TLC Notes

Thin Layer Chromatography (TLC) Notes

Not Voodoo Home / Notes on Special Topics / Column Chromatography Notes / Comments?

The Cospot: During reaction monitoring, a typical TLC plate has three lanes: the reactant, the reaction mixture, and a "cospot" where reaction mixture was spotted directly on top of reactant. The cospot is important for reactions where reactant and product have similar Rfs. Also, in some cases the reaction mixture affects the appearance of the reactant during chromatography. The reactant can appear to be gone, but in actuality just looks different on TLC under the reaction conditions. This situation is easily identified by a cospot.

Preparatory TLC: Full-size plates with a thick layer of silica are used for preparatory separation by thin layer chromatography.

Up to 100 mg of compound is deposited in a horizontal thin line at the bottom of the plate, and the plate is run in the appropriate solvent system as usual.
The product is (ideally) located by UV visualization, and marked with a pencil.
A razor blade is used to scrape the silica containing the product off the plate.
The silica is placed in a fritted funnel and flushed with a polar solvent (like EtOAc). The pure product can be isolated from the filtrate.

(How to Make Capillary Spotters: coming soon)

Solvent Systems: see Notes on Column Chromatography.

Visualization Methods (Including TLC Stains/Dips)

Ultraviolet: Look at the plate under the light first when working with compounds with conjugated double bond systems.
Iodine: Shake with powdered I2. You can then heat the plate to remove the iodine stain, and use a liquid TLC stain as usual.
Anisaldehyde (great for carbonyl groups):
1. EtOH (200 mL); H₂SO₄ (10 mL); p-anisaldehyde (10 mL)
2. EtOH (250 mL); H₂SO₄ (2.5 mL); p-anisaldehyde (15 mL)
3. EtOH (425 mL); H₂SO₄ (16 mL); p-anisaldehyde (8-12 mL); HOAc (5 mL): mix everything but p-anisaldehyde, which you add when the mixture has cooled to rt. Store refrigerated.
Ceric Ammonium Molybdate (great for hydroxy groups):
1. Ce(SO₄)₂ (cerium sulfate: 5.0 g);(NH₄)₆Mo₇O₂₄.4H₂O (ammonium molybdate: 25.0 g); conc. H₂SO₄ (50 mL); H₂O (450 mL)
2. Ce(NH₄)₄(SO₄)₄ 2H₂O (ceric ammonium sulfate: 4.0 g); ammonium molybdate (10 g); conc. H₂SO₄ (40 mL); H₂O (360 mL)
3. Ceric ammonium sulfate (0.5 g); ammonium molybdate (12 g); conc. H₂SO₄ (15 mL); H₂O (235 mL)
Ninhydrin (great for amines): Dissolve 0.3g ninhydrin in 100 ml of n- butanol; add 3 ml AcOH.
Phosphomolybdic Acid (generally useful): 10% in Ethanol
Potassium Permanganate (generally useful):
1. KMnO₄ (1g); Na₂CO₃ (2g); H₂O (100 mL)
2. KMnO₄ (3g); K₂CO₂ (20g); 5 percent NaOH (5 mL); H₂O (300 mL)

For more information see The Black Group TLC Page

Commercially Available Plates for TLC

Silica gel, alumina, reverse-phase silica gel, large plates for preparatory scale.

see also: Troubleshooting Thin Layer Chromatography and the comprehensive page (including troubleshooting) here, TLC How to