Question

Why is 100% hexane not a good mobile phase for finding the polarity of aspirin and acetaminophen using thin layer chromatography?

Answer 1

First, let us explain TLC:

TLC stands for Thin Layer Chromatography; it is an analytical technique in which we exploit each sample's affinity toward different polar/nonpolar solvents.

We use a stationary phase: a phase which does NOT moves, as the name implies. It is typically the "thin layer" or the "plate". It is commonly to use silica gel plates, which are polar, but any species could be used, such as paper.

We also use a mobile phase: as the name implies, this phase will "move" and will carry along the path all the components from the solvent. Many times, you will be able to split in several components the "mixture" of the sample. Therefore, they can be seen in the stationary phase (plate). Typically, we allow the mobile phase to go up to the "Rfinal" mark, or final reference. This is the distance travelled by the solvent (mobile phase). It starts at solvent level, it is R0, therefore.

Rf = distance traveled by sample / distance traveled by solvent

Then, "Rf" value allows us to understand the affinities of anaytes towards the solvent.

As Rf increases, this implies higher affinity toward the mobile phase ( since it is carried all along the mobile phase)

If Rf is low, this implies a low affinity toward the mobile phase; therefore higher affinity toward the stationary phase, since it sticks to the plate rather than travel upwards.

hexane is very "nonpolar" it will not allow aspirin and/or acetaminophen will not have an affinity towards the mobile phase

they will be much likely bind due to polar-polar interactions to the stationary phase

therefore, Rf values will be pretty low for boths, for isntance 0.02 vs. 0.04 or so

This is not good, since we are not using the complete front.

We need to find a sutiable mobile phase, such a mix of hexane - acetone or hexane - ethanol

This will increase polarity of mobile phase, therefore, Rf vlaues can be seen between 0<Rf < 1 range easier