Question

After a Organic Lab of TLC I never understood the actual science behind the experiment.

Why is it important to have an evenly packed column free of any air pockets?

2. Why was it necessary to add DCM in order to get our mixture to completely dissolve?

3. Why did the yellow compound remain mostly stationary until the DCM was run through the column?

4. What was the purpose of running a TLC of our fractions? 5. Can Rf values ever be greater than 1? Equal to 1?

6. Why is it important that the TLC jars are kept closed so that no solvent evaporates? (Hint: Do the solvents evaporate at the same rate?)

Answer 1

1. If column is not evenly packed and have some air pocket, the compound will get trapped in the air gap. So, the mobile phase can't bring down the compound uniformly.

2. The mixture should dissolve uniformly and make a uniform slurry and load in column, so that the mobile phase bring down the compounds (in the mixture), by dissolving the molecules as per their polarity.

3. Its not about color of the compound. The yellow color might be a more polar compound (containg amine or carboxylic group).

4. To check the purity of the fraction. TCL will say, how many spots you are getting in the fraction. If it is a single spot then you got a pure compound. If the TCL has more than one spot, then you are getting a mixture of compounds in the fraction which you have eluated through the column.

The Rf will always be less than or equal to 1

5. If TLC jar are left open, the the solvent will evaporate. Suppose you are developing the TLC with mixing of hexane and ethylacetate in some ratio (at a particular polarity). If the jar is left open the hexane will evaporate (since less boiling solvent) and the polarity will change which might land up u a high polar solvent system. Which will mix two or more compound and give a single spot.