Factors Affecting the Boiling Point of Molecules

1. Polarity

• More polar molecules have higher boiling points.
• Stronger interactions between polar molecules increase boiling points.

2. Size

• Larger molecules have higher boiling points.
• Example: Alkanes (methane, ethane, propane to decane) are non-polar, yet their boiling points increase with size:
  • Methane: -162°C (natural gas)
  • Butane: 0°C (lighter fluid)
  • Hexane: ~70°C (paint thinner)
  • Decane/Octane/Nonin: >100°C (gasoline)
• Larger non-polar molecules exhibit increased boiling points due to greater dispersion forces.
  • Example: Wax (alkane chains 18-22 carbons) has boiling points of 200-300°C.
• Dispersion forces:
  • Random electron fluctuations cause weak partial charges.
  • Many weak interactions throughout a large molecule add up to strong forces.

3. Geometry

• Affects how well molecules pack together.
• Well-packed molecules increase boiling points.
• Poorly packed molecules decrease boiling points.
• More discussion on geometry in context of cis/trans configurations on lipids.

Additional Notes

• Intermolecular forces affect both boiling and melting points.
• Example of small vs. large diatomic molecules:
  • Fluorine (F2) is gaseous at room temp.
  • Iodine (I2) is solid at room temp, due to larger size and increased interactions.

Summary

• Main factors affecting boiling/melting points:
  1. Polarity (most significant)
  2. Size (also significant)
  3. Geometry (less significant than polarity and size)