Nothing is more central to organic chemistry than the concept of the chemical bond. As a professional organic chemist, the past eighteen years of my life has been spent learning how to make (and occasionally, how to break) chemical bonds. A bond is the “glue” that holds a pair of atoms into a tight arrangement, forming a molecule. A water molecule, for example, has two chemical bonds. Oxygen is at the center of the molecule, with a hydrogen atom on either side that is bound to the central oxygen. This forms water, or H-O-H, with the hypens representing the chemical bonds. Learning how these bonds form requires a deep understanding of bond strength. A strong interaction between the two atoms will lead to a strong bond, whereas a weak interaction leads to a flimsy and tenuous bond.
Similar to a molecular bond are the bonds between two adjacent molecules. While these intermolecular interactions are never as strong as intramolecular bonds, they can still play a significant role. Up until now, it’s been understood that alkanes (a class of molecules such as propane, which contain simply carbon and hydrogen) only had access to very weak intermolecular bonds. The reason lies in a principle called “electronegativity”. It states that the strongest interactions form between a positive charge and a negative charge; opposites attract. As you get further and further away from this idealized situation, the bond loses strength. At the opposite end of the spectrum you have the interactions between the hydrogens on adjacent alkanes. They really don’t have much going for them; they are the weakest intermolecular interactions studied by organic chemists.
I’ve always been very comfortable with this description of chemical bonding. However, there are oddities in the organic literature which pop up from time to time. Polyhedral alkanes – alkanes whose molecular structure takes the form of a polyhedron, such as a cube – show much increased intermolecular attraction. No-one has really understood why. That’s why I was so interested in a recent publication in the journal Nature: Chemistry which outlined a new way of studying these molecules. By placing the molecules into computer program, the authors were able to apply a custom-designed algorithm. They discovered that the spherical shape of the polyhedrons allows the hydrogen atoms to form interactions with multiple hydrogen partners nearby. It forms a three-dimensional spider web lattice of bonds. Each bond individually is very weak (just as a single silk strand is weak), but together, they form a strong and resilient framework. This result explains why the polyhedral alkanes require so much more energy to vaporize than the linear straight-chain alkanes, such as octane.
This discovery is important to me, as an organic chemist, because it drills home several important lessons. We can’t afford to ignore scattered examples of puzzling behavior, and we have to constantly push our technology to the breaking point in order to learn more about chemical behavior. Even though Linus Pauling published his acclaimed “Nature of the Chemical Bond” in the 1920’s, it’s only today that we’re able to explain some of the more intricate aspects of bonding theory. It’s a lesson that I’m going to take to heart as I go forward in my studies of science.
The source of this article can be found at:
Echeverria, J. “Dihydrogen contacts in alkanes are subtle, but not faint”. Nature: Chemistry 2011, 3, 323-330.