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CHEM 121 Studioby Learn4Less · UBC CHEM 121

CHEM 121 glossary

Dipole–dipole forces

Attractions between polar molecules, where the δ+ end of one molecule attracts the δ− end of a neighbour. They add to the dispersion forces, so a polar molecule usually boils higher than a nonpolar one of similar size.

Dipole–dipole forcesPolar molecules line up so the partial negative end of one is next to the partial positive end of the next; the dotted lines show the attractions between neighbouring molecules.δ+δ−δ+δ−δ+δ−δ+δ−δ+δ−δ+δ−δ+ end of one molecule attracts δ− end of the nextonly between polar molecules (e.g. HCl, CH₂O)

Polar molecules line up δ+ to δ−.

Example

Acetone (CHX3COCHX3\ce{CH3COCH3}, 58.1 g/mol) and butane (CX4HX10\ce{C4H10}, 58.1 g/mol) have almost the same mass, but polar acetone boils at 329 K and nonpolar butane at 273 K.

See alsoPolar moleculeDipole momentIntermolecular forcesHydrogen bonding