Physical Properties of Ketones and Aldehydes

Physical Properties of Aldehydes and Ketones

The physical properties of aldehydes and ketones, including boiling point, melting point, and solubility, are significantly influenced by the presence of the carbonyl (C=O) group and the resulting intermolecular forces.

Boiling Point

  • Comparison to Alkanes, Alkenes, and Ethers: Aldehydes and ketones have boiling points that are *higher* than alkanes, alkenes, and ethers with the same number of carbon atoms. This is due to the presence of the polar carbonyl group, which allows for stronger dipole-dipole interactions.
  • Comparison to Alcohols: Aldehydes and ketones have boiling points that are *lower* than alcohols with the same number of carbon atoms. This is because aldehydes and ketones *cannot* form hydrogen bonds with themselves (they lack an O-H bond). While the carbonyl group is polar, it is not as effective at intermolecular interactions as the -OH group.
  • Effect of Size: Boiling points increase with increasing molecular size (number of carbon atoms) due to stronger London dispersion forces.
  • Branching: Branching *decreases* the boiling point, similar to other organic compounds.

Melting Point

  • Trends: Melting point trends for aldehydes and ketones are influenced by factors like molecular packing and symmetry, similar to other organic compounds. There is no simple trend comparing them to alkanes, alkenes, ethers, or alcohols.

Solubility

  • Solubility in Water: Small aldehydes and ketones (up to about four or five carbon atoms) are moderately soluble in water. The polar carbonyl group can interact with water molecules through dipole-dipole interactions and some hydrogen bonding with the water's hydrogens. However, as the carbon chain length increases, the nonpolar character of the alkyl groups becomes dominant, and water solubility *decreases*.
  • Solubility in Organic Solvents: Aldehydes and ketones are soluble in most common organic solvents.

Summary Table (Illustrative Trends)

Compound Type Boiling Point Trend Water Solubility Trend Key Intermolecular Force
Alkane Increases with size Insoluble London Dispersion
Alkene Similar to alkanes Insoluble London Dispersion (sometimes weak dipole-dipole in *cis*)
Alkyne Slightly higher than alkanes/alkenes Insoluble London Dispersion
Ether Lower than alcohols, similar to alkanes Slightly soluble (small), Insoluble (large) Dipole-Dipole, London Dispersion
Epoxide Slightly higher than corresponding alkenes Slightly soluble (small), Insoluble (large) Dipole-Dipole, London Dispersion
Aldehyde Higher than alkanes, alkenes, ethers; lower than alcohols Moderately soluble (small), Insoluble (large) Dipole-Dipole, London Dispersion
Ketone Higher than alkanes, alkenes, ethers; lower than alcohols Moderately soluble (small), Insoluble (large) Dipole-Dipole, London Dispersion
Alcohol Much higher Soluble (small), Insoluble (large) Hydrogen Bonding
Key Takeaways
  • Aldehydes and ketones have boiling points higher than alkanes, alkenes, and ethers due to dipole-dipole interactions, but lower than alcohols because they cannot form hydrogen bonds with themselves.
  • Small aldehydes and ketones are moderately water-soluble, but larger ones are not.
  • Aldehydes and ketones are soluble in organic solvents.