An Undergraduate Organic Experiment Using Thin-Layer Chromatography Taylw 6. Jones and Tappey H. Jones United States Naval Academy, Annapolis, MD 21402 In spite of the more sophisticated chromatographic techniques now available to the chemist in the research lahoratory, thin-layer chromatography (TLC) continues to be a powerful techniaue. Whereas i t is most often used to follow the progress of ieactions, perhaps its greatest utility is as a simple, convenient, and inexpensive method of analyzing multicomponent mixtures. This technique illustrates the principles of chromatographic separations, and thus, lends itself to undergraduate instruction. Although in recent years the separation and identification by comparison with authentic samples of drugs',Z, dyes3, facial lipids4, and caffeine in commercial products5 have been described in THIS JOURNAL, these would not serve ourpurposes. Our experience has been that a student is much less likely to comprehend and retain ~ r i n c i ~ l that e s are illustrated with comnounds with which he is infamiliar, and the experiments Lited employ compounds with functional groups that are not discussed until the second semester of a conventional undergraduate organic chemistry sequence. We were desirous of introducing TLC and the general theory of chromatographic separations in the first semester. T o that end we have devised a n undergraduate experiment that involves separation of derivatives of aliphatic compounds, 2,4-dinitrophenylhydrazones.
Materials Eavtmnn Chromnfogram Sheet I:IIRl Silica Gel :I-Heptanonr (13315-I.Aldrirh Chemira. Cu.) Ihllanal II3IO.:~28.4,Aldrirh Chemical CoI I.:rhyl Ievulinate t12.262-9,Aldrich Chemical CO.I Diethyl ether Petroleum ether, low-hoiling Equipment 4 Disposable micropipets (Corning, 5-pL, 7099-5) 4 Pasteur pipets, @/,-in. 1Pipet bulb Ruler Procedure In thirexprrimenr thr~tudenfprepares the 2.4-dinitrophenylhydro~une~ oi three cnrbonyl wmpuunds: 3-heptanonr (11,hurand (lit, and rrhyl levulinate ~IIII. The student is alro riven an unknown mixture of the three compounds or some combination thereof. The 2,4-dinitrophenylhyd~azine solution is prepared by adding 2 mL of concentrated H2SO& to a 25-mL Erlenmeyer flask containing 0.4 g of 2.4-dinitrophenylhydrazine.~ Water (3 mL) is added dropwise until solution is complete. The resulting warm solution is diluted with 10 mL of 95%ethanol.
Thin-layer chromatDgrsm of three known carbonyi compounds and an unknown mixture.
compound in less than 0.5 mL of 95% ethanol until no further reaction occurs. It is important not to add excess reagent. Addition of 2 mL of water and 1 mL of ether to the mixture is followed hv transfer of the ether solution with a clean . oioet . to a second test ruhe. 'l'hc prorrdurr dcscribrd ahuve is repeated for the rrmaining knownsand rhr unknown.Thr rntireexprriment will require ekht test tub-. SITP%S 10 the amdcnt rhr dangers assoriared with miss contamination through failure to use clean pipets! Using four micropipets the respective ether solutions are spottedon a line parallel to the hottomedgeof an approximately 2.5 X 4.0-in. chromatographic plate. Overspotting will result in streaking of the components. The plate is developed using a 9:l mixture of petroleum ether to ether. Screw-cap jars containing a piece of solvent-saturated filter paper serve as convenient developing tanks. When the solvent front is near the top ofthe plate, the plate is removed and the position of the solvent marked on the plate. Since the compounds absorb in thevisible region of the spectrum, they can he readily located without further chemical treatment.
Results From the chromatogram the student can calculate the Rr values (the distance the leading edge of the spot is from the origin divided by the distance the solvent is from the origin). A comoarison of the values obtained for the three knowns with t& values for t h e unknown mixture allows ready identification of the components. A representative chromatogram is shown in the figure.l T h e corresponding Rt values are as follows: 3-heptanone
' Chasar, D. W., and Toth. G. B., J. CHEM.EDUC.. 51.22 (1974).
Newman, R. L., J. CHEM. EDUC., 49,834 (1972). Brinkman, U. A. Th., and De Vries, G., J. CHEM. EDUC., 49, 545
11972).
The oreoaration of the 2.4-dinitroohenvlhvdrazine , , , ~ ~ derivatives ~ , . . of rhr three known compoundr and rhr unknown mixture is carr~edout bymrrf~~l.dr
