CBSE Class 12 Chemistry: Aldehydes & Ketones — Notes 2026

Structure and Nomenclature

• Aldehyde: −CHO group; carbonyl carbon attached to H and organic group; methanal (HCHO), ethanal (CH₃CHO)
• Ketone: >C=O; carbonyl between two carbon atoms; propanone (CH₃COCH₃)
• IUPAC: parent chain includes C=O; suffix -al (aldehyde), -one (ketone); number from closest end to C=O

Preparation Methods

• Oxidation: 1° alcohol → aldehyde (PCC, Cr₂O₃); 2° alcohol → ketone (Na₂Cr₂O₇/H₂SO₄)
• Rosenmund reduction: RCOCl + H₂ (Pd/BaSO₄) → RCHO + HCl; Baeyer-Villiger: ketone → ester with peracid
• Ozonolysis: alkene + O₃ → ozonide → RCHO + R₂CO (reductive workup) or RCOOH + R₂C=O (oxidative)

Nucleophilic Addition Reactions

• C=O is electrophilic (carbonyl C); Nu⁻ attacks → tetrahedral intermediate → protonation → product
• Addition of HCN: gives cyanohydrin; RCHO + HCN → RCH(OH)CN (α-hydroxy nitrile)
• Grignard: RMgX + RCHO → 2° alcohol; R₂CO → 3° alcohol; HCHO → 1° alcohol

Distinction Tests

• Tollens' test (silver mirror): aldehydes oxidised to carboxylate; ketones do not respond
• Fehling's test: Benedict's/Fehling's solution (Cu²⁺); aliphatic aldehydes give brick red precipitate; aromatic aldehydes do not
• Iodoform test: CH₃CO− group → CHI₃ (yellow precipitate); acetaldehyde, acetone, ethanol (oxidised to acetaldehyde)

Aldol Condensation

• Self-condensation of aldehydes with α-H in base: 2 CH₃CHO → CH₃CH(OH)CH₂CHO (aldol)
• Aldol: β-hydroxy carbonyl compound; on heating → α,β-unsaturated carbonyl (aldol dehydration)
• Cross-aldol: two different carbonyl compounds; mixture of products; industrial use in synthesis

Cannizzaro Reaction

• Aldehydes without α-H (HCHO, benzaldehyde) undergo disproportionation in base
• 2 RCHO + NaOH → RCOO⁻Na⁺ + RCH₂OH; one oxidised, one reduced
• Industrial use: HCHO → methanol + formate; benzaldehyde → benzyl alcohol + benzoic acid

CBSE Board Focus

• Aldehydes & Ketones: 6–8 marks; Tollens'/Fehling's/iodoform tests, aldol condensation mechanism
• Numerical/mechanism: nucleophilic addition steps for HCN addition to ethanal
• Distinguish aldehyde from ketone: 4 tests; at least 2 with equations in CBSE