Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 15.7 Hydrogen bonding and properties-Study Notes - New Syllabus

15.7 Physical Properties and Solubility of Aldehydes and Ketones

Aldehydes and ketones contain the polar carbonyl group:

\( \mathrm{C=O} \)

The polarity of this group strongly influences their physical properties and solubility behaviour.

(i) Aldehydes and Ketones Do Not Form Intermolecular Hydrogen Bonds

Reason

• Hydrogen bonding requires:
  • A highly electronegative atom (N, O or F)
  • A hydrogen directly bonded to it

Aldehydes and ketones contain oxygen atoms, but:

• They do not contain \( \mathrm{O-H} \) bonds.
• Therefore, molecules cannot hydrogen bond to each other.

Intermolecular Forces Present

• Permanent dipole–dipole attractions
• London dispersion forces

Effect on Physical Properties

• Boiling points are:
  • Higher than alkanes of similar Mr (due to polarity)
  • Lower than alcohols of similar Mr (no hydrogen bonding)

Example Comparison

• Propanone has higher boiling point than propane.
• Propanone has lower boiling point than propan-1-ol.

(ii) Aldehydes and Ketones Can Form Hydrogen Bonds with Water

Reason

• Oxygen atom in carbonyl group has lone pairs.
• These lone pairs can hydrogen bond with water molecules.

Hydrogen Bond Formation

• Water donates hydrogen bond.
• Carbonyl oxygen accepts hydrogen bond.

Effect on Solubility

• Lower aldehydes and ketones are highly soluble in water.
• Solubility decreases as carbon chain length increases.

Reason for Decreasing Solubility

• Hydrocarbon chain is non-polar.
• Larger non-polar region reduces overall interaction with water.

Examples

• Methanal and ethanal are very soluble.
• Larger ketones become less soluble.

Summary of Effects

Key Features

• Aldehydes and ketones are polar molecules.
• Cannot hydrogen bond to themselves.
• Can hydrogen bond with water.
• Boiling points intermediate between alkanes and alcohols.
• Smaller molecules are highly water-soluble.