Aldehydes & Ketones 

Structure of Carbonyl Group

• Carbonyl group (>C=O) consists of one σ-bond and one π-bond.
• Carbon is sp²-hybridised; oxygen uses p-orbital.
• π-electron cloud lies above and below the plane of the σ-bond.
• Bond is polar: C (δ⁺) – O (δ⁻).

Aldehydes & Ketones

• Aldehyde: R–CHO
• Ketone: R–CO–R′
• They are functional isomers of each other.

Nomenclature

• Suffix –al for aldehydes
• Suffix –one for ketones
• Examples:
  • But-2-en-1-al
  • Butane-2,3-dione
  • Benzophenone
  • 2-Methylcyclohexanone

General Methods of Preparation

1. Dry Distillation of Calcium Salts

• Calcium formate → Formaldehyde
• Calcium acetate + calcium formate → Acetaldehyde

2. Rosenmund Reduction

• Only aldehydes are formed.
• Acid chloride + H₂ → Aldehyde
• Catalyst: Pd–BaSO₄ (poisoned)
• Formaldehyde and ketones are not obtained.

3. From Nitriles (via Grignard Reagent)

• R–CN + RMgX → Ketone (after hydrolysis)
• HCN + RMgX → Aldehyde (after hydrolysis)

4. Hydroboration–Oxidation of Alkynes

• Terminal alkynes → Aldehydes
• Internal alkynes → Ketones

5. Wacker’s Process

• Ethene → Acetaldehyde
• Catalysts: PdCl₂, CuCl₂, O₂

6. Oxidation of Methyl Benzene

• Toluene → Benzaldehyde (controlled oxidation)

7. Etard Reaction

• Toluene + CrO₂Cl₂ → Benzaldehyde
• Solvent: CCl₄

Physical Properties

• Polar molecules due to C=O group
• Exhibit dipole–dipole interactions
• Ketones have slightly higher boiling points than isomeric aldehydes

Chemical Properties

Nucleophilic Addition Reactions

• Carbonyl carbon is electrophilic.
• Reaction proceeds via tetrahedral intermediate.
• Reactivity order:
  HCHO > Aldehydes > Ketones
• Reactivity decreases with +I effect and steric hindrance.

Important Nucleophilic Additions

• NaHSO₃ addition: Aldehydes > Ketones
• Alcohol:
  • Aldehyde → Hemiacetal → Acetal
  • Ketone → Hemiketal → Ketal
• Diols: Cyclic ketals formed
• HCN: Cyanohydrin formation (pH 9–10)

Addition of Ammonia Derivatives

• NH₂OH → Oxime
• NH₂NH₂ → Hydrazone
• Phenylhydrazine → Phenylhydrazone
• 2,4-DNP → 2,4-dinitrophenylhydrazone
• Semicarbazide → Semicarbazone

Oxidation of Aldehydes & Ketones

Strong Oxidising Agents

• KMnO₄, K₂Cr₂O₇, HNO₃
• Aldehydes → Carboxylic acids

Weak Oxidising Agents (Tests)

• Tollen’s reagent: Silver mirror test (aldehydes)
• Fehling’s solution: Brick-red Cu₂O precipitate
• Benedict’s solution: Positive for aliphatic aldehydes

Haloform Reaction

• Methyl ketones give CHX₃ (haloform)

Reduction Reactions

To Alcohols

• LiAlH₄, NaBH₄
• Aldehydes → 1° alcohols
• Ketones → 2° alcohols

To Alkanes

• Clemmensen reduction: Zn–Hg / HCl
• Wolff–Kishner reduction: NH₂NH₂, KOH, glycol

Reactions Involving α-Hydrogen

Aldol Condensation

• Aldehydes/ketones with α-H form aldol in presence of dilute NaOH.
• Aldol may dehydrate to form α,β-unsaturated compound.

Cross Aldol Condensation

• Occurs between two different carbonyl compounds.
• Gives mixture of self- and cross-aldol products.

Electrophilic Substitution (Aromatic)

• –CHO and –CO– groups are meta-directing.
• They are deactivating groups.
• Example: Nitration of benzaldehyde → m-nitrobenzaldehyde