Aldehydes, Ketones and Carboxylic Acids
Why This Chapter Matters
Carbonyl compounds have 8-12 marks in JEE. Named reactions (Aldol, Cannizzaro, Clemmensen), nucleophilic addition mechanism, and acid/ester reactions are all tested.
Core Concepts
1. Preparation Methods
Aldehydes (RCHO):
From alcohols: RCH₂OH →(PCC or KMnO₄ limited/Cr₂O₃/Al₂O₃) RCHO
Ozonolysis of alkenes: RCH=CHR' → RCHO + R'CHO (with Zn/H₂O)
Gattermann-Koch: Ar-H + CO + HCl →(AlCl₃) Ar-CHO
Rosenmund reduction: RCOCl + H₂ →(Pd/BaSO₄) RCHO
Ketones (RCOR'):
From secondary alcohols: oxidation.
Friedel-Crafts acylation: Ar-H + RCOCl →(AlCl₃) Ar-CO-R
2. Nucleophilic Addition to C=O
π bond breaks, nucleophile attacks electron-deficient C.
Nu⁻ + C=O → Nu-C-O⁻ → Nu-C-OH (after protonation)
Aldehydes more reactive than ketones (less steric hindrance + carbonyl C more electrophilic)
Reactions:
Addition of HCN: RCHO + HCN → RCH(OH)CN (cyanohydrin) — important intermediate
Addition of Grignard: RCHO + R'MgX → R-CH(OH)-R' (gives secondary alcohol)
3. Named Reactions (Must Know for JEE!)
Aldol condensation: α-carbon attacks carbonyl of another molecule. Forms β-hydroxy carbonyl compound.
2CH₃CHO →(NaOH, cold) CH₃CH(OH)CH₂CHO (aldol) →(heat) CH₃CH=CHCHO (crotonaldehyde)
Cannizzaro reaction: Aldehydes WITHOUT α-hydrogen disproportionate in strong base.
2HCHO + NaOH → CH₃OH + HCOONa (formaldehyde → methanol + formate)
2PhCHO + NaOH → PhCH₂OH + PhCOONa (crossed Cannizzaro with formaldehyde)
Clemmensen reduction: C=O → CH₂ (acidic conditions, Zn-Hg/HCl)
Wolff-Kishner reduction: C=O → CH₂ (basic conditions, N₂H₄/KOH)
Tollen's test: RCHO + [Ag(NH₃)₂]⁺ + OH⁻ → RCOO⁻ + Ag↓ (silver mirror)
Distinguishes aldehyde from ketone (aldehydes give positive Tollen's)
Fehling's test: RCHO + Cu²⁺ (blue) → RCOO⁻ + Cu₂O↓ (brick red)
Works for aliphatic aldehydes. Aromatic aldehydes (benzaldehyde) do NOT give Fehling's test.
4. Carboxylic Acids (RCOOH)
Acidic nature: RCOOH ⇌ RCOO⁻ + H⁺
pKa typically 4-5 (stronger than carbonic acid, weaker than HCl)
EWG increase acidity: Cl₃CCOOH >> Cl₂CHCOOH > ClCH₂COOH >> CH₃COOH
Reactions:
With NaOH: RCOOH + NaOH → RCOONa + H₂O (neutralisation)
With SOCl₂: RCOOH → RCOCl (acid chloride) — most reactive derivative
Esterification: RCOOH + R'OH ⇌(H⁺) RCOOR' + H₂O (reversible, Fischer esterification)
Decarboxylation: RCOONa + NaOH →(CaO, heat) R-H + Na₂CO₃ (soda lime)
PYQs
2024: Which aldehyde does NOT give Aldol condensation?
HCHO (formaldehyde) — no alpha H. Also, benzaldehyde (C₆H₅CHO) has no α-H.
2023: Distinguish between ethanal (CH₃CHO) and propanone (CH₃COCH₃) using ONE chemical test.
Tollen's test: ethanal gives silver mirror. Propanone (ketone) does not.
Alternatively: Fehling's test — ethanal gives brick red ppt, propanone does not.
2022: Arrange in order of increasing boiling point: CH₃CHO, CH₃CH₂OH, CH₃COOH
CH₃CHO < CH₃CH₂OH < CH₃COOH
(Aldehyde: dipole-dipole | Alcohol: H-bonding | Acid: stronger H-bonding + dimer formation)