One of the first things we learn about in Gas Chromatography school is that the retention time of an analyte is inversely related to it’s boiling point (Boiling Points of Alcohols); A lower boiling point corresponds to a longer retention time. There are other factors, of course, but this felt like a good opportunity to go back to the fundamentals and discuss the origins of boiling points and how they relate to chemical structure. To make the discussion more concrete, let’s consider a series of alcohols and see what kind of trends in boiling points of alchols we can figure out.
isopropyl alcohol 82
n-amyl alcohol (n-pentyl alcohol) 138
iso-butanol (2-methyl-1-propanol) 108
t-butanol (2-methyl-2-propanol) 83
Boiling point boils down to one simple characteristic – how much does a molecule of a given species want to be next to an identical molecule, versus being in the air? If we remember our general chemistry lectures, it’s not surprising that this depends on the various interactions that govern how much molecules are attracted to one another. Often these are grouped into the following way:
Van der Waals
For the series of organic alcohols, we effectively have two things to consider. There’s no ionic interaction in any of these, so the differences in boiling point are effectively due to how well the molecules can line up next to one another, and how big the carbon chains are. If this seems too simple, read on!
Comparing n-amyl alcohol (n-pentyl alcohol) and 2-pentanol, the only structural difference is the position of the hydroxyl group along the backbone. However, they have boiling points that differ by about 20 degrees C. In many introductory courses, the fact that all of the intermolecular interactions are strongest at shorter distances is often neglected (maybe it’s too much to discuss early on in a chemistry curriculum!). What this means for the boiling point of 2-pentanol and n-pentanol is that the molecule who can pack more closely also interacts more strongly, and therefore increases the boiling point. This same rule is usually stated along the lines of “branching in organic molecules decreases the boiling point.” This same effect is also seen in the boiling points of isobutanol and t-butanol, as the more branched t-butanol has a lower boiling point.
Next consider the boiling points of isopropyl alcohol, 2-ethyl-1-hexanol and isobutanol. The only difference among these is the length of a few of the carbon chains. Isobutanol has a slightly higher boiling point than isopropanol, but the much larger 2-ethyl-1-hexanol has a boiling point 80 degrees higher! This is due to the relatively larger carbon chains in the branches, which give rise to a larger Van der Waals interaction between molecules.
Often people will say that the Van der Waals interaction is the weakest of the four intermolecular interactions. While this is certainly true on a small scale, the Van der Waals interaction is in some cases the most important, because it is always present. This is especially true when the molecules begin to get large. The fact that it’s always present means that in larger molecules, the interactions on an atom-by-atom basis add up pretty quickly, and we end up with molecules with very high boiling points. In fact, in the case of the simple hydrocarbons, we have no polar interactions, and we end up with boiling points that are effectively dependent only on the size of the carbon chain. Rather than being some meaningless interactions, the Van der Waals forces are strong enough to increase the boiling point of n-alkyl carbon chains from 36 degrees for propane to 216 for dodecane. Eventually the interactions are strong enough to make even the plain carbon chains solid at room temperature!
It’s always a good idea to spend a little bit of time remembering the fundamentals behind something so simple as a boiling point. With the internet constantly at our fingertips, it’s easy to look up basically any melting point, and forget exactly what contributes to the number on the screen. The simple fact is that the number on the screen represents how strongly two molecules (actually much more than two) want to stay close together, and this depends on their structure and the nature of their interaction. Most people are good about remembering the strength of the dipole-dipole and hydrogen bonding interactions, but sometimes the other factors surrounding what makes something boil get forgotten. Just remember that the branching of molecules often hinders their ability to physically get closer together and therefore interact strongly together. It’s equally important to keep in mind that the criminally underrated Van der Waals interaction is a big player on the field. Even though it’s secondary to the others in terms of strength of individual interactions, it has a tendency to add up quickly in larger systems!