Aldehydes & Ketones Lecture Notes (CH4001/CH4005/PY4140)

Aldehydes & Ketones - Lecture Notes

Overview of Key Reactions

  • Hydration of Carbonyl Compounds:
    • Aldehydes and ketones can react with water to form hydrates.
    • This leads to the formation of a hemiacetal (from aldehyde) or a hemiketal (from ketone).

Hemiacetals and Hemiketals

  • Formation:
    • Similar to hydration, but involves reaction with alcohols.
    • Important to note that the product can revert, with equilibrium generally favoring the reactants (left side).
  • Stability:
    • Generally unstable; notable exception occurs in sugars, where they can form more stable structures.

Continuous Reaction Leading to Acetals and Ketals

  • Mechanism:

    • Hemiacetals and hemiketals can further react where remaining -OH is replaced by -OR from an alcohol, leading to:
    • Acetal from hemiacetal
    • Ketal from hemiketal

  • Condition for Stability:

    • More stable when the product is cyclic (R groups in alcohols connect or when using a diol).

Reaction with Ammonia and Derivatives

  • Process:
    • Involves nucleophilic addition across the C=O bond, followed by dehydration leading to formation of a C=N bond.
  • Optimal pH:
    • Best at pH 4:
    • Too acidic: Amine gets protonated, losing nucleophilic character.
    • Not acidic enough: Dehydration step fails.

Examples of Nucleophilic Addition

  • Consider nucleophile NH2X, with notable cases:
    • X = H (NH3): produces imine
    • X = R’ (amines): produces another imine
    • X = OH (hydroxylamine): produces oxime
    • X = NH2 (hydrazine): produces hydrazone
    • X = NHC6H5 (phenylhydrazine): produces phenylhydrazone
    • X = NHC6H3(NO2)2 (2,4-dinitrophenylhydrazine):
    • Forms a 2,4-dinitrophenylhydrazone, used as Brady’s reagent for qualitative testing of aldehydes/ketones (forms orange/red precipitate).

Oxidation of Carbonyl Compounds

  • Further Oxidation:
    • Aldehydes can oxidize to carboxylic acids using agents such as:
    • CrO_3 ,
    • KMnO_4 ,
    • H2CrO4 .
  • Ketones:
    • Do not undergo simple further oxidation without losing an alkyl group.

Reduction of Carbonyl Compounds

  • Process of Reduction:

    • Aldehydes reduce to primary alcohols and ketones to secondary alcohols
    • Common reducing agents:
    • NaBH_4
    • LiAlH_4 , followed by aqueous acid.

  • Mechanism:

    • Occurs via nucleophilic addition of hydrogen across the C=O bond, may lead further reduction to CH_2 in some cases depending on reagents used.

  • Considerations:

    • Choice between reducing methods can depend on the presence of other functional groups sensitive to acid or base conditions.