Dipole–Dipole Interaction
The electrostatic attraction between the positive end of one polar molecule and the negative end of another.
Formula / equation
E ∝ μ₁μ₂ / r³
Unit
kJ/mol
In depth
These forces act in addition to dispersion forces and are typically 5–25 kJ/mol, so polar molecules boil higher than non-polar molecules of similar mass. Thermal motion partly randomises the alignment, so the effect weakens as temperature rises.
Examples in the real world
Acetone (58 g/mol) boils at 56 °C while butane (58 g/mol) boils at −0.5 °C.