Which is more polar, benzoin or benzil? Explain how the TLC plates tell you the answer. Does the answer make sense when you look at the structures of benzoin and benzil? Explain.
The Correct Answer and Explanation is:
Correct Answer:
Benzoin is more polar than benzil. This conclusion can be drawn by analyzing the results from thin layer chromatography (TLC) plates. On a TLC plate, the more polar a compound is, the more strongly it interacts with the polar stationary phase (commonly silica gel), and thus it travels a shorter distance, producing a lower Rf value. In contrast, less polar compounds interact weakly with the stationary phase and travel farther, producing a higher Rf value.
Explanation:
When comparing benzoin and benzil on a TLC plate developed using a moderately polar solvent system, benzoin will exhibit a lower Rf value, while benzil will show a higher Rf value. This observation indicates that benzoin is more polar because it adheres more strongly to the polar silica gel and migrates less during development.
The polarity difference makes sense when examining their chemical structures. Benzoin contains both a hydroxyl group (–OH) and a ketone group (C=O). The hydroxyl group significantly increases polarity due to its ability to form hydrogen bonds with the stationary phase on the TLC plate. Hydrogen bonding is a strong polar interaction that causes benzoin to stick to the silica gel and migrate slowly.
Benzil, on the other hand, contains two ketone groups but lacks the hydroxyl group present in benzoin. While the carbonyl groups contribute some polarity, ketones cannot engage in hydrogen bonding as donors, which limits their polar interactions compared to benzoin. As a result, benzil interacts less strongly with the stationary phase and travels farther on the TLC plate.
Thus, both the experimental TLC results and structural analysis confirm that benzoin is more polar than benzil. The TLC plate visually reflects this through the lower Rf value of benzoin and the higher Rf value of benzil, consistent with their structural differences.
