London Dispersion Forces (College Board AP® Chemistry): Revision Note

What Are Intermolecular Forces?

• Intermolecular forces are attractions between separate particles, such as molecules, or ions and molecules

• All intermolecular interactions are electrostatic

  • This means that they involve attractions between positive and negative species, much like ionic bonds

• Intermolecular forces are generally weaker than intramolecular forces, such as ionic bonds, covalent bonds and metallic bonds

• Intermolecular forces are responsible for the physical properties of covalent molecules, including:

  • Boiling point

  • Melting point

  • Density

  • Viscosity

  • Solubility

• For example, the stronger the intermolecular forces in a substance, the higher its boiling point (melting points usually follow the same trend)

Intermolecular vs intramolecular forces

London Dispersion Forces

• London dispersion forces are Coulombic attractions between temporary, fluctuating (instantaneous) dipoles

• They act between all molecules, polar and nonpolar, and are often the strongest net intermolecular force between large molecules

• The average distribution of electrons in a nonpolar molecule is symmetrical

  • This means that it has no permanent dipole moment

• However, at any instant, the electrons in an atom or molecule may be unevenly spread, creating an instantaneous dipole

  • One side develops a partial negative charge, δ− and the other side develops a partial positive charge, δ+

• This dipole then induces a dipole in a neighboring particle, and the two attract weakly

Instantaneous and induced dipoles

• The strength of London forces is influenced by polarizability

• Polarizability describes how easily the electron cloud of a molecule can be distorted to give an instantaneous dipole

• The greater the polarizability, the more easily the electron cloud can be distorted

  • This results in a stronger dispersion force

• Polarizability increases with the number of electrons and the size of the electron cloud, and is enhanced by pi bonding

  • So dispersion forces are stronger between molecules with more electrons: justify this with the number of electrons, not molar mass

• For example, the boiling points of the halogens and the noble gases increase down each group, because dispersion forces are the only intermolecular forces in these substances

Boiling points of halogens and noble gases

• Isomers have the same number of electrons, so their shape decides the contact area between molecules

  • A more branched, more compact molecule has less contact area with its neighbors, so its dispersion forces are weaker

• For example, pentane, 2-methylbutane and 2,2-dimethylpropane all have the same molecular formula, C₅H₁₂, and the same molar mass, 72.15 g/mol

  • However, they differ in the arrangement of their atoms

  • Pentane has no branching and is the least compact

  • 2-methylbutane has one branch, making it more compact than pentane

  • 2,2-dimethylpropane has two branches, making it more compact than 2-methylbutane

  • This increase in branching/compactness lowers the boiling point from pentane (36 °C) to 2-methylbutane (28 °C) to 2,2-dimethylpropane (9.5 °C)

Branching and contact area