Joseph K. Myers
2-24-04
(Separation of benzyl alcohol and methyl benzoate: see [1].)
This technique is used to separate compounds from each other, such as pigments in spinach.
In the process, a thin layer (~.25mm) of an adsorbent such as alumina is coated onto a glass plate, foil, etc. A solution of the material to be separated is placed in spots onto the lower end of the plate. This is inserted into a vessel with a small amount of developing solvent (various choices may be made).
The solvent moves upwards through the layer, carrying constituents of the mixture with it. These separate into individual lines or spots, which themselves are detectable visually or with appropriate methods.
By the suitable choices of material for the layer and for its preparation, the technique can be applied to great advantage. A relatively clean separation is made with some degree of speed.
(See [2].)
(See reference books for some criteria for the purity of substances, which guide the use of methods of separation.)
The uses are as a purification method, as well as to analyze and find a solvent system bested suited for running a column with column chromatography.
Because the presence of the compound is revealed in the complexion of spots, either under regular or ultraviolet light, the thin-layer technique may also be used to monitor the progress of a reaction.
The main disadvantage is the small sample sizes that can be obtained.
This time, the silica on a glass plate is the stationary phase. The mobile phase is the solvent.
None.
| Compound | MW | mp(C) | bp | fp | d(g/cm3) |
|---|---|---|---|---|---|
| benzyl alcohol | 108.138 | -15.4 | 205.31 | ? | 1.0419 |
| methyl benzoate | 136.149 | -12.4 | 199 | ? | 1.0837 |
Notes: hexanes and ethyl acetate were used in the solvent mixture. However, a different mixture may be used.
For separation of cis and trans azobenzenes:
(Note that these instructions may vary from the goal of a particular experiment. The motivation of these instructions is to identify solvent mixtures that will separate syn and anti / cis and trans azobenzenes using thin-layer / gel chromatography.)
Prepare the developing chamber by placing a folded filter paper lengthwise in a wide-mouth jar. As directed, prepare the solution of solvents: benzyl alcohol and methyl benzoate.
Add an amount of the resultant eluent to the developing chamber so it forms a 1-cm layer on the bottom of the container. Screw the cap on tightly and shake well to saturate the interior with vapors of the solvent.
Obtain the three or four 3x10 cm strips of silica gel chromatogram sheets (they should be without a fluorescent indicator). Handle only by the sides to avoid contaminating with body oils.
Place two pencil dots symmetrically about 1 cm from the ends of the sheet.
Use a capillary pipet to apply a small spot of of the solution in toluene.
Repeat for each strip, and allow to dry.
Place a strip into the developing chamber, with both spots above the solvent level.
Allow the solvent to move to within 2-3 mm of the top of the strip before withdrawing the strip.
Repeat the process for each additional strip using a different eluting solvent.
Note the number of spots arising from each of the original two.
For separation of methyl benzoate and benzyl alcohol:
Use a similar method as above, using a mixture of methyl benzoate and benzyl alcohol to be dropped on the TLC plate, instead of the previous mixture.
The goal is to find the optimal solvent mixture: the choices used varied for each group, and are shown in the table of results.
| Group (drawer #) | % Hexane (A) / Ethyl acetate (B) | Height of A (cm) | Height of B | Difference | Height of Solvent | Rf (A) | Rf (B) |
|---|---|---|---|---|---|---|---|
| 1 (1-19) | 98/2 | .6 | 1.9 | 1.3 | 9.0 | .0667 | .211 |
| 2 (25-43) | 95/5 | .6 | 3.1 | 2.5 | 8.8 | .068 | .352 |
| 3 (49-67) | 90/10 | 1.0 | 3.5 | 2.5 | 9.0 | .111 | .389 |
| 4 (73-91) | 85/15 | 1.6 | 4.3 | 2.7 | 9.0 | .178 | .478 |
| 5 (97-115) | 80/20 | 2.1 | 3.8 | 1.7 | 9.0 | .233 | .422 |
| 6 (121-133) | 75/25 | 2.8 | 4.7 | 1.9 | 9.0 | .311 | .522 |
Approximately an 85/15 solvent mixture of hexane and ethyl acetate provides the greatest separation.
1. Gilbert JC, Martin SF. Experimental Organic Chemistry. 2002. 3rd Edition. p166-175.
2. Beyer H, Walter W. Organic Chemistry. 1997. Translated 22nd Edition. p8.