Last Updated: May 2026
NEET Chemistry Aldehydes Ketones and Carboxylic Acids 2027 (NCERT Class 12 Chapter 12) is a 6-mark chapter on average — 1–2 direct + 1 mechanism + 1 conversion question per paper. This 1,800-word guide covers the IUPAC nomenclature, preparation methods, electrophilic addition (with mechanism), nucleophilic addition (with detailed arrow-pushing), oxidation/reduction reactions, and 40 practice problems including 6 conversions.
1. Carbonyl Functional Group
The C=O group with C bonded to one or two H atoms forms aldehyde (R–CHO); C bonded to two carbons forms ketone (R–CO–R’). Carboxylic acids contain –COOH. The C=O is polar (δ+ on C, δ- on O), making it electrophilic at C and nucleophilic at O.
2. IUPAC Nomenclature
| Compound | Common | IUPAC |
|---|---|---|
| HCHO | Formaldehyde | Methanal |
| CH3CHO | Acetaldehyde | Ethanal |
| CH3COCH3 | Acetone | Propan-2-one |
| C6H5CHO | Benzaldehyde | Benzaldehyde |
| HCOOH | Formic acid | Methanoic acid |
| CH3COOH | Acetic acid | Ethanoic acid |
| C6H5COOH | Benzoic acid | Benzoic acid |
3. Methods of Preparation
3.1 Aldehydes
- Oxidation of primary alcohol (controlled, with PCC or CrO3-pyridine).
- Dehydrogenation of primary alcohol over Cu/300°C.
- Rosenmund reduction: RCOCl + H2/Pd-BaSO4 → RCHO.
- Stephen reaction: RCN + SnCl2/HCl + H2O → RCHO.
- Etard reaction: toluene + CrO2Cl2 → benzaldehyde.
- Gattermann-Koch: benzene + CO/HCl + AlCl3 → benzaldehyde.
3.2 Ketones
- Oxidation of secondary alcohol (CrO3, Na2Cr2O7).
- From acid chloride + RCdCl (Grignard with Cd salt).
- Friedel-Crafts acylation (aromatic ketones).
3.3 Carboxylic Acids
- Oxidation of 1° alcohol or aldehyde (KMnO4 or K2Cr2O7).
- From nitrile (acid hydrolysis: RCN + H2O → RCOOH).
- From Grignard + CO2 → RCOOH.
4. Nucleophilic Addition — Mechanism
Step 1: Nucleophile (Nu⁻) attacks δ+ C of C=O. Step 2: Tetrahedral alkoxide intermediate forms. Step 3: Protonation of O⁻ by H+ → addition product. Aldehydes more reactive than ketones because (i) less steric hindrance, (ii) less +I effect on carbonyl C.
Examples of Nucleophilic Addition
- HCN + RCHO → RCH(OH)CN (cyanohydrin)
- NaHSO3 + RCHO → RCH(OH)SO3Na (bisulphite addition — purification)
- NH2-G + RCHO → RCH=N-G (imines, with –G = OH/NH2/NHR/NHC6H5)
- Alcohol + RCHO → RCH(OR’)2 (acetals)
5. Important Named Reactions
| Reaction | Reagent | Product |
|---|---|---|
| Cannizzaro | Conc. NaOH (no α-H aldehydes) | Alcohol + carboxylate |
| Aldol condensation | Dilute NaOH/Ba(OH)2 (α-H aldehydes/ketones) | β-hydroxy aldehyde/ketone → α,β-unsaturated |
| Clemmensen reduction | Zn-Hg/HCl | C=O → CH2 |
| Wolff-Kishner reduction | NH2NH2/KOH/glycol | C=O → CH2 |
| Tollens’ | Ag(NH3)2OH | Silver mirror (aldehydes only) |
| Fehling’s | Cu(II) tartrate complex | Red Cu2O ppt (aliphatic aldehydes) |
| HVZ (acids) | X2/red P | α-halo carboxylic acid |
| Decarboxylation | Soda lime (NaOH+CaO) | R-COOH → R-H + CO2 |
6. Acidity of Carboxylic Acids
pKa ≈ 3.5–5 (much more acidic than alcohols, pKa ≈ 16). Reasons: (i) resonance stabilisation of conjugate base (carboxylate), (ii) partial double-bond character of C–O bonds in carboxylate. Order: HCOOH > CH3COOH > propanoic. Halogenated acids (Cl, F) are stronger; α-halo > β-halo > γ-halo (–I effect distance).
7. Tests to Distinguish
- Tollens’ test: aldehydes give silver mirror; ketones don’t.
- Fehling’s test: aliphatic aldehydes give red ppt; aromatic aldehydes (benzaldehyde) and ketones don’t.
- Iodoform test: methyl ketones (CH3-CO-R) and ethanol give yellow CHI3.
- 2,4-DNP test: confirms C=O in both aldehydes and ketones.
8. 40 Practice Problems (Sample 10)
- Cannizzaro reaction is given by: (a) Acetaldehyde (b) Formaldehyde (c) Propanone (d) Both (a) and (c) — (b)
- Tollens’ reagent: (a) AgNO3+NH4OH (b) CuSO4 (c) Fehling A+B (d) DNP — (a)
- Most acidic among: (a) HCOOH (b) CH3COOH (c) C2H5COOH (d) CH3CH2CH2COOH — (a)
- Aldol product of acetaldehyde: (a) Acetone (b) Crotonaldehyde (c) 3-hydroxybutanal (d) Acetic acid — (c)
- Wolff-Kishner reduction converts C=O to: (a) CH-OH (b) CH2 (c) C-Cl (d) C-NH2 — (b)
- Most reactive towards nucleophilic addition: (a) HCHO (b) CH3CHO (c) CH3COCH3 (d) C6H5CHO — (a)
- Iodoform test gives positive result with all EXCEPT: (a) Ethanol (b) Acetaldehyde (c) Acetone (d) Methanol — (d)
- HVZ reaction is given by: (a) Aldehydes (b) Ketones (c) Carboxylic acids with α-H (d) Esters — (c)
- Etard’s reagent: (a) CrO2Cl2 (b) PCC (c) KMnO4 (d) NaBH4 — (a)
- Decarboxylation of sodium acetate gives: (a) Methane (b) Ethane (c) Ethanol (d) Acetic acid — (a)
Frequently Asked Questions
Q1. NEET 2027 weightage of this chapter?
3–4 questions = 12–16 marks. Top-5 organic chapter.
Q2. Mechanism vs reaction — what to memorise?
Memorise both for nucleophilic addition, aldol, Cannizzaro. Memorise reactions only (not mechanism) for Tollens’, Fehling’s, iodoform.
Internal Resources
[cg_quiz id=”neet-aldehydes-ketones-2027″]