dipole–dipole interaction
/ DY-pole /
Drop a handful of tiny bar magnets onto a table and shake gently: they shuffle around until each magnet's north end sits near a neighbor's south end. Polar molecules do almost the same thing. A molecule with a slightly positive end and a slightly negative end behaves like a little electrical magnet, and these line up to attract one another — that lining-up is the dipole–dipole interaction.
A dipole–dipole interaction is the attraction between molecules that each carry a permanent dipole, meaning their charge is permanently lopsided because of an uneven sharing of electrons. The positive end of one molecule is drawn to the negative end of the next. Because real molecules tumble and jostle with thermal motion, the average attraction is what survives, and it grows stronger as the dipoles get bigger and the temperature gets lower.
Why it matters: dipole–dipole forces help explain why polar substances like acetone boil higher than nonpolar ones of similar size, and why polar molecules dissolve more readily in polar solvents. The honest caveat is that for many molecules the ever-present dispersion force is actually larger; dipole–dipole attraction is an extra contribution layered on top, not the whole story.
Acetone and propane have nearly the same molecular weight, yet acetone boils 100 °C higher because its molecules are polar and grip one another through dipole–dipole attraction.
Permanent dipoles add an extra pull between polar molecules.
Dipole–dipole interaction requires permanent dipoles, so it acts only between polar molecules. Nonpolar molecules have no permanent dipole and so feel only dispersion forces — but every polar molecule feels dispersion too, on top of its dipole–dipole attraction.