Well, maybe it's not always used as a verb, but one molecule can hydrogen bond to another. Hydrogen bonding as well as dipole-dipole and London dispersion forces are all forces (actions) between two separate molecules. To say that a molecule has a hydrogen bond would be in error. Molecules can hydrogen bond with another molecule. It typically takes at least two or more molecules to hydrogen bond.
Intermolecular forces, sometimes called secondary forces, are between two molecules. Remember that an interstate highway goes from one state to another just as intermolecular forces are from one molecule to another. Intramolecular means within one molecule. Intramolecular forces are bonds (covalent, polar covalent, etc).
Intermolecular forces of most importance to organic chemists typically include the following.
Hydrogen bonding is the strongest of these three, while London dispersion forces are the weakest. Simply knowing the ordering of the strength of these forces and mechanistically how they work can help you make a lot of predictions in chemistry (and help you earn an A).
Predictions overview
For hydrogen bonding to occur the molecule must have O-H or N-H bonds. For example, ammonia and water both can hydrogen bond with themselves and with each other. Any time you see a structure with OH or NH you must think "Hydrogen Bonding". An OH is called a "hydroxyl group".
Draw the hydrogen bonding interaction between a water molecule and an ammonia molecule?
Notice how one molecule behaves as the hydrogen bonding donor and one molecule the hydrogen bonding acceptor.
Tertiary amines like trimethyl amine have low bp/mp than primary and secondary amines since they cannot hydrogen bond.
Hydrogen bonding is really just a strong dipole-dipole interaction. Most other dipole-dipole interaction forces are weaker than hydrogen bonding. For a molecule to undergo dipole-dipole interactions with another molecule, it must be polar. Any molecule with an unsymmetrical distribution of heteroatom (i.e. O, N, Cl, Br, I) could be polar. For a molecule to be polar, one end must clearly have more charge than the other. The ketone (acetone) is polar and therefore can do dipole-dipole and London, but not H-bonding. Acetone is polar by virtue of its highly polarized C=O double bond. Using resonance, we can illustrate the polarity of acetone.
Acetone has a similar molecular weight as 1-propanol, but since it does not have any OH bonds, it can not H-bond and therefore has a lower bp. Acetone can only do dipole-dipole and London dispersion which are both weaker than H-bonding.
The following example illustrates the effect of the different intermolecular forces on boiling point.
All molecules can do London dispersion forces. London dispersion forces are the weakest of the intermolecular forces. When I learned about these forces my high school chemistry teacher required us to call them "instantaneous dipole - induced dipole" interactions. When I asked her why she proclaimed "because thats all you need to know to understand how it works". She was correct and I've never forgot how it works.
Here is a mental picture of how it works. Imagine the molecules collides into another molecule or its container and all of its electrons kind of slosh over to one side. This is an instantaneous dipole. The other molecule it collided into has all its electrons forced to its other side (the induced dipole). This is the strongest with larger molecules and atoms.
Question: What type of intermolecular forces are important for acetic acid shown below.
Show AnswerAcetic acid has an OH (hydroxyl group) so hydrogen bonding is important, it is polar so dipole-dipole would be important as well.
Question: Rank the following molecules from lowest to highest boiling point.
Show AnswerLondon dispersion forces are the key factor here, as the molecules lack OH or NH groups (no hydrogen bonding) and are non-polar, so they cannot engage in dipole-dipole interactions. For London dispersion forces, larger molecular size leads to stronger intermolecular forces. Therefore, the ranking in terms of boiling point is: propane < butane < pentane.