You're welcome Meri, but I think I need to revise my statements about the solubilities of salicylic acid and aspirin! According to the Merck Index, salicylic acid has a water solubility of 1 gram per 460 mL of water, temperature not stated but presumably 25°C, while aspirin, which I thought was less soluble, is stated to have a water solubility of 1 gram per 300 mL at 25°C, which is actually more soluble. Converting to units of moles per liter, aspirin still beats salicylic acid by a small amount, 0.0185 M to 0.0157 M.
We routinely prepare aspirin from salicylic acid in our college chemistry labs where I teach, and afterward the students take a small amount of their product and place it in a test tube with a few milliliters of water to check for the presence of the phenol group of salicylic acid with ferric chloride solution (it forms a dark purple color with salicylic acid, but does not give any color with pure aspirin). They also do the same thing with salicylic acid, but I have always observed that more of it dissolves compared to aspirin. That is, a small "spatula tip" amount of salicylic acid dissolves pretty well, but with aspirin, undissolved solid remains. But apparently that's not really true! Since the amounts were not actually weighed, the salicylic acid may just be giving the appearance of being more soluble when it actually isn't.
Well, live and learn! We can also rationalize why aspirin can be more soluble that salicylic acid. From the standpoint of intramolecular hydrogen bonding, both –OH groups in salicylic acid are tied up to some extent, rendering them less available for hydrogen bonding with solvent water molecules. This would hinder water solubility. In aspirin, with the acetyl group present, one –OH group is no longer available, but the carboxylic acid –OH is still is present and available to hydrogen bond with water. But intramolecular hydrogen bonding is no longer possible in aspirin. So, rationalizing the relative solubilities, the intramolecular hydrogen bonding in salicylic acid must compete significantly with the solvent water molecules, hindering its solubility, while in aspirin, even with only the one carboxylic acid –OH group, is more soluble because its –OH group is not hindered by competing intramolecular hydrogen bonding.
Steve
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