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Physical and Chemical properties of Carbonyl Compounds (Aldehydes and ketones)

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Physical Properties of Carbonyl Compounds

Molecular Structure

  • The carbonyl group consists of a carbon atom double-bonded to an oxygen atom, creating a site of high electron density.

  • In aldehydes, the carbonyl group is bonded to a hydrogen atom and an alkyl or aryl group. In ketones, it is bonded to two alkyl or aryl groups.

Polarity

  • The carbon-oxygen bond is highly polar due to the electronegativity difference between the atoms, resulting in a significant dipole moment.

Boiling Points

  • Aldehydes and ketones have higher boiling points than hydrocarbons of similar molecular weight due to dipole-dipole interactions.

  • Their boiling points are lower than alcohols and carboxylic acids, as they cannot form hydrogen bonds as effectively.

Solubility

  • They are generally soluble in organic solvents and somewhat soluble in water.

  • Lower molecular weight aldehydes and ketones are more water-soluble because they can form hydrogen bonds with water molecules.

Chemical Properties

Nucleophilic Addition

  • The electrophilic carbon in the carbonyl group is susceptible to attack by nucleophiles.

  • This forms the basis of many reactions, such as with amines, hydrides, and cyanides.

Oxidation and Reduction

  • Aldehydes are easily oxidized to carboxylic acids by oxidizing agents like KMnO₄ or K₂Cr₂O₇.

  • Ketones are more resistant to oxidation but can be reduced to alcohols using reducing agents like NaBH₄ or LiAlH₄.

Formation of Hemiacetals and Acetals

  • Aldehydes and ketones can react with alcohols to form hemiacetals and acetals under acid catalysis.

Aldol Condensation

  • Aldehydes and ketones can undergo aldol condensation in the presence of a base or acid, forming β-hydroxy aldehydes or ketones.

Cannizzaro Reaction

  • Aldehydes without an α-hydrogen undergo the Cannizzaro reaction with strong bases, producing a mixture of a carboxylic acid and an alcohol.

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