Dipole-Dipole Interactions (College Board AP® Chemistry)

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

Dipole-Dipole Interactions

Dipole-dipole interactions between molecules of acetonitrile, CH₃N in solid and liquid states.

• We can see the effect of dipole-dipole interactions in polar molecules by comparing the boiling points of acetonitrile and propane, C₃H₈

  • Acetonitrile is a polar molecule with a molecular weight of 41 amu that boils at 355 K

  • Propane is a nonpolar molecule with a molecular weight of 44 amu that boils at 213 K

• Dispersion forces are present in both molecules

  • Since both molecules have a similar molecular weight, the strength of dispersion forces will be similar

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

• Generally, the strength of dipole-dipole interaction is dependent on the magnitude of the dipole and the proximity of the molecules