Organic Chemistry: Aldehydes, Ketones and Carboxylic Acids

1. Introduction

Aldehydes (-CHO) and ketones (C=O with two alkyl/aryl groups) are carbonyl compounds. Carboxylic acids (-COOH) are the most common organic acids. These are among the most versatile functional groups.

2. Aldehydes and Ketones

2.1 Nomenclature

Aldehydes end in -al, ketones in -one. Aldehyde gets priority numbering (CHO = C1).

2.2 Preparation

1. Oxidation of 1° alcohols (→ aldehyde) and 2° alcohols (→ ketone).
2. Ozonolysis of alkenes.
3. Friedel-Crafts acylation (for aromatic ketones).
4. From acyl chlorides (Rosenmund reduction for aldehydes).

2.3 Nucleophilic Addition Reactions

1. Addition of HCN: RCHO + HCN → Cyanohydrin. Used in synthesis of α-hydroxy acids.
2. Addition of NaHSO₃: Crystalline bisulphite addition product.
3. Addition of Grignard reagent: → Alcohols.
4. Addition of alcohols: → Hemiacetal → Acetal.

2.4 Reduction

Clemmensen reduction (acidic): C=O → CH₂ (with Zn-Hg/HCl). Wolf-Kishner reduction (basic): C=O → CH₂ (with NH₂NH₂/KOH).

2.5 Oxidation

Aldehydes oxidise easily to carboxylic acids. Ketones require strong oxidising conditions.

Tollens' test: Aldehyde + [Ag(NH₃)₂]⁺ → Silver mirror. Fehling's test: Aldehyde + Cu²⁺ (alkaline) → Red Cu₂O precipitate. Iodoform test: CH₃CO- group or CH₃CH(OH)- gives yellow CHI₃ precipitate.

2.6 Distinction Tests for Aldehydes and Ketones

'Tollens' and Fehling's tests both distinguish aldehydes from ketones. However, aromatic aldehydes (like benzaldehyde) do not give Fehling's test — they give Tollens' test.'

3. Aldol and Cannizzaro Reactions

3.1 Aldol Condensation

Two aldehydes/ketones with α-hydrogen atoms form β-hydroxy carbonyl compounds in dilute base.

3.2 Cannizzaro Reaction

Aldehydes without α-hydrogen (HCHO, C₆H₅CHO) undergo self-oxidation and reduction in concentrated base to give alcohol and acid.

4. Carboxylic Acids

4.1 Preparation

1. Oxidation of 1° alcohols and aldehydes.
2. Hydrolysis of nitriles.
3. From Grignard reagent (with CO₂).

4.2 Acidity

Carboxylic acids are more acidic than phenols. Electron-withdrawing groups increase acidity; electron-donating groups decrease it.

4.3 Reactions

1. Esterification: RCOOH + R'OH → RCOOR' + H₂O.
2. Reduction: RCOOH → RCH₂OH (with LiAlH₄).
3. Decarboxylation: RCOONa + NaOH(CaO) → RH + Na₂CO₃.
4. Formation of acyl chlorides, amides, and anhydrides.

5. Worked Problems

Problem 1: How can you distinguish between acetaldehyde and acetone? Solution: Tollens' test — acetaldehyde gives silver mirror, acetone does not. Iodoform test — both give yellow precipitate (both have CH₃CO- group).

Problem 2: Write the products of aldol condensation of ethanal. Solution: 2CH₃CHO → CH₃CH(OH)CH₂CHO (3-hydroxybutanal) → crotonaldehyde (on heating).

6. Common Mistakes

'Students often think the Cannizzaro reaction occurs for all aldehydes. It only works for aldehydes without α-hydrogen atoms. Aldehydes with α-hydrogen undergo aldol condensation.'

7. ISC Exam Focus

8. Self-Test Questions

1. Describe the mechanism of the aldol condensation reaction with an example.
2. Explain why carboxylic acids are stronger acids than phenols.
3. Distinguish between formaldehyde and acetaldehyde using suitable tests.
4. Write the product when benzaldehyde reacts with (i) conc. KOH (ii) NH₂NH₂/KOH.
5. Complete: CH₃CH₂COOH + SOCl₂ → ? + PBr₃ → ? + NH₃ → ?

9. Quick Revision — Distinction Tests

'Always use freshly prepared Tollens' reagent. Old reagent may form explosive silver azide. Fehling's test is negative for aromatic aldehydes like benzaldehyde.'