Extra Credit Question I am running the reaction shown below with two reactants,

Question

Extra Credit Question I am running the reaction shown below with two reactants, A and B. To see if my reaction is done, I take a small sample and run a TLC. I used a solvent system of 4:1 hexanes: ethyl acetate to run my TLC. The sample from the reaction is labeled “Ru”. I also spot my reactants to compare Rf values. OH 5 mol % Dy(OTf)3 MeCN 30 min. Product R 0.80 a. When the reaction is done, the reactants should be consumed and everything left should be product. Is my reaction R0.48 R 0.48done, or should I let it keep going? How do vou know? Rf = 0.21 Rf 0.21 b. Rank the compounds in increasing polarity Rxn

Explanation / Answer

First, a brief info on 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.

a)

From the TLC results;

we have 3 spots on it, which is products + reactants

The reaction shows it is not 100% complete, since reactants are still present

therefore, we must continue iwth the reaction.

b)

high Rf values --> nonpolar

low Rf values --> polar ( due to affinity to the stationary phase)

then

A most polar, then B , least polar is product

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