Question

Caffeine is quite polar. In an initial experiment, caffeine had an Rf value of 0 (stuck on the baseline). What would you do to get caffeine to move off baseline?

Answer 1

Ans. The movement of a molecule during paper chromatography or thin layer chromatography is due to its relative solubility in the developing solvent. The developing solvent is usually a mixture of polar and non-polar solvents.

The stationary phase (cellulose of paper or silica of TLC) is also polar.

As mentioned, caffeine is also polar.

When placed on the baseline of TLC plate (say, we are conducting TLC experiment), caffeine would strongly be retained on the polar surface of stationary phase. Use of a non-polar solvent or a solvent of relatively low polarity does not interact (dissolves) the polar molecules of caffeine, therefore caffeine does not move upward along with the solvent. As a result, an Rf value of 0 might be obtained for the specified conditions.

Correction: The only way to let caffeine move along with the solvent is to increase its relative solubility in the developing solvent. So, use a relatively more polar solvent like dichloromethane, acetic acid or even water in the developing solvent. Presence of such polar solvent in the developing solvent dissolve caffeine in and the molecule moves along the height of solvent front.

5% acetic acid (polar solvent) in ethyl acetate (non-polar solvent) is preferred for isolation of caffeine using TLC. If gives an Rf value greater than 0.