1. Electron Density:
* Phenol: The hydroxyl group on the phenol ring is highly electron-rich due to resonance. This makes the oxygen atom more nucleophilic and susceptible to electrophilic attack by acylating agents.
* Carboxylic Acid: The carboxylic acid group, while containing an oxygen atom, is less electron-rich due to the electron-withdrawing effect of the carbonyl group.
2. Steric Hindrance:
* Phenol: The phenol group is less sterically hindered than the carboxylic acid group. The hydrogen atom attached to the phenolic oxygen is smaller than the carboxylic acid group's carbon and oxygen atoms. This makes the phenol group more accessible to the acylating agent.
* Carboxylic Acid: The carboxylic acid group is larger and more sterically hindered, making it less likely to undergo acylation.
3. Resonance Stabilization:
* Phenol: The acylation product of phenol is stabilized by resonance, further favoring the reaction. The resonance structures distribute the positive charge throughout the ring, making it more stable.
* Carboxylic Acid: The acylation product of the carboxylic acid group does not have the same degree of resonance stabilization as the phenol acylation product.
4. Reaction Conditions:
* Acetylation: In acetylation reactions (using acetic anhydride), the use of a base like pyridine helps deprotonate the phenolic hydroxyl group, making it even more nucleophilic and favoring acylation at the phenol.
In summary:
The combination of higher electron density, less steric hindrance, and resonance stabilization makes the phenol group in salicylic acid more reactive towards acylation compared to the carboxylic acid group. This is why acylation preferentially occurs at the phenol group.
