Physical properties and structure

1. Relationship Between Structure and Properties

• The structure of an organic compound affects its physical properties.
• This includes:
  • Boiling points
  • Melting points
  • Vapour pressure
• Intermolecular forces between molecules influence these properties.

2. Intermolecular Forces in Organic Compounds

• Intermolecular forces act between different molecules and are called Van der Waals forces.
• There are three main types of intermolecular forces:
  1. Induced dipole forces (weakest)
  2. Dipole-dipole forces
  3. Hydrogen bonds (strongest)

2.1 Induced Dipole Forces

• Also called London dispersion forces.
• Occur when electrons move, creating temporary charge imbalances.
• Causes weak attraction between molecules.
• Found in non-polar molecules, e.g. propane $(C₃H₈).$
• Leads to low boiling points as these forces are easily broken.

2.2 Dipole-Dipole Forces

• Found in polar molecules where one side is slightly positive and the other slightly negative.
• The opposite charges attract each other, creating a stronger force than induced dipole forces.
• Example: Propanal $(C₃H₆O).$

2.3 Hydrogen Bonds (Strongest)

• A strong type of dipole-dipole force.
• Occurs when H is bonded to a highly electronegative atom $(O, N, or F).$
• Present in alcohols (ethanol, methanol) and carboxylic acids.
• Increases boiling points significantly (e.g. water and ethanol).

3. Examples of Intermolecular Forces

Example	Intermolecular Forces Present
Propane $(C₃H₈)$	Induced dipole forces
Water $(H₂O)$	Hydrogen bonds
Ethanol $(C₂H₅OH)$	Hydrogen bonds and dipole-dipole forces
Methanoic acid $(HCOOH)$	Hydrogen bonds (stronger than in alcohols)

4. Key Takeaways

• Weaker forces → Lower boiling points.
• Stronger forces → Higher boiling points.
• Hydrogen bonding is the strongest intermolecular force and significantly impacts physical properties.