1. Aldol Condensation (Wurtz, 1872)

• Condition: Dilute NaOH (or dilute acid)
• Requirement: Alpha-hydrogen MUST be present
• Substrate: Aldehydes or ketones with alpha-H
• Products:
  • Stage 1 (addition): Beta-hydroxy carbonyl compound (aldol)
  • Stage 2 (dehydration, heat): Alpha,beta-unsaturated carbonyl compound
• Key example: 2 CH3CHO → CH3CH(OH)CH2CHO → CH3CH=CHCHO + H2O

2. Cannizzaro Reaction (Cannizzaro, 1853)

• Condition: Concentrated NaOH
• Requirement: NO alpha-hydrogen
• Substrate: HCHO, C6H5CHO, (CH3)3CCHO
• Products: RCOO-Na+ (oxidized) + RCH2OH (reduced) — disproportionation
• Crossed Cannizzaro: HCHO is preferentially oxidized

3. Haloform Reaction / Iodoform Test

• Condition: I2 + NaOH (or NaOI)
• Requirement: CH3CO- group (or compound oxidizable to one)
• Products: CHI3 (yellow iodoform, CHI3 SMILES: IC(I)I) + RCOONa
• Stoichiometry: 3 I2 + 3 NaOH per CH3CO- group
• Positive compounds: CH3CHO, CH3COR, C6H5COCH3, CH3CH2OH, (CH3)2CHOH

4. Clemmensen Reduction (Clemmensen, 1912)

• Condition: Zn-Hg amalgam + conc. HCl, reflux (ACIDIC)
• Substrate: Aldehyde or ketone (C=O)
• Product: Methylene (CH2) — complete deoxygenation

5. Wolff-Kishner Reduction (1911-1946)

• Condition: NH2NH2 + KOH in ethylene glycol, heat (BASIC)
• Intermediate: Hydrazone (C=NNH2)
• Product: Methylene (CH2) — complete deoxygenation
• Preferred when acid-sensitive groups are present

6. Tollens' Test (Tollens, 1882)

• Reagent: [Ag(NH3)2]+ (ammoniacal AgNO3)
• Positive result: Silver mirror (Ag deposited)
• Selective for: Aldehydes ONLY
• Equation: RCHO + 2[Ag(NH3)2]+ + 2OH- → RCOO- + 2Ag↓ + 4NH3 + H2O