Intermolecular Forces

Dipoles are super important in determining the strength of forces between molecules.
Think of dipoles like tiny magnets. Just like magnets align themselves so that opposite poles touch and like poles repel, dipoles do too.  Since opposite charges attract and like charges repel, the dipoles will align so that the negative charges of one molecule are adjacent to the positive charges of another. This creates an attractive force.

The greater the magnitude of these dipoles, the greater will be the attractive intermolecular force, and the greater will be the boiling point.  And remember what determines the magnitude? Electronegativity differences! (note 1).  Therefore dipoles largely determine boiilng points. Another way of saying this is that boiling points increase with polarity.

Quickly, here’s the 4 important intermolecular forces, from strongest to weakest (see the diagram!)

• ionic. Not strictly an intermolecular force, but when full charges are present these will have the highest boiling points. There are such things as liquid salts, and their boiling points are extremely high. Look for full charges.
• hydrogen bonding . oxygen (electronegativity 3.5) bound to hydrogen (electronegativity 2.2) . Look for OH or NH groups.
• dipole dipole -look for carbon bound to electronegative elements such as O or Cl.
• london dispersion forces Hydrocarbons (C-H bonds) have small electronegativity differences and small dipoles. They are attracted to each other through transient dipoles called London dispersion forces. Look for: hydrocarbons.

Thanks for reading!
James

Note 1 – Electronegativity differences make up the most part of the dipole, but symmetry of the molecule also plays a role.  In certain symmetrical molecules like CCl4 the individual dipoles  cancel each other out, making the molecule non-polar.