Dipole-Dipole Forces

Do you remember how electronegativity works? I posted about it, so you should. Here, just for you, I'll link back to that post. Electronegativity is necessary to understand polarity. And polarity is how these forces operate. An organic molecule with a functional group that contains an electronegative atom such as oxygen or nitrogen will likely have a permanent dipole. The heteroatom pulls negative charge toward itself (because it's electronegative, obviously). As described in the previous post, regions of the molecule can have differential charge. But a permanent dipole is much stronger than the fleeting changes responsible for London dispersion. Because of this, molecules that have dipole-dipole interaction experience stronger intermolecular forces than ones that have only London dispersion. Compounds with this property are said to be polar and ones that do not are non-polar. Consider these examples...

Acetone
Condensed structure: OC(CH₃)₂
It has a permanent dipole that looks like this.
Those Greek letters represent partial charge. The electronegative oxygen pulls electron density toward itself, so it is the center of negative charge. The region opposite it, lying near the central carbon, is the most positively charged region of the molecule.

Carbon dioxide
Condensed structure: CO₂
This time, when we draw the arrow through one oxygen, the other oxygen cancels it out. Despite having a highly electronegative element, carbon dioxide is non-polar.

Whether a solvent is polar or non-polar tells chemists a lot about its potential uses, and some reactions need one type of solvent or the other. Acetone is a well-known polar solvent, but water is the best known in this category. Many organic compounds such as n-hexane are commonly used as non-polar solvents. That's all for these at the moment. Just remember that in order for dipole-dipole forces to occur, the compound(s) must have permanent dipoles, and that dipole-dipole forces are usually much stronger than London dispersion. Of course, a molecule can have both. But the stronger forces are considered to override the weaker ones for all practical purposes that I've encountered.