Question

One student ran a TLC of an unknown compound on a silica gel plate and the distance from the starting line to the solvent front was 4 centimeters. the compound being analyzed had Rf of 0.25. Another student was analyzing another unknown compound by TLC on an identical plate but the distance from the starting line to the solvent front was 5 cm and the distance from the starting line to the compound was 1.25 cm. Are the students compounds the same? If not what additional experiments could be used to distinguish them?

Answer 1

The distance a compound moves in TLC is dependent on the stationary phase, mobile phase as well as the distance traveled by the solvent on the plate Since irrespective of the distance between the solvent front and the base, for the same mobile and stationary phases, the distance moved by a compound relative to the solvent front is constant, the parameter retention factor or Rf was established. It was given as:

So, the Rf of the unknown compound run by the second student will be:

Rf = 1.25/5 = 0.25,

which is also the Rf of the compound analyzed by the first student. Therefore, the compounds of both the students are identical according to TLC.

However, more than one molecule can have the same relative polarity and so show the same Rf for a particular mobile phase and stationary phase. So, to rule out any false-positives, structural analysis like NMR and IR spectra of the two compounds should be recorded and confirmed thus if they are indeed identical.