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Preparative column chromatography is one of the most valuable techniques for separating compounds in the synthetic laboratory. Unfortunately, this technique finds little use in the teaching laboratory because teachers must choose between an obsolete version, gravity column chromatography, and newer techniques such as flash chromatography and medium-pressure chromatography, which rely on expensive specialized glassware and require the use of hazardous pressurized columns. Another, little-known preparative technique, dry-column flash chromatography (1), offers the separation power of the newer techniques but is considerably safer and less expensive. It is also easy to learn and use. Since dry-column flash chromatography is not covered in any of the popular undergraduate laboratory texts, we describe here an inexpensive adaptation of this technique suitable for the teaching laboratory, which we have used in our large general chemistry and organic chemistry classes.
General Procedure
The dry-column flash procedure resembles many other types of column chromatography in that one "packs" a column, loads the sample, and elutes the column. It is unique, however, in that the column (i) consists of a "dry" bed of silica gel placed in a sintered glass funnel, (ii) is eluted by using suction, and (iii) is drained dry after each fraction. These features make it much easier to pack the column, and students do not have to worry about their columns going "dry". As a result, first-time users generally have no difficulty obtaining satisfactory results, and it is easy to get separations that rival those of analytical TLC.
Equipment
The column consists of silica gel (Merck grade 60, inhalation hazard!) packed inside a 4-cm diameter mediumporosity sintered glass funnel (Kimax 60 mL X 40 M). The rest of the apparatus consists of 16 x 150 mm test tubes and standard vacuum filtration glassware (Fig. 1). The eluting solvent needs to be a mixture of nonpolar and polar solvents; we use hexane and ethyl acetate for all experiments. The advantages of this combination are twofold: the two solvents have similar vapor pressures, and use of the same solvent pair in all experiments facilitates recycling.
Packing the Column
The funnel is loosely filled to...