Question

2. In the previous lab session, you were asked to predict which dibenzoylethylene isomer was the most polar based on its structure. Was your prediction supported by the observed melting points and TLC Rf values of the two isomers?

Answer 1

dibenzoylethylene --> least polar is the Symmetrical , since poles cancel each out

dibenzoylethylene --> most polar is the one which is not Symmetrical, since the dipoles are "felt" stornger in the moecule, since they are not cancelled.

Then

dibenzoylethylene (polar) --> has higher Intermolecular forces, therefore, expect a higher melting point than the dibenzoylethylene (nonpolar) species.

Also, for Rf values in the TLC; recall that:

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.

In this case

dibenzoylethylene (polar) should have a lower Rf value if we used polar stationary phase + nonpolar mobile phase

dibenzoylethylene (nonpar) shoul have a higher Rf value since it has higher affinity towards nonpolar solvents