Predict the Order of benzoin and benzil on a TLC plate. Which would travel further (higher Rf value)? Explain! (6 pts) Why do polar compounds move slower up the silica TLC plates than nonpolar compounds? Why does increasing the polarity of the mobile phase increase the rate at which compounds move up the plate? (6 pts) 12 cm 15 cm The figure on the right shows a typical TLC plate. (6 pts) Label the origin line and solvent front. What is an Rf Value and how do you calculate it? Calculate the Rf value for compound E. John
The Correct Answer and Explanation is:
Order of Benzoin and Benzil on a TLC Plate
Benzil will travel further up the TLC plate than benzoin. Therefore, benzil will have a higher Rf value compared to benzoin.
Explanation
Benzoin is more polar because it contains both a hydroxyl (OH) group and a carbonyl (C=O) group. In contrast, benzil has two carbonyl groups but lacks the hydroxyl group, making it less polar overall. Silica gel, which is the stationary phase in most TLC plates, is highly polar. Polar compounds interact more strongly with the polar stationary phase and therefore move more slowly. Nonpolar compounds interact less with the stationary phase and travel further, resulting in a higher Rf value. Thus, benzil, being less polar, travels further than benzoin.
Why Polar Compounds Move Slower on Silica TLC Plates
Polar compounds form stronger interactions such as hydrogen bonding or dipole-dipole interactions with the polar silica gel. These interactions cause them to stick to the stationary phase, making them move slower up the plate. Nonpolar compounds have weaker interactions with silica and thus travel faster.
Effect of Increasing Polarity of the Mobile Phase
When the mobile phase (solvent) is made more polar, it competes with the compounds for interaction with the silica gel. This weakens the interactions between the compounds and the stationary phase, causing even polar compounds to move faster. As a result, increasing solvent polarity increases the rate at which compounds ascend the plate.
Labeling TLC Plate and Rf Calculation
- The origin line is the baseline where the sample is spotted, usually at the bottom.
- The solvent front is the furthest point the solvent reached at the top.
Rf Value Definition and Calculation
Rf (Retention factor) = Distance traveled by compound ÷ Distance traveled by solvent front
For compound E:
Distance traveled by compound = 12 cm
Distance traveled by solvent front = 15 cm
Rf = 12 ÷ 15 = 0.80
