PM153 Lecture 3 - Non-covalent forces and forces between molecules

Non-covalent forces

  • Weaker than covalent or ionic bonds (10-50 times weaker)

  • Allow for tweaking of molecular shapes and interactions

  • Electrostatic in nature generally

  • Requires close proximity to have an effect

  • Main cause: non-polar nature of bonds and electronegativity

  • Consists of

    • Dispersion forces

    • Hydrophobic forces

    • Permanent dipole interactions

    • Hydrogen bonds

    • Ionic interactions

Dispersion forces

  • Electrons move within their orbitals

  • An uneven distribution causes electrical charge in the molecule

  • A negative area caused in this way can attract a positive area, causing an induced dipole

  • Characteristics of dispersion forces

    • Short lived

    • Very weak

    • Only work over very short distances

    • Prevalence is due to shape and size:

      • How many electrons are around to cause the force and how close can they get to each other

  • They are important as:

    • Any molecule can exhibit them as they are not limited by nature of the molecule

    • Can occur between two non-polar molecules

Hydrophobic forces