Hi, well, for one thing the water solublity is greatly reduced when salicylic acid is converted to aspirin. This is because there is less opportunity for hydrogen bonding (dipole-dipole interactions) between the aspirin molecule and the solvent water molecules.
Salicylic acid has two –OH groups, one in a carboxylic acid group (–COOH) and the other as a phenol group (–OH attached to the benzene ring). Both of these groups hydrogen bond with water molecules very well, enhancing the solubility.
In aspirin, the phenol –OH group is "blocked" by replacing the H with an acetyl group (–COCH3). With the acetyl group now present, the –OH group is no longer available for hydrogen bonding, with the result that aspirin is less soluble in water.
You may also see references to "intramolecular" hydrogen bonding in salicylic acid, which occurs between the –COOH group and the –OH group in the same molecule, which is no longer possible after the –OH hydrogen is replaced with the acetyl group in aspirin.
Steve
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