Consider compounds trans-4-tert-butylcyclohexanol and 4-tert-butylcyclohexanone below. Using silica gel TLC plates, which of these two compounds will have the higher Rf value when dichloromethane is the developing solvent? Why?
Does it possibly have to do with the polarity?
Yes, it does. Is dichloromethane a polar solvent?
yes it is a polar solvent
So the polar solvent holds more tightly to the polar solute; the less polar solute has nothing to hold it back so it swishes through the TLC plate.
How would you know which one is the polar solute?
Yes, the relative polarity of the compounds can influence their Rf (retention factor) values in thin-layer chromatography (TLC). The Rf value is a measure of how far a compound travels on TLC plates relative to the solvent front. Compounds with higher polarity tend to have lower Rf values, while less polar compounds have higher Rf values.
In this case, we have trans-4-tert-butylcyclohexanol and 4-tert-butylcyclohexanone. To determine which compound will have the higher Rf value using dichloromethane as the developing solvent, we need to compare their relative polarities.
Comparing the structures, we can see that both compounds contain the tert-butyl group (-C(CH3)3) which is bulky and nonpolar, contributing to the nonpolarity of these molecules. However, there is a difference in the functional groups present in these compounds.
Trans-4-tert-butylcyclohexanol contains an -OH (hydroxyl) group while 4-tert-butylcyclohexanone contains a >C=O (carbonyl) group. The carbonyl group makes 4-tert-butylcyclohexanone a more polar molecule compared to trans-4-tert-butylcyclohexanol. This increased polarity is due to the electronegativity difference between oxygen and carbon, resulting in a partial positive charge on the carbon atom and a partial negative charge on the oxygen atom.
Since the dichloromethane is a nonpolar solvent, it will likely interact more favorably with the less polar compound, trans-4-tert-butylcyclohexanol. As a result, trans-4-tert-butylcyclohexanol will have a higher Rf value compared to 4-tert-butylcyclohexanone.
To confirm this prediction experimentally, you can perform the thin-layer chromatography using silica gel TLC plates and dichloromethane as the developing solvent. Spot the compounds on the TLC plate and allow the solvent to migrate. Once the solvent front reaches the desired distance, remove the plate and visualize the spots under UV light or using suitable detection methods. Measure the distance traveled by each compound and calculate their respective Rf values.