Consider the compounds 4-tert-butylcyclohexanol and 4-tert-butylcyclohexanone. Using silica gel plates, which of the two compounds will have the higher Rf value when ethyl acetate is the developing solvent? Which will have the higher Rf value when dichloromethane is the developing solvent? Give reasoning for each case.
How do I go about figuring this out? Should I pay attention to the polarity of each compound?
To determine which compound will have a higher Rf value when using different developing solvents, you can indeed consider the polarity of the compounds along with the polarity of the solvents.
The Rf value (retardation factor) is a ratio between the distance traveled by a compound and the distance traveled by the solvent front. It helps to determine the relative affinity of a compound for the stationary phase (the silica gel of the TLC plate) compared to the mobile phase (the developing solvent). The higher the Rf value of a compound, the more it tends to interact with the mobile phase and move up the plate.
Now let's analyze the two compounds:
1. 4-tert-butylcyclohexanol:
This compound contains a hydroxyl group (OH) which makes it quite polar due to hydrogen bonding and dipole-dipole interactions. Silica gel plates, being polar stationary phases, tend to retain polar compounds such as alcohols. Therefore, the Rf value of 4-tert-butylcyclohexanol is expected to be relatively lower compared to less polar compounds when using a polar solvent like ethyl acetate.
2. 4-tert-butylcyclohexanone:
This compound contains a carbonyl group (C=O) which increases its polarity compared to a simple alkane. However, the polarity of ketones such as 4-tert-butylcyclohexanone is still lower than that of alcohols due to the absence of hydrogen bonding. When using a polar solvent like ethyl acetate, 4-tert-butylcyclohexanone is expected to have a higher Rf value compared to 4-tert-butylcyclohexanol due to its comparatively lower polarity.
Based on these properties, we can conclude:
When using ethyl acetate as the developing solvent:
- 4-tert-butylcyclohexanol, being more polar, is expected to have a lower Rf value.
- 4-tert-butylcyclohexanone, being less polar, is likely to have a higher Rf value.
When using dichloromethane as the developing solvent:
Dichloromethane is a nonpolar solvent. In this case, both compounds are less polar compared to the solvent. However, since 4-tert-butylcyclohexanone exhibits slightly higher polarity compared to 4-tert-butylcyclohexanol due to the carbonyl group, it is expected to have a slightly higher Rf value.
In summary:
- 4-tert-butylcyclohexanol will have a higher Rf value when using dichloromethane as the developing solvent.
- 4-tert-butylcyclohexanone will have a higher Rf value when using ethyl acetate as the developing solvent.