Question

Carboxylic acids are more acidic than phenols because

• A. Formation of dimers
• B. Intermolecular hydrogen bonding
• C. More covalent nature
• D. More resonance stabilisation of their conjugate base

Hint:

When comparing acidity, always look at the stability of the conjugate base. Factors that stabilize the conjugate base (like resonance, induction, electronegativity) will increase the acidity of the parent compound. For resonance, equivalent contributing structures provide the greatest stabilization.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
The acidic strength of an organic compound is directly proportional to the stability of the conjugate base formed after the loss of a proton ($H^{+}$).
Step 2: Key Formula or Approach:
Compare the resonance structures of the carboxylate ion and the phenoxide ion to determine which effectively disperses the negative charge better.
Step 3: Detailed Explanation:
1. Carboxylate Ion ($RCOO^{-}$): The negative charge is delocalized equally over two highly electronegative oxygen atoms. The two resonance structures are equivalent, providing a very high degree of stability.

2. Phenoxide Ion ($C_{6}H_{5}O^{-}$): The negative charge is delocalized over one oxygen atom and the carbon atoms of the benzene ring. Since carbon is significantly less electronegative than oxygen, the delocalization onto the ring is less effective at stabilizing the charge compared to delocalization between two oxygens. Moreover, the resonance structures are non-equivalent.
Because the carboxylate ion is much more stable than the phenoxide ion, carboxylic acids release protons more readily than phenols.
Step 4: Final Answer:
The higher acidity is due to the greater resonance stabilization of the carboxylate conjugate base.