Thin-layer chromatography (TLC) is a simple technique used to determine the progress of a reaction. It is based on the principle that inter-molecular forces (particularly hydrogen bonding and dipole forces) vary among compounds with different substituents. The TLC plate is a small rectangular glass plate coated with a layer of Si(OH)₂.

The actual technique is carried out as follows. A line is drawn on the TLC plate in pencil to indicate the starting point, and columns are also marked for the different groups, usually the original reactant and a sample from the reaction (and a third column with known product, if available). A sample of each group is taken using a capillary tube and lightly dabbed (“”spotted””) onto the TLC plate in the proper column. The plate is then inserted into a container with a small amount of non-polar solvent, just so that when the TLC plate rests, the solvent line is just below the pencil-drawn starting line. As the solvent rises by capillary action, it takes the spotted chemicals with it, thereby creating a competing system of forces between the solvent and polar TLC plate. Eventually, the chemicals will fall out of the solvent and remain adhered to the TLC plate some distance above the initial starting point. Organic compounds can be visualized by UV light or iodine staining.

A student carried out the reaction shown below and monitored its progress via TLC plates, also shown below. R = reactant, Rxn = sample from reaction vessel.

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