in the hands of beginning students. The Experiment'
Prepare the 0.05 M 2,4-dinit.rophenylhydrazinereagent by dissolving 35.0 g of 2,4-dinitrophenylhydrazine in a mixture of 3500 ml of methanol and 70 ml of concentrated hydrochloric acid. After standing with occasional shaking for a t least 30 min, filter into an amber bottle. Preparation of the 2,4-Dinrtrophenylhydrazones
Mix 0.5 g of the unknown with 50 ml of the methanolic 2,4-dinitrophenylhydrazine reagent in a 125-ml Erlenmeyer flask. Precipitation is rapid and practically complete within about 10-15 min in reactions with cyclic and low-molecular weight unbranched
1 A more detailed description of the experimental procedure may be obtained from the authors.
Volume 47, Number 10, October 1970
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703
acyclic ketones. If no precipitate appears within 10-15 min, add 2 M aqueous hydrochloric acid dropwise, with adequate mixing until a very faint turbidity forms.2 When precipitation seems complete, immerse the flask in an ice bath for a t least 20 min before filtering. Wash the precipitate three times with small amounts of an ice-cold mixture of methanol-2 M aqueous hydrochloric acid (5: 1). Set aside a few milligrams of the crude product for a m p det,ermination. Recrystallize the remainder from a n appropriate solvent and, after air-drying each sample, determine the mp of the crude and recrystallized d e r i v a t i ~ e . ~ Preparation of the Semicarbazone
Add 0.5 g of the unknown t o a mixture of 2 ml of 25Yv [w/v] aqueous semicarbazide hydrochloride and an equal volume of 37.5Yv [w/v] aqueous sodium acetate trihydrate, in an 18 X 150mm test tube, which is immersed and has been heated in a steam bath for 1.5-2 min. If the ketone dissolves t o give a clear solution and/or if a white precipitate forms on stirring, continue heating in the steam bath for 4 min longer. If the ketone fails t o dissolve within 30 sec of stirring and heating, add 1 ml of 9.5'34 ethanol t o the hot mixture, stir and continue heating in the bath for 20-30 sec. Continue adding 1 ml increments of ethanol in this fashion until the carbonyl compound dissolves completely; about 1 4 ml of the ethanol may be required. Do not add more alcohol than is necessary. After solution occurs, continue heating for an additional 4 min. Then add ice and water until the test tube is almost completely full. Stir, separate the white semicarbazone by suction filtration, and wash the product three times with small portions of ice-cold water. Save a few milligrams of the derivative for a mp and recrystallize the rest from a suitable solvent. Airdry the crude and recrystallized products before determining their m p ' ~ . ~ The amount of aqueous hydrochloric acid required may be as little as 0.9 ml or as much as 10 ml. Derivatives which do not precipitate unless the aqueous acid is added usually melt below 100°C whereas compounds which precipitate rapidly in its absence generally melt above this temperature. The student should compare the mp of his recrystallized product with those listed in the Tables and should then report the mp's of his crude and recrystallized derivative to his laboratory instructor. If his results and conclusions are approved, all but a small amount of the recrystallized material should be turned in to the instructor. Subsequently, small amounts of the two (or more, if the possibilities are not to be limited to this number) reference compounds for mp determinations are placed separately on a clean sheet of unruled white paper. The student should observe the mp's of these compounds and those of their mixtures a t the same time. The results of all six mp determinations and the identity of the unknown ketone should be given in his report. Students sometimes fail to dry their products completely; melting ranges then are broad and much lower than those of the pure dry substances.
704
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Journal of Chemical Education
Discussion
The following procedure has been used successfully in providing reference compounds for the mp determination while at the same time, having each student unknowingly check the mp of his recrystallized product and that of a classmate. No unknown is distributed unless a t least one other ketone, whose derivative has an almost identical mp, is also assigned to someone else in the same (or another) class. If the student reports a mp for his recrystallized compound which deviates from that listed in the tables by more than 2OC, he is asked to repeat the mp d e t e r m i n a t i ~ n . ~When his mp is satisfactory and he has limited the identity of his unknown to the structures which are possible, the instructor reduces the number to two and gives him a small amount of each reference compound. One is actually his own product; the other was prepared by a classmate. IIe is instructed to observe the mp of each of the "reference" compounds, as well as that of each admixed with his unknown, a t the same time. Rlixtures of unlike derivatives give such large depressions in this experiment that students have had no trouble identifying their unknown. Our students have been required to prepare only one derivative, usually the dinitrophenylhydrazone. Semicarbazones are usually reserved for student,^ with better laboratory technique since these derivatives are usually more soluble in the recrystallizing solvents. This difficulty may be surmounted by having the students prepare these derivatives on a larger scale (twice to four times that given in the experimental procedure). This may be desirable, too, if larger amounts of reference compounds are desired for subsequent classes. Literature Cited (1) ROBERTSON, G. R., AND JACOBS, T. L., "Laboratory Practice of Organic Chemistry" (4th Ed.), The Macmillan Co., Inc., New York, 1962, pp. 17Q-1811. - . --- .
(2) COLEMAN, G. H., WAWZONEK, S., AND BUCKLES,R. E., "Laboratory Manual of Organic Chemistry" (2nd Ed.), Prentice-Hall, Inc., Englewood Cliffs, N. J., 1962, pp. 7-8. (3) J. S.. "Oreanic Chemistrv: An Exverimental Awvror-h " . . SWINEHART. ~ p p l e t o n - c e n t u r ycrofts (~ivision-of ~ e r e d i t hCorp.), N&v York, 1969, pp. 77-8. (4) FIESER,L. F., "Organic Experiments" (2nd Ed.), Raytheon Education Co., Lexington. Mass., 1968, p. 52. (5) PEARSON, R. E.. J. CHEM.EDUC.,46, 692 (1969). L. B., AND WINGROVE, (6) ROBERTS, R. M., GILBERT,J. C., RODEWALD, A. S., "An Introduction to Modern Experimental Organic Chemistry," Holt, Rinehart and Winston, Inc., New York, 1969, p. 328. lVl.,
