explain the strength of acid for o-salicylic acid pka-2,2
Answers
Explanation:
Here’s an “organic chemistry” look at the question. (Others have answered the more basic (pun intended?) question of which is more acidic based on the given pKa values).
Salicylic acid and aspirin (acetylsalicylic acid) are derivatives of benzoic acid. Salicylic acid has ortho -OH, while aspirin has ortho -OCOCH3. -OH is more pi-electron-donating than -OCOCH3. This raises the free energy of the carboxylate ion (by negative-negative charge repulsion) in salicylate more than in acetylsalicylate. Thus one would think that salicylate is the stronger conjugate base, and therefore salicylic acid would be the weaker acid.
What I have not yet accounted for is that -OH forms a hydrogen bond with -COOH, and an even stronger hydrogen bond with -COO(-). This *lowers* the free energy of the carboxylate ion in salicylate more than acetylsalicylate. This would make salicylate the weaker conjugate base, and therefore salicylic acid is the stronger acid. Apparently this effect is stronger than the effect of pi electron donation.