Learnosha CBSE 12
Chemistry Aldehydes, Ketones and Carboxylic Acids 4 Marks
Appeared in: 2022 (3 sets)
Read the passage given below and answer the questions that follow : Aldehydes, ketones and carboxylic acids are some of the important classes of organic compounds containing carbonyl group. These are highly polar molecules due to higher electro-negativity of oxygen relative to carbon in the carbonyl group. Aldehydes are prepared by dehydrogenation or controlled oxidation of primary alcohols and controlled reduction of acyl halides. Ketones are prepared by oxidation of secondary alcohols and hydration of alkynes. Aldehydes and ketones undergo nucleophilic addition reactions onto the carbonyl group but carboxylic acid does not undergo nucleophilic addition reaction. The alpha ($\alpha$) – hydrogens of aldehydes and ketones are acidic. Therefore aldehydes and ketones having at least one $\alpha$-hydrogen undergo Aldol condensation.
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CBSE Class 12 Chemistry: Marking Scheme Solution

Question: Explain why carboxylic acids do not undergo nucleophilic addition reactions like aldehydes and ketones, and identify the condition for Aldol condensation.

Step 1 (Conceptual Basis): [1 Mark]

The carbonyl carbon in carboxylic acids is involved in resonance. The lone pair of electrons on the oxygen atom of the $–OH$ group participates in resonance with the carbonyl group, which significantly reduces the electrophilicity of the carbonyl carbon.

$$R-C(=O)-OH \longleftrightarrow R-C(-O^-)=O^+-H$$

Step 2 (Step-by-Step Reasoning): [1 Mark]

  • In aldehydes and ketones, the carbonyl carbon is highly electrophilic due to the strong $-I$ effect of the oxygen atom, making it susceptible to nucleophilic attack.
  • In carboxylic acids, the resonance structure delocalizes the positive charge on the carbonyl carbon, making it less electrophilic.
  • Furthermore, the $–OH$ group is protonated in acidic media or the carboxylate ion ($RCOO^-$) is formed in basic media, both of which deactivate the carbonyl carbon toward nucleophilic addition.

Step 3 (Final Concluded Answer): [1 Mark]

Conclusion: Carboxylic acids do not undergo nucleophilic addition because the resonance stabilization of the carboxyl group reduces the electrophilicity of the carbonyl carbon. Condition for Aldol Condensation: Aldehydes and ketones must possess at least one $\alpha$-hydrogen atom to undergo Aldol condensation, as these hydrogens are acidic and can be removed by a base to form a nucleophilic enolate ion.

💡 EL10 Concept Insight: Think of the carbonyl carbon as a "magnet" that attracts nucleophiles; in carboxylic acids, the oxygen atom "shares" its electrons with that magnet, making it much less attractive to incoming particles compared to aldehydes or ketones.

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