Aldehydes, Ketones and Carboxylic Acids (JEE)
1. Introduction to Carbonyl Compounds
Aldehydes and ketones contain the carbonyl group ($\ce{C=O}$).
Carboxylic acids contain the carboxyl group ($\ce{-COOH}$).
The carbonyl carbon is sp² hybridized and electrophilic.
2. Structure of Carbonyl Group
$\ce{C=O}$ bond consists of one $\sigma$ bond and one $\pi$ bond.
Oxygen is more electronegative, hence carbon becomes partially positive.
3. Nomenclature (IUPAC)
| Compound | Suffix |
|---|---|
| Aldehyde | -al |
| Ketone | -one |
| Carboxylic Acid | -oic acid |
4. Methods of Preparation of Aldehydes
- Oxidation of primary alcohols
- Rosenmund reduction
- Ozonolysis of alkenes
$\ce{R-COCl + H2 ->[Pd/BaSO4] R-CHO}$
5. Methods of Preparation of Ketones
- Oxidation of secondary alcohols
- Friedel–Crafts acylation
$\ce{RCOCl + AlCl3 -> R-CO-R'}$
6. Preparation of Carboxylic Acids
- Oxidation of aldehydes
- Hydrolysis of nitriles
$\ce{R-CN + 2H2O -> R-COOH + NH3}$
7. Physical Properties
- Lower aldehydes are soluble in water
- Carboxylic acids have high boiling point due to H-bonding
8. Chemical Properties – Nucleophilic Addition
Aldehydes and ketones undergo nucleophilic addition reactions.
$\ce{R-CHO + HCN -> R-CH(OH)-CN}$
9. Reduction Reactions
$\ce{R-CHO ->[NaBH4] R-CH2OH}$
$\ce{R-CO-R' ->[LiAlH4] R-CH(OH)-R'}$
10. Oxidation Reactions
Aldehydes are easily oxidized, ketones are resistant.
$\ce{R-CHO + [O] -> R-COOH}$
11. Tollens’ Test
Aldehydes reduce Tollens’ reagent to give silver mirror.
$\ce{R-CHO + 2[Ag(NH3)2]^+ -> R-COO^- + 2Ag}$
12. Fehling’s Test
Aliphatic aldehydes give red precipitate of $\ce{Cu2O}$.
13. Aldol Condensation
Aldehydes or ketones with $\alpha$-hydrogen undergo aldol condensation.
$\ce{2CH3CHO ->[NaOH] CH3CH(OH)CH2CHO}$
14. Cannizzaro Reaction
Aldehydes without $\alpha$-hydrogen undergo disproportionation.
$\ce{2HCHO + NaOH -> HCOONa + CH3OH}$
15. Acidity of Carboxylic Acids
Carboxylic acids are acidic due to resonance stabilization of conjugate base.
Electron-withdrawing groups increase acidity.
16. Hell–Volhard–Zelinsky Reaction
$\ce{R-CH2-COOH ->[Br2/P] R-CHBr-COOH}$
17. Esterification
$\ce{R-COOH + R'-OH ->[H^+] R-COOR' + H2O}$
18. Decarboxylation
$\ce{R-COONa ->[NaOH/CaO] R-H + Na2CO3}$
19. Comparison: Aldehydes vs Ketones
| Property | Aldehyde | Ketone |
|---|---|---|
| Oxidation | Easy | Difficult |
| Tollens Test | Positive | Negative |
20. Common JEE Traps
- Confusing oxidation behavior of aldehydes & ketones
- Ignoring presence of $\alpha$-hydrogen
- Forgetting resonance effect on acidity
21. Final Revision Checklist
You have mastered this chapter if you can:
- Identify functional groups
- Predict reactions correctly
- Differentiate aldehydes & ketones
- Explain acidity trends
- Solve JEE reaction-based questions