Aldehydes, Ketones and Carboxylic Acids – JEE Mains Chemistry

• by Zenith LMS
• Updated April 2025

1. Introduction

This chapter covers the structure, preparation, physical and chemical properties of three important classes of organic compounds: aldehydes, ketones, and carboxylic acids. These contain the carbonyl group (>C=O) or carboxyl group (–COOH).

2. Classification and Nomenclature

• Aldehydes: Carbonyl group at the end of a carbon chain. Named by replacing –e with –al (e.g., ethanal).
• Ketones: Carbonyl group within the chain. Named by replacing –e with –one (e.g., propanone).
• Carboxylic Acids: Contain –COOH. Named by replacing –e with –oic acid (e.g., ethanoic acid).

3. Methods of Preparation

• Aldehydes and Ketones: Oxidation of alcohols, ozonolysis of alkenes, hydration of alkynes.
• Carboxylic Acids: Oxidation of aldehydes, nitriles, or primary alcohols.

4. Physical Properties

• Aldehydes and ketones are polar and have dipole-dipole interactions.
• Carboxylic acids form hydrogen bonds → higher boiling points and solubility in water.

5. Chemical Reactions

• Nucleophilic Addition Reactions: Addition of HCN, NaHSO₃, Grignard reagents to aldehydes and ketones.
• Oxidation: Aldehydes → acids; ketones → resistant to mild oxidizing agents.
• Reduction: Aldehydes and ketones → alcohols; acids → alcohols via LiAlH₄.
• Reactions of Acids: Formation of acid chlorides, esters, and amides; decarboxylation reactions.

6. Tests and Identification

• Tollen’s Test: Silver mirror test for aldehydes.
• Fehling’s Test: Red precipitate confirms aldehydes.
• 2,4-DNP Test: Forms yellow/orange ppt. with aldehydes and ketones.
• Sodium bicarbonate Test: CO₂ effervescence indicates carboxylic acid.

7. Important Reactions

• Aldol Condensation: Reaction between two aldehydes/ketones with α-hydrogens under basic conditions.
• Cannizzaro Reaction: Aldehydes without α-H undergo disproportionation.
• Hell-Volhard-Zelinsky Reaction: Halogenation of carboxylic acids at α-position.

8. Uses and Applications

• Aldehydes: Formaldehyde in disinfectants, acetaldehyde in perfumes and dyes.
• Ketones: Acetone as a solvent in nail polish remover, paint thinners.
• Carboxylic Acids: Acetic acid in vinegar, benzoic acid as a food preservative.