Problem 20 - A Complete Resource Book in Chemistry for JEE Main

Question

Carboxylic acids are more acidic than phenol and alcohol because of (a) formation of dimers (b) highly acidic hydrogen (c) resonance stabilization of their conjugate base (d) intermolecular hydrogen bonding

Step-by-Step Solution

Answer

Carboxylic acids are more acidic due to (c) resonance stabilization of their conjugate base.

Step 1: Identify the Problem

The question asks why carboxylic acids are more acidic compared to phenol and alcohol. The answer lies in the stability of the conjugate base formed after the acid loses a proton (H⁺).

Step 2: Understand Conjugate Base Stability

When carboxylic acids lose a proton, they form a carboxylate ion. The stability of this ion is crucial for determining the acidity of the original acid. A more stable conjugate base means a stronger acid.

Step 3: Analyze Resonance Stabilization

The carboxylate ion formed from a carboxylic acid is resonance-stabilized. This means that the negative charge can be delocalized over the two oxygen atoms, resulting in a stable structure. This delocalization makes the carboxylic acid more acidic.

Step 4: Compare with Phenol and Alcohol

In phenol, when it loses a proton, the phenoxide ion is formed which also has resonance stabilization, but not as effective as carboxylate ions because the charge is shared with the less electronegative carbon atoms of the benzene ring. In alcohols, there is hardly any resonance stabilization after losing a proton, making them the least acidic.

Step 5: Conclusion Based on Resonance

Given the options, (c) resonance stabilization of their conjugate base, is the primary reason carboxylic acids are more acidic compared to phenol and alcohol.

Key Concepts

Conjugate Base Stability

Understanding conjugate base stability is essential when assessing the acidity of different compounds. When an acid like a carboxylic acid loses a proton, it forms a conjugate base known as a carboxylate ion. The stability of this ion directly influences the acidity level of the carboxylic acid.

Why? Because acids are more willing to donate a proton if their remaining form (the conjugate base) is stable. The more stable this ion, the stronger the acid. For carboxylic acids, the resulting carboxylate ion benefits significantly from high stability, which greatly increases its acidity in comparison to other compounds like phenols and alcohols.

Resonance Stabilization

Resonance stabilization plays a pivotal role in the acidity of carboxylic acids. When a carboxylic acid donates a proton, it transforms into a carboxylate ion, a structure remarkably stabilized by resonance.

This process involves the delocalization of the negative charge over the two oxygen atoms in the carboxylate ion. Essentially, resonance stabilization spreads out the negative charge, creating resonance structures that lead to an exceptionally stable ion. This concept of charge delocalization is why carboxylic acids are markedly more acidic as the charge is not just localized on one oxygen atom but shared through resonance structures.

Comparison with Phenol and Alcohol

Comparing carboxylic acids with phenol and alcohol helps further highlight their superior acidity. When a phenol loses a proton, it forms a phenoxide ion. While this ion does achieve some resonance stabilization, it is generally less effective than the carboxylate ion.

In phenol, the negative charge is partially delocalized into the ring, which involves carbon atoms, making it less favorable compared to full oxygen delocalization in carboxylates.

• Phenol: Partial delocalization, less stable conjugate base.
• Alcohol: Negligible resonance stabilization, resulting in the least acidic nature.

Carboxylic acids considerably outperform both phenols and alcohols in acidity due to the superior resonance stabilization of their conjugate bases.