dipole-dipole interactions
Summary
Dipole-dipole interactions are attractions between the partial positive end of one polar molecule and the partial negative end of another. For molecules of similar size and mass, they typically make boiling points higher than dispersion forces alone would.
Detail
Meaning: A polar molecule has a net dipole moment, which arises when polar bonds are arranged asymmetrically. Neighboring polar molecules tend to orient so the δ+ end of one sits near the δ− end of another, and the net electrostatic attraction holds them together. This is one kind of intermolecular (van der Waals) force, alongside London dispersion forces. Hydrogen bonding is a especially strong form of dipole-dipole attraction, seen when H is bonded to N, O, or F. Stronger intermolecular attractions usually mean higher boiling and melting points, a higher enthalpy of vaporization, and lower vapor pressure.
On the MCAT: A passage often gives boiling points for compounds of similar molar mass, such as an aldehyde or ketone versus an alkane, and asks you to explain or predict the order. A typical ranking for similar size is ionic > hydrogen bonding > dipole-dipole > dispersion only. The idea also supports "like dissolves like": polar solutes tend to dissolve in polar solvents because dipole-dipole attractions replace those lost between solute molecules.
Common confusion: Polar bonds do not guarantee a polar molecule. CO₂ and CCl₄ have polar bonds, but their symmetry cancels the dipoles, so they have no net dipole and attract mainly through dispersion forces. Also, compare size before deciding: a large nonpolar molecule can have stronger total dispersion forces than a small polar one.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.