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For a pictorial how-to: http://www.chem.ucla.edu/~bacher/General/30BL/tips/TLC1.html
1. Figure out which solvent system puts your starting reagent at an Rf of 0.3-0.4.
good starting points
polar compounds: 100% EtOAc or 5% MeOH/dichloromethane
normal compounds: 10-50% EtOAc/Hexane
nonpolar compounds: 5% EtOAc/hexane, 5% ether/hexane, 100% hexane.
2. Prepare a chamber with about 1/2 cm of your solvent mixture in it.
3. Get a TLC plate (1.5-2 cm wide and ~5 cm high is a good size) and put three pencil dots in a horizontal row at the bottom, ABOVE the 1/2 cm mark. When you put the plate in the chamber, these dots should not be submerged in the solvent.
4. With a capillary pipette, put a sample of your starting material (diluted to a concentration similar to the reaction mixture) at the leftmost dot, and at the middle dot. Your sample spot should be small enough that it does not spread to the other pencil dots.
Note: If you have more than one starting material, you'll need more than three dots.
5. With a capillary pipette, put a sample of your reaction mixture at the middle and rightmost dot. The middle dot is a "cospot"- to see why this is important, see TLC notes.
6. Run the TLC until the solvent front has almost (but not quite) reached the top of the plate.
7. Look under a UV lamp and circle the UV-active spots with a pencil.
8. Dry the plate on either a hot plate or in the air, until the solvent is gone.
9. Dip into an appropriate TLC stain, and heat on a hot plate. You may have to try a few before you find the best stain for your compound.
Tip: Dipping a hot plate into KMnO4 is not a good idea, but dipping a hot plate into anisaldehyde works well.
See also: TLC notes, Troubleshooting TLC, Chromatography Notes.