(a) How does carboxylate ion get stabilised by resonance? Explain the structure.

(b) Carboxylic acids are more acidic than phenols. Explain.

Carboxylate ions are formed when carboxylic acids lose a proton (H⁺). The stabilization of these ions through resonance is crucial for understanding their properties.

Resonance Stabilization of Carboxylate Ions

When a carboxylic acid donates a proton, it transforms into a carboxylate ion, which has the general structure RCOO^-. This ion features two oxygen atoms bonded to a carbon atom. The key to its stability lies in resonance.

Structure and Resonance

The carboxylate ion can be represented by two resonance structures:

• In the first structure, one oxygen carries a negative charge, while the other oxygen is double-bonded to the carbon.
• In the second structure, the positions of the double bond and the negative charge are swapped.

This delocalization of the negative charge between the two oxygen atoms allows the carboxylate ion to be more stable than if the charge were localized on a single atom. The resonance forms contribute equally to the overall structure, resulting in a lower energy state.

Comparing Acidity: Carboxylic Acids vs. Phenols

Carboxylic acids are generally more acidic than phenols due to their ability to stabilize the negative charge after losing a proton.

Reasons for Greater Acidity

• Resonance in Carboxylate Ions: As mentioned, carboxylate ions benefit from resonance stabilization, which disperses the negative charge effectively.
• Inductive Effect: The presence of electronegative atoms in carboxylic acids can further stabilize the carboxylate ion through the inductive effect, enhancing acidity.
• Phenolic Structure: In contrast, phenols, while they can also stabilize a negative charge through resonance, do not have the same level of charge delocalization as carboxylate ions.

As a result, carboxylic acids are more likely to donate a proton, making them stronger acids compared to phenols.