Dipole-Dipole Interactions

Dipole-dipole interactions are a consequence of permanent dipole moments in polar molecules
The dipole–dipole force is an attractive intermolecular force
- This is because polar molecules tend to arrange themselves so that the positive end of one molecule is near the negative end of another
Repulsions can also occur when the positive (or negative) ends of two molecules are in close proximity
To illustrate this force, consider the way the molecules of acetonitrile, CH₃CN are packed in the liquid and solid states
- In the solid state, they are arranged with the negatively charged nitrogen end of the molecule aligned with the positively charged —CH₃ end
- In the liquid state, however, as a result of less restriction in the movement of molecules, the arrangement is disordered
  - This means both attractive and repulsive forces are present at any instant

dipole-dipole

Dipole-dipole interactions between molecules of acetonitrile, CH₃N in solid and liquid states.

We can see the effect of dipole-dipole interactions in polar molecules by comparing the boiling points of acetonitrile and propane, C₃H₈
- Acetonitrile is a polar molecule with a molecular weight of 41 amu that boils at 355 K
- Propane is a nonpolar molecule with a molecular weight of 44 amu that boils at 213 K
Dispersion forces are present in both molecules
- Since both molecules have a similar molecular weight, the strength of dispersion forces will be similar
However, the additional presence of a dipole-dipole force in acetonitrile gives it a higher boiling point
Generally, the strength of dipole-dipole interaction is dependent on the magnitude of the dipole and the proximity of the molecules

Dipole-Dipole Interactions (College Board AP Chemistry)