Question

We did Knoevenagel-Robinson experiment in organic lab. To check for the product's purity (ethyl 3-coumarincarboxylate), we used the thin layer chromatography technique. We dissolved a small amount of the solid crystals in dichloromethane and added a drop of the solution to a plate. Then, we ran the chromatography. We used two mobile phases in two different beakers. One contained 10% ethyl acetate/hexane solution and the other contained 20% ethyl acetate/hexane solution. We put a plate containing a drop of the solution of dichloromethane and the knoevenagel crystals in each of those beakers. The spot in the 20% ethylacetate/hexane travelled further than the one in 10%. My questions: 1) why did we dissolve the knoevenagel product in dichloromethane at first? 2) why did the spot in the 20% travelled further? 3) how does this technique test the purity of the product? (our professor told us that if we have only one spot at the end of the chromatography test, then the product is pure.)

Answer 1

1. The reason for dissolving your Knoevenagel product in dichloromethane (DCM) is that DCM is a low boiling solvent and when you put your product crystals along with DCM on TLC plate, DCM evaporate faster and it doesn't interfere in the elution (elution is nothing but running the TLC plate on the mobile phase in a TLC chamber) .  

2. The difference in the distance traveled by the two spots depends on the polarity of the solvents you have used for the mobile phase during elution. Between ethyl acetate and hexane, ethyl acetate is more polar than hexane. If you use 20% ethyl acetate:hexane mixture, the polarity of this mixture is more than the 10% ethyl acetate:hexane mixture. If polarity of the mobile phase is more, the compound under study will have more interactions with the mobile phase than the stationary silica phase and hence compound travel more in the polar mobile phase.

3. In general, in TLC (thin layer chromatography) technique, each compound will show a spot on the TLC plate at different Rf values. If your product is pure with only one compound in it (without any impurities), it will show only one spot on TLC plate otherwise if it is impure there will be more than one spot on TLC plate.