Notes on Aldehydes and Ketones

Key Concepts

Carbonyls

• Compounds with a C=O bond.
• Can be either aldehydes or ketones.

Aldehydes

• C=O bond at the end of the chain with an H attached.
• Names end in '-al' (e.g., ethanal: CH₃CHO).

Ketones

• C=O bond in the middle of the chain.
• Names end in '-one' (e.g., propanone: CH₃COCH₃).

Solubility

• Smaller carbonyls are soluble in water due to hydrogen bonding capabilities.
• Pure carbonyls are attracted by permanent dipole forces, not hydrogen bonds.

Reactions

Comparison with Alkenes

• C=O bonds are stronger than C=C and do not easily undergo addition reactions.
• C=O bonds are polarized; oxygen is more electronegative, attracting nucleophiles.

Oxidation

• Primary alcohols can be oxidized to aldehydes and then to carboxylic acids.
• Tertiary alcohols do not oxidize.
• Secondary alcohols oxidize to ketones, which do not further oxidize.
• Potassium dichromate (K₂Cr₂O₇) is an oxidizing agent for alcohols and aldehydes.
  • Aldehydes oxidize to carboxylic acids.
  • Ketones do not oxidize further.

Example Reaction

• Aldehyde + [O] → Carboxylic Acid
  • Reagents: Potassium dichromate (VI) solution and dilute sulfuric acid.
  • Conditions: Heat under reflux.
  • Full equation: 3CH₃CHO + Cr₂O₇²⁻ + 8H⁺ → 3CH₃CO₂H + 4H₂O + 2Cr³⁺
• Observation: Orange dichromate ion (Cr₂O₇²⁻) reduces to green Cr³⁺ ion.

Use of Fehlings Solution and Tollens Reagent

• Tests for the presence of aldehyde groups.
• Fehlings Solution: Converts aldehydes to carboxylic acids, observed by red precipitate of Cu₂O.
• Tollens Reagent: Converts aldehydes to carboxylic acids, observed by silver mirror.

Reduction of Carbonyls

• Reducing agents: NaBH₄ (sodium tetrahydridoborate) or LiAlH₄ (lithium tetrahydridoaluminate).
  • Aldehydes reduce to primary alcohols.
  • Ketones reduce to secondary alcohols.
• Reagents: NaBH₄ in aqueous ethanol.
• Conditions: Room temperature and pressure.
• Catalytic hydrogenation possible with hydrogen and nickel catalyst.

Addition of Hydrogen Cyanide to Carbonyls

• Forms hydroxynitriles.
• Reagent: Potassium cyanide (KCN) and dilute sulfuric acid.
• Conditions: Room temperature and pressure.
• Mechanism: Nucleophilic addition.
• KCN supplies nucleophilic CN⁻ ions; H₂SO₄ supplies H⁺ ions.
• HCN can be used but is toxic; KCN/NaCN are preferred despite their own toxicity.
• Reaction results in formation of racemate due to equal chance of enantiomer formation, leading to no optical activity.