Allen M. Schoffstall and James D. Specht University of Colorado Colorado Springs, Colorado 80907
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Vacuum Distillation Experiment
Common organic laboratory techniques are being introduced more frequently as parts of project experiments. For example, vacuum distillation has been included as a product purification step of synthetic sequences.' We wish to report an experiment, somewhat different from these, which includes vacuum distillation. The primary purpose of the experiment is to provide the student with a feeling similar to that of a researcher who is attempting to separate by distillation a mixture containing a product of unknown structure for identification. A certain amount of authenticity must be sacrificed by the use of commercially available chemicals, however. The method requires distillation of a mixture of two' liquids. The student is given the structure of the lower boiling material only. The procedure allows latitude in assigning students different materials to be distilled, while at the same time, treating the main distillation component in each case as an unknown product. The unknown is assigned a structure by the student based upon spectral data obtained from the purified product after distillation. The student is then furnished with the molecular formula of the product and is asked to propose the most reasonable synthetic scheme for converting starting material to product, together with alternative routes and their disadvantages. More difficult synthetic schemes are reserved for capable students. The experiment was assigned in a laboratory course for chemistry majors where all siudents used the same vacuum system on a rotational basis. As an alternative an~roach.the instructor mav wish to tell the student sdniething more about the before the student performs the distillation. It may seem more realistic to inform the student of a synthetic sequence employed to make the product. The structure of the product might be revealed to the student before the experimental work is done. The student could then be asked to propose other synthetic routes
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MOHRIQ,J. R., AND NECKERS,D. C., "Laboratory Experiments in Organic Chemistry," Reinhold, New York, 1968, pp. 50-52. BALDWIN,J., "Experimental Organic Chemistry," McGraw-Hill, New York, 1965, p. 100. NELSON,K. L., AND THOMPSON, E., "L&boratory Projects in Organic Chemistry," Allyn and Bacon, Boston, 1966, p. 83. % T h einstructor will want to supplement this procedure with directions to suit the local situation. Copies of detailed instructions used by the authors are available upon request. Use of standard taper glassware and a fraction collector cut the normal
for this purpose. All materials used can be eluted st an oven temperature of 175- and carrier gas flow rate of 30 ml per minute except for triethttnolamine. ST., Rec. trav. chim., 65, 'VEER, W. L. C., AND GOLDSCHMIDT, 793 (1946).
Pairs of Liquids Distilled Starting material
Boiling pointa
Product
Ethylene glycol
198'
Triethanolamine
Beneonitrile
190.7'
PBenaylpyridine -~
Aniline n-Butyl phenyl ether 1,3-Dibromw nmnme
184" 210"
Indoline Isoamyl sdicylate
167'
1,5-Pentanediol
~
Boiling point* 278' s t 150
17 mm' 180-l0 at 31 mm4 220° 276-7' s t 743 mm 239"
*Boiling points are taken from "Handbook of Chemistry and Physics," (48th Ed.), The Chemical Rubber Company, Cleveland, Ohio, 1967, and correspond to 1 atm. unless otherwise noted.
to the product. We have found that retention of the product as an unknown creates much student interest in this experiment and does not seem to adversely affect a student's ability to accomplish the distillation. Procedure Obtain from your instructor a mixture of 10 ml of a. lower boiling liquid and 30 ml of a. higher boiling liquid, the name of the lower boiling material and the approximate boiling range of the higher boiling, unknown product a t reduced pressure. Set up the distillation apparatus. Adjust the system pressure to 4-6 mm, so that t,he starting material will distil between 50' and 10O0C. Take a distillate cut after all of the starting material has been distilled. Adjust the system pressure to between 0.1 and 2.0 mm and take a second distillate cut. Repeat and take a third cut. This should represent the main product fraction. Be careful not to heat the distillation pot to dryness.% Run ir spectra. of each of the distillate fractions. By eamparing spectra, be certain that little or no starting material is present in the main product fraction.' Run an nmr spectrum of the main product fraction or obtain a. spectrum from your instructor. Obtain the molecular formula of the urodnet from o r i t . A ~ C I RI S ~ T I I C ~ I I Tto P the pn>duct. Propose what J ~ f Ir d to be the best wulh~sibof the product from ?ow synthexer fm purposes of startiw matrrinl, %< well ns alt~r~tative discussion with the rest of the class.
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Discussion Separation of starting material from product can be accomplished well in all of the cases mentioned above without use of a fractionating column. Some of the pairs of starting materials and products used in our experiments are listed in the table. The most challenging unknowns for identification are 4-benzylpiperidine and Cbenzylpyridine. A student can be expected to narrow possible structures for the piperidine to either 3-benzylpiperidine or 4-benzylpiperidine. Upon reference to Chemical Abstracts, the student can locate a facile synthesis for the Cisorner,' but not for the Volume 47, Number 7, July 1970 / 539
3-isomer. The synthesis of 4-benzylpyridine is also r e p ~ r t e d . ~The nmr spectrum of an alternative structure, 2-benzylpyridine, is availabhs Syntheses may be proposed for the other unknowns using reactions listed in most elementary organic textbooks. Starting materials for particular unknowns were chosen not because they represented the best precursors. Rather, they were chosen because they can be related to the unknowns by one or more reasonable steps, because they can be distilled above 50°C but below 100°C a t 4-6 mm, because they are stable, unoffensive sub"'NMR Spectra Catalog," Volnme 2, Varian Associates, Pa10 Alto, California, Spectrnm nnmber 593.
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Journal of Chemical Education
stances, and because they are available commercially. Likewise, no particular consideration was given to credulity of the presence of starting material mixed with product in proposed syntheses. The student is not expected to propose a synthetic route in which the starting material can survive throughout so as to contaminate the final product. We plan to add other pairs of liquids to those listed above for future use. The experiment will be assigned this year early in the second semester of a two semester introductory laboratory course that meets six hours per week. It was assigned midway in the second semester last year, and was handled in stride by the students.
