Separation and IR Analysis of a Mixture of Organic Compounds Evan M. Thompson and John Almy California State College. Stanislaus Turlock. CA 95380 Most organic lahoratory texts give lengthy descriptions of the theoretical aspects of fractional distillation and provide adequate discussion of the theory and practice of gas-liquid chromatography (GLC). However, few published laboratory exercises combine the two separation techniques so that GLC can he used as an analyticalmethod to monitor the progress and completeness of a hulk separation by fractional distillation.' Furthermore, very few published exercises use distillation to separate a mixture of liquid components which are then identified.2 We wish to present a procedure which we have assigned to students in our vear-seouence oreanic laboratorv which not only combines hactionh distillation with gas-iquid chromatography hut also includes infrared analysis. The objectives of this experiment are threefold: (1) to introduce students to fractional distillation and analysis of each fraction by GLC; (2) to induce them to decide if the purity of each fraction is sufficient for infrared analysis; and (3) to involve them in a series of steps (hy use of the boiling point and infrared spectrum) which culminates in identification of two liquid orzanic "unknowns." With a class of 12 students, the experiment is accomplished in a single 3-hour period. By the time the students are assigned the experiment described below, they have utilized the GLC to measure the ~ r o d u c tdistributions of several reactions. and thev have prrfurmed w e r s l unknown identifiratiuns by the derivative and i p t ~ t r u (IR l and Nhllt) method. Using our laboratory text4as a basis, the student sets up and performs a fractional distillation to separate 10 g of a two-component (15)mixture in which the components boil at least 20 degrees apart. Our mixtures are made up of one low boiling component (c&Aohe&e, t-butyl alcohol, ethanol, propanol, ethyl acetate, dioxane, 3-pentanone, n-prapyl alcohol, or 2-butanone)and one high boiling component (I-butanol.. cvclohexanol. isoamvl alcohol.. cvclohexanone. . , chlurobt.orenr, Irrc~md~enzene, rydnhtxyl;mme, or I-prntnnd~. Studrnts.trt. tolu that each oftheir twc,compc~nenuhosa~xcarbons o r lrss per nwlrrulc dnd that nucumpment h d s w e r IROO. They nrr instructrd tu make carelul Iwding pomr mrasurmwnts and wmakr wrc that the rut lor the high hiling frartkm is taken well after itz h t d i n a point isestohliihrd. ~Yirldsdrenut uicd t t r nsscr* the grade in this experiment.) The student cheeks each component for purity by GLC (a six-foot column containing 8%Carbowax1540ondiatomiteat llO°C).At this point the student needs to decide if the purity of each component is sufficientto proceed with infrared analysis. (Normally the purity is 97% or greater with the 5-in. fractionating columns whichare used.) Students who have difficulty arriving at this decision are asked if their infrared spectrum of one fraction would be affected seriously by small amounts of the other fraction. They also are asked how they will deal with impurities when they analyze each infrared spectrum. The students then run infrared spectra on each component, neat, between salt plates. A t this point, the laboratory work is terminated
and the students now assemble their data. The students'knowledge of infrared spectrum interpretation has been learned from the lecture as well as laboraton, and identification is made normallv .eaoerience . hy tlrct srrking distinyui.;hing al,iorptims I(:-H nliphaticandarc,. matir. ('=S and C=C, ('=O, ;~nmstirring. C 0 atld N-H r w 0-H). From thir, tentative functional rnrups are dedurrd. ISrudents are free to make simple chemical tests to confirm functional groups.) Comparisons of the general features of the student's spectrum with those in the Aldrich Catalog5normally supply confirmation of the general functional group classifications. The boiling point taken during distillation is a key to the final identification. Discussion The exercise described above brines toeether manv of the techniques used in the organic lahor&xG while simple distillations are introduced earlier in the academic year, they are incorporated in lengthy preparative procedures. The present exercise, however, gives the student a sense of the applicability of distillation procedures to many separation problems. After a number of laboratory exercises which are designed to "prove" concepts which are presented in lecture, students appreciate tasks which emphasize practical techniques. T h e students' use of GLC checks their work; the use of unknowns gives the students the independence from set procedures. The use of the infrared spectrophotometer and the interpretation of spectral data gives students practice in this area, particularly if it appears that one fraction is contaminated with the other. Our ex~eriencewith this exercise has been that the students get a feeling of accomplishment, particularly when their sDectra match those found in ~ u h l i s h e dworks.5 While other separations such as extraction and other spertrophotometric anulvses (such as N M R J are nosihle.. the . Drccedure descril~cd in this experiment is the most convenient and effective for our laboratory.
' (a)Auk A . J. CHEM.EDUC..41. 436 (1964);(b)Rosenblan. D. H.. and Davis. G. T.. ''Laboratory Course in Organlc Chemistry." Allyn and Bacon. Boston. 1973, p. 218. (a) Schoffstall,A. M., and Specht, J. D., J. CHEM. EDUC.,47, 539 (1970);(b)Schatz, P. F., J. CHEM. EDVC.,56, 480 (1979);(c)Roberts, R. M., et al.. "An Introduction to Modern Experimental Organic Chemistry." Holt, Rinehart and Winston, New York, 1974, pp. 104. 155. 198. Our actual experimental procedure (2 pp.) is available on request: Department of Chemistry, California State College, Stanislaus, 800 Monte Vista Avenue, Turlock, CA 95380. Moore, James A., and Dalfymple, David L.. "Experimental Memods in Organic Chemistry." 2nd Ed., W. B. Saunders Co., Philadelphia.
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1976.
Pouchart, C. J.. "The Aldrich Library of Infrared Spectra," 2nd Ed., Aldrich Chemical Co., Milwaukee, 1975.
Volume 59
Number 7
July 1982
617
