Is Aspirin polar or non-polar? In my thin layer chromatography lab, the aspirin traveled far up the silica gel plate, which would mean it’s non-polar. But looking at the chemical structure, it looks to me like it is polar. Why would it travel up the plate so far if it is a polar molecule, or is it not polar for some other reason?
The correct answer and explanation is:
Correct Answer: Aspirin is a polar molecule, but it has limited polarity.
Aspirin, or acetylsalicylic acid, is a molecule that contains both polar and non-polar regions. Its chemical structure includes polar functional groups such as a carboxylic acid (-COOH) and an ester (-COOCH₃), which can participate in hydrogen bonding and dipole-dipole interactions. These groups contribute to the overall polarity of the molecule. However, the rest of the aspirin molecule includes a large aromatic benzene ring, which is non-polar. This combination gives aspirin partial polarity, making it amphipathic — having both polar and non-polar characteristics.
In thin layer chromatography (TLC), the movement of a compound on a silica gel plate is influenced by its polarity relative to the stationary and mobile phases. Silica gel is highly polar and interacts strongly with polar substances through hydrogen bonding and dipole interactions. Non-polar substances interact less with the polar stationary phase and therefore travel further up the plate with the mobile phase, which is usually non-polar or moderately polar.
If aspirin traveled far up the silica plate in your experiment, this suggests that under the chosen solvent conditions, it interacted weakly with the polar silica gel. This does not mean aspirin is entirely non-polar. It means the non-polar character of aspirin may dominate in the specific solvent system used. For example, in a non-polar mobile phase, compounds with more non-polar character will migrate further, even if they contain some polar groups.
Therefore, while aspirin is chemically polar due to its functional groups, its behavior on the TLC plate reflects a balance between polar and non-polar interactions. Its movement suggests that the non-polar aspects of its structure allowed it to travel more with the solvent, especially if a non-polar solvent was used.