Dipole-dipole
Summary
A dipole-dipole force is the intermolecular attraction between the δ+ end of one polar molecule and the δ− end of another. For molecules of similar size, it is usually stronger than London dispersion alone, so polar compounds tend to have higher boiling points and lower vapor pressures.
Detail
Meaning: A permanent dipole exists when polar bonds do not cancel by symmetry, leaving one end partially negative (δ−) and the other partially positive (δ+). Neighboring polar molecules orient with opposite partial charges close together, producing a net attraction. This is an intermolecular force, far weaker than a covalent or ionic bond. Hydrogen bonding is a stronger special case, with H bonded to N, O, or F.
On the MCAT: Rank boiling points, melting points, or vapor pressures. For molecules of similar molar mass, the more polar one usually has the higher boiling point and lower vapor pressure (acetone boils far above butane, which has nearly the same mass). You may need to judge polarity from geometry, or predict that polar solutes dissolve in polar solvents.
Common confusion: Polar bonds do not guarantee a polar molecule. CO₂, CCl₄ and BF₃ have polar bonds but no net dipole, so they have no dipole-dipole forces, only dispersion. Also, every molecule has dispersion forces, and a much larger nonpolar molecule can outweigh a small polar one.
Sources
- Chemistry: The Central Science
- Chemistry: A Molecular Approach
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.