In the Laboratory www.JCE.DivCHED.org • Vol. 81 No. 7 July 2004 • Journal of Chemical Education 1023 The general aspects and practical applications of thin- layer chromatography (TLC) have been discussed thoroughly in the literature (1, 2). TLC methods are successfully used in many fields of research and development such as clinical medicine, forensic chemistry, biochemistry, and pharmaceu- tical analysis (3). This chromatographic technique is relatively inexpensive and has found widespread application as a simple, reliable, and quick analytic tool. In this inquiry-based activity the usefulness of TLC in the organic laboratory is explored. The experiment demon- strates simple aspects of chemistry (hydrogen bonding and polarity) and applications of TLC. The activity follows the backward design described by Wiggins and McTighe (4). This design is an assessment of students’ proficiency and under- standing of hydrogen bonding and polarity as applied to TLC. The goal is that students will understand TLC and be able to apply it to an unknown situation. This is accomplished by a simulated industrial application at the end of the ex- periment. Initially students develop a uniform method to dis- tinguish individual compounds and then apply this method to an unknown sample. Students are not able to tackle the application without understanding the principles covered ear- lier in the activity. Procedure and Results Demonstration Since this is the first time many students will be exposed to TLC, the instructor demonstrates the proper setup and execution of a TLC experiment during the prelab lecture. The activity is not intended for students to perfect the technique, so the instructor includes important details such as keeping the level of the mobile phase in the chamber lower than the spots on the plates. If this occurs the sample leaches into the eluent (solvent) and faulty results are obtained. Students are also cautioned not to leave plates in the chamber after the solvent front has reached the top edge. Neglecting to remove the plates will lead to diffusion of the sample in all direc- tions, which will cause the spots to appear broad and ill-de- fined. Part 1: Understanding R f The experiment is divided into four parts. In the first part, students spot three plates of different lengths with the same compound and use the same eluent. The plates are de- veloped (development is stopped when the eluent has almost reached the top of the plate) and the distances traveled are determined. The students discover that the longer the plate, the further the spot travels. The fact that the same compound is used in all three runs should spur the students to see the need for determining a systematic relationship comparing the distances traveled for the different plate lengths. Compari- son of the ratios of the distance the analyte travels to the dis- tance the eluent (solvent) travels for each plates regardless of plate size leads to the discovery of retention factor, R f (Fig- ure 1 and eq 1): distance traveled by analyte distance tr aveled by eluent (solvent) = R f (1) Not all students are able to identify this important relation- ship, and it is important that the instructor participates in guiding them to this discovery. Part 2: Examining Hydrogen Bonding During the second part of the experiment students dis- cover that separation in TLC arises from the sample’s ability to hydrogen bond to the silica gel on the plate. Six com- pounds of comparable molar mass, each containing either an ester, ketone, ether, alcohol, or carboxylic acid functional group, are developed on the same plate using eluent A (Table 1). Students’ results show that the ketone, ether, and ester Thin-Layer Chromatography: W The “Eyes” of the Organic Chemist Hamilton Dickson, Kevin W. Kittredge,* and Arlyne M. Sarquis Department of Chemistry and Biochemistry, Miami University–Middletown, Middletown, OH 45042; *kittrekw@muohio.edu Figure 1. Determination of the retention factor, R f . solvent front distance traveled by eluent origin distance traveled by analyte e n a x e H d n a e t a t e c A l y h t E f o s e g a t n e c r e P . 1 e l b a T C d n a , B , A s t n e u l E n i t n e u l E e t a t e c A l y h t E % s e n a x e H % A 5 1 5 8 B 0 3 0 7 C 0 6 0 4