Dipole-Dipole Interactions (College Board AP® Chemistry): Revision Note

Dipole-Dipole Interactions

• Dipole-dipole interactions are a consequence of permanent dipole moments in polar molecules

• The dipole-dipole force is an attractive intermolecular force

  • This is because polar molecules tend to arrange themselves so that the positive end of one molecule is near the negative end of another

• Repulsions can also occur when the positive (or negative) ends of two molecules are in close proximity

• To illustrate this force, consider the way the molecules of acetonitrile, CH₃CN are packed in the liquid and solid states

  • In the solid state, they are arranged with the negatively charged nitrogen end of the molecule aligned with the positively charged —CH₃ end

  • In the liquid state, however, as a result of less restriction in the movement of molecules, the arrangement is disordered

    • This means both attractive and repulsive forces are present at any instant

Solid and liquid acetonitrile

Dipole-dipole interactions between molecules of acetonitrile, CH₃CN, in the solid and liquid states

• Comparing the boiling points of acetonitrile and propane, C₃H₈, shows the effect of dipole-dipole interactions

  • Acetonitrile is a polar molecule with a molar mass of 41 g/mol that boils at 355 K

  • Propane is a nonpolar molecule with a molar mass of 44 g/mol that boils at 231 K

• Dispersion forces are present in both molecules

  • Both molecules have a similar number of electrons (22 and 26), so they are similarly polarizable and their dispersion forces are similar

• However, the additional presence of a dipole-dipole force in acetonitrile gives it a higher boiling point

• The strength of dipole-dipole interactions depends on the magnitudes of the dipoles and on the molecules' relative orientation