Question: Why carboxylic acid does not give nucleophilic addition reactions like aldehydes and ketones?
Step 1 (Governing Principle): [0.5 Mark]
The reactivity of the carbonyl group ($>C=O$) towards nucleophilic addition depends on the magnitude of the positive charge on the carbonyl carbon. In carboxylic acids, the lone pair of electrons on the oxygen atom of the $-OH$ group participates in resonance with the carbonyl group.
The resonance structures are represented as:
$$R-C(=O)-OH \longleftrightarrow R-C(-O^-)=O^+-H$$Step 2 (Step-by-Step Reasoning): [0.25 Mark]
- Due to resonance, the carbonyl carbon atom becomes less electrophilic compared to the carbonyl carbon in aldehydes and ketones.
- Furthermore, the $-OH$ group donates electron density to the carbonyl carbon, which significantly reduces the partial positive charge ($\delta^+$) on the carbon atom.
Step 3 (Final Concluded Answer): [0.25 Mark]
Because the electrophilicity of the carbonyl carbon is greatly reduced by resonance, it cannot effectively attract a nucleophile. Therefore, carboxylic acids do not undergo nucleophilic addition reactions.
💡 EL10 Concept Insight: Think of the carbonyl carbon as a magnet that attracts nucleophiles; in carboxylic acids, the oxygen atom "shares" its extra electrons with that magnet, making it much weaker and less attractive to incoming nucleophiles.