Intermolecular Forces
Intermolecular forces define the interactions between separate molecules. They are what hold the lipids in our cellular membranes together and explain why water is a liquid at room temperature while methane is a gas. This is in contrast with intramolecular bonds which compose the molecules themselves. In comparison, intermolecular bonds are much much weaker than their intramolecular counterparts.
London Dispersion
The weakest of them all is the London dispersion force AKA van der Waals forces (VDW). London forces result from the charge fluctuations in atoms due to changes in the position of orbiting electrons. This creates transient partially positive and partially negative charges within a molecule. Since all of the neighboring molecules have the same thing going on these transient charges attract one another causing the molecules to stick together.
Furthermore every molecule or compound has electrons in it so everything has London Dispersion forces. Since non-polar molecules don’t have any fixed partial positive or partial negative charges these are the only intermolecular forces they get. London dispersion forces take hardly any energy to break which is why methane, ethane, butane, and propane are all gases at room temperature.
However, larger nonpolar molecules such as dodecane are liquids at room temperature. So what’s different? We didn’t change the type of intermolecular force but how many we had. Since dodecane is much longer than the other alkanes listed above it has more surface area and thus more London dispersion forces holding it together. While each individual London interaction is super weak they are additive so 12 or 20 in a row ends up leading to a robust intermolecular interaction.
This also means that branched molecules and cis-conformation molecules both of which have decreased surface areas will experience weaker London forces.
Dipole-Dipole
While all molecules will have London dispersion forces only some will have dipole-dipole interactions. Why? Dipole-dipole interactions result from fixed partially positive and partially negative charges found within molecules. Instead of originating from the random fluctuations of electrons, these fixed dipoles arise from differences in electronegativity. This means that in order for a molecule to have a dipole-dipole interaction it must have atoms of differing electronegativities and the proper overall geometery.
It is important to note the role geometry plays in dipole-dipole interactions. For example, a CCl4 molecule has atoms with differing electronegativities but it isn’t polar. While CHCl3 would be polar. The difference is their shape. Here the poles created by four chloride atoms all cancel out resulting in no net dipole. In contrast, the three chloride atoms only partially cancel each other out. This results in a net dipole where one side of the molecule has a partial negative charge and the other side has a partial positive charge.
Hydrogen Bonding
Hydrogen bonding is really just a special and particularly stable form of dipole-dipole interaction. These are the strongest of the intermolecular bonds and form between hydrogen bond donors and hydrogen bond donors. Hydrogen bond donors are hydrogens attached to oxygen, nitrogen, or fluorine. While ydrogen bond donors are the oxygen, nitrogen, and fluorine atoms themselves.