Question

Assume you have a mixture of benzophenone, diphenylmethanol and biphenyl that you have placed (spotted) on a TLC reverse phase gel (non-polar stationary phase) plate. If you developed the plate in methanol (mobile phase) which compound would have the highest R_f value? Explain your reasoning to support your answer.

Answer 1

RF value - is the distance travelled by any given component divided by the distance travelled by the solvent front used in the experiment. For a given system at a known temperature, it is a characteristic of the component and can be used to identify components present in it.

Chromatographic separations take advantage of possibility that substances will differently between two phases, a mobile phase and a stationary phase.

In thin layer chromatography , or TLC, the mobile phase is a liquid (methanol) and the stationary phase is a solid absorbent (reverse phase gel).

Theory of Thin Layer Chromatography - In thin layer chromatography , a solid phase, the adsorbent (the stationary phase) is a gel on a solid support, as a thin layer (about 0.25 mm thick). Thin plates of glass are the most common support, but plastic and aluminum can also be used.

In TLC, the mixture starts as a small spot near the bottom of the plate and the solvent (mobile

phase) carries the compounds up the plate as it travels up from the bottom by a force called as capillary action.

In most cases, the stationary phase (adsorbent) is very polar and the mobile phase (eluant) is fairly non-polar, but here the case is reverse, that is the stationary phase is non -polar but the mobile phase is polar thus the name reverse phase gel. Molecules that are more non polar stick to the non- polar stationary phase more than polar molecules which are carried along in the mobile phase (methanol). seperation occurs because some molecules spend longer time standing still and adsorbed in the stationary phase while others choose to move on.

Rf value = distance traveled by substance divided by

                 the distance traveled by solvent front

While having the same Rf value (under the same conditions) does not prove that two substances are the same, having different Rf values demonstrates that they are different.

Biphenyl would have the highest Rf value as it is least polar and is easily carried by the mobile phase , methanol.