Factors that have an influence on the physical properties of organic compounds

1. Influence of Intermolecular Forces

• Weaker intermolecular forces (e.g. induced dipole-dipole forces) result in:
  • Lower boiling and melting points.
  • Higher vapour pressure (molecules escape easily).
• Stronger intermolecular forces (e.g. hydrogen bonding) result in:
  • Higher boiling and melting points.
  • Lower vapour pressure.

2. Influence of Functional Groups

• Hydroxyl (-OH) groups (found in alcohols and carboxylic acids) lead to:
  • Hydrogen bonding → Strong intermolecular forces.
  • Higher boiling points.
• Functional groups in alkanes, alkenes, alkynes, haloalkanes, aldehydes, ketones, and esters:
  • No hydrogen bonding.
  • Weaker forces (dipole-dipole or induced dipole).
  • Lower boiling points.

3. Influence of Chain Length

• Shorter carbon chains:
  • Fewer intermolecular forces.
  • Lower boiling and melting points.
  • Higher vapour pressure.
• Longer carbon chains:
  • More sites for intermolecular forces.
  • Higher boiling and melting points.
  • Lower vapour pressure.

4. Influence of Molecular Mass

• Smaller molecular mass:
  • Weaker intermolecular forces.
  • Lower boiling and melting points.
  • Higher vapour pressure.
• Larger molecular mass:
  • Stronger intermolecular forces.
  • Higher boiling and melting points.
  • Lower vapour pressure.

5. Influence of Branching in Carbon Chains

• More branching (alkyl groups):
  • Creates a more compact molecular shape.
  • Smaller surface area for intermolecular forces.
  • Weaker van der Waals forces → Lower boiling points.
• Fewer branches (longer straight chains):
  • Greater surface area for intermolecular forces.
  • Stronger van der Waals forces → Higher boiling points.

Key Takeaways

• Intermolecular forces, functional groups, molecular size, and chain structure all influence physical properties.
• Stronger forces → Higher boiling & melting points, Lower vapour pressure.
• Weaker forces → Lower boiling & melting points, Higher vapour pressure.