I. INTERMOLECULAR FORCES (IMFs)—These are weak forces between molecules that rely on small electrostatic interactions.
A. HYDROGEN BONDING—Hydrogen atoms, when bonded to certain elements create unusually large intermolecular attractions.
1. Hydrogen is an exceptional element in that when it forms a covalent bond, its electron is held to one side of the nucleus leaving the other side completely exposed.
2. Any approaching negatively charged group can get very close to the hydrogen nucleus and this produces an unexpectedly large electrostatic attraction.
3. These electrostatic attractions are exaggerated when H is bonded to a more electronegative element that is small enough to allow a significant intermolecular interaction, i.e., fluorine, oxygen, or nitrogen. Such exaggerated, intermolecular, electrostatic attractions are called hydrogen bonds.
4. The occurrence of hydrogen bonds has the following two important consequences, both of which are explained by the increased attraction between molecules caused by hydrogen bonding, making it more difficult to separate them.
i. It gives substances that contain them anomalously high boiling points, as shown in the following table.
| Hydrogen Halide | Normal Boiling Pt, oC |
| HF* | 19 |
| HCl | –85 |
| HBr | –67 |
| HI | –35 |
*High BP attributed to hydrogen bonding
ii. Substances that contain them tend to have increased viscosity.
As soon as you see a hydrogen atom directly connected to N, O, or F, think “hydrogen bonding,” BUT be careful! Just because a chemical formula has hydrogen and one of those elements present, it does not necessarily mean that hydrogen is connected DIRECTLY to N, O, or F. Check to see which atoms are connected to one another. For example, H3COCH3 does not have hydrogen bonding (hydrogen atoms bonded to carbon atoms), but CH3CH2OH, does (hydrogen atoms bonded to oxygen atoms).
5. Hydrogen bonds are the strongest of the (weak) intermolecular forces.
B. DIPOLE–DIPOLE FORCES—Permanent dipoles in adjacent molecules attract one another.
1. Molecules with polar bonds (caused by differences in electronegativity) and dipoles that do not cancel will have permanent dipoles.
2. When molecules that have permanent dipoles approach one another, they will arrange themselves so that the negative and the positive ends of the molecules attract one another.
3. The attractions are called dipole–dipole forces.
4. Dipole–dipole interactions are intermediate in strength in terms of the (weak) intermolecular forces.
C. LONDON DISPERSION FORCES (LDFs)—These are very small electrostatic interactions between molecules when no permanent dipoles are present.
1. London dispersion forces are small electrostatic forces that are caused by movement of electrons within the covalent bonds of molecules that would otherwise have no permanent dipole.
2. As one molecule approaches another, the electrons of one or both are temporarily displaced owing to their mutual repulsion. This movement causes small, temporary dipoles to be set up that attract one another. These attractions are called London dispersion forces.
3. These dispersion forces increase with the number of electrons in the molecule. This leads to more dispersion forces, greater attraction and therefore higher melting and boiling points among molecules with larger numbers of electrons when compared to molecules with smaller numbers of electrons.
4. London dispersion forces are the weakest of the (weak) intermolecular forces.
Recall that ALL substances will have some degree of London dispersion force, but text questions usually focus on recognition of a single IMF. You should immediately recognize hydrogen bonding whenever H is directly connected to N, O, or F and dipole–dipole interactions when there are significant differences in electronegativity and the dipoles do not cancel. Molecules with very small or no differences in electronegativity will only exhibit LDFs as their IMFs.