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 Acidity:
The acidity of a compound is determined by its ability to donate a proton (H\(^+\)). The strength of an acid depends on the stability of the conjugate base formed after the proton is donated. A more stable conjugate base corresponds to a stronger acid.
Step 2: Comparing the Conjugate Bases of Carboxylic Acid and Phenol:
Carboxylic Acid: A carboxylic acid (R-COOH) loses a proton to form a carboxylate ion (R-COO\(^-\)). \[ \text{R-COOH} \rightleftharpoons \text{R-COO}^- + \text{H}^+ \] The carboxylate ion is stabilized by resonance. The negative charge is delocalized over two highly electronegative oxygen atoms. The two contributing resonance structures are equivalent, which leads to very effective stabilization.
\[ \text{R-C(=O)-O}^- \leftrightarrow \text{R-C(O}^-\text{)=O} \] Phenol: A phenol (Ar-OH) loses a proton to form a phenoxide ion (Ar-O\(^-\)). \[ \text{Ar-OH} \rightleftharpoons \text{Ar-O}^- + \text{H}^+ \] The phenoxide ion is also stabilized by resonance. The negative charge on the oxygen is delocalized into the benzene ring, specifically at the ortho and para positions. However, in these resonance structures, the negative charge resides on less electronegative carbon atoms, and the aromaticity of the ring is disturbed. These structures are less stable contributors compared to the primary structure with the charge on oxygen.
Step 3: Detailed Explanation:
The key difference lies in the effectiveness of the resonance stabilization.
- In the carboxylate ion, the negative charge is shared equally between two identical and highly electronegative oxygen atoms. This delocalization is very effective.
- In the phenoxide ion, the negative charge is primarily on one oxygen atom, with minor delocalization onto carbon atoms of the ring. The resonance structures are not equivalent, and placing a negative charge on a less electronegative carbon atom is less favorable than placing it on oxygen.
Because the carboxylate ion is more stabilized by resonance than the phenoxide ion, the equilibrium for the dissociation of a carboxylic acid lies further to the right than that for a phenol. This makes carboxylic acids stronger acids than phenols.
Step 4: Final Answer:
Carboxylic acids are more acidic than phenols due to the greater resonance stabilization of their conjugate base (carboxylate ion) compared to the conjugate base of phenol (phenoxide ion).