To the average person, the word “aromatic” usually means “pleasingly fragrant”. For chemists too, it held this meaning originally. Over time, chemists found that certain organic compounds were much more stable (unreactive) than was normal for similar chemicals. Because many of these molecules were also quite pungent, the chemists began to refer to them as aromatic compounds. Over time, it was discovered that dyes and other compounds also had similarly stable chemical structures in them, but the name was already established and so the term aromatic came to apply to a whole class of compounds, some smelly, some not.
What exactly are the requirements for a compound to be considered aromatic? Citing from McMurray’s “Organic Chemistry” (1992), Erich Huckel, a German physicist back in 1931, said that “a molecule can be aromatic only if it has a planar, monocyclic system of conjugation with a p orbital on each atom and only if the p orbital system contains 4n + 2 pi electrons, where n is an integer.” That set of requirements can be broken down into simpler terms.
According to Huckel, to be aromatic a molecule must:
1. have one flat (planar) ring
2. have a p orbital on every atom in the ring
(which means sp2 hybridization, if you know your subject)
3. be conjugated
(which generally means alternating single and double bonds, but not always, since lone pairs and empty p orbitals also work)
4. have 4n + 2 pi electrons
(that’s 2, 6, 10, 14, 18 if you weren’t sure)
For the most part we still accept Huckel’s requirements. The only difference is that we also know that there are polycyclic aromatic molecules that have more than one ring (but are still planar). Naphthalene (think mothballs) is an example, and it sure is smelly too.
The main interest in aromatic compounds comes from their increased stability compared to otherwise similar molecules. The reason for this added stability comes from the arrangement of p orbitals. When the requirements for aromaticity are met, the result is a ring (or rings) of atoms which all have a p orbital that points straight up and down. All of these p orbitals are side by side with one another, and overlap, creating a large ring above and below the plane of the atoms where the electrons can flow freely. (In a magnetic field, the electrons actually do follow this circular path, just as they would in a loop of wire.) Creating one large space for the electrons to “inhabit” is an energetically favorable arrangement. To carry out any chemical reaction on the aromatic ring, you have to disrupt that ring of pi electrons (all chemical reactions deal with the electrons), and that requires extra energy on top of the energy required for normal chemical reactions. As a result, the aromatic structure is more stable less reactive than comparable molecules.
For non-chemists, think of the electrons as children, and the ring of p orbitals as a playground with all their friends playing a favorite game. Compare that to the child who is playing alone in the backyard, and ask yourself which child it will be easier to get to come in to dinner. The child at the playground will be much more resistant, for not only is he/she enjoying the game more, but leaving will also ruin the game for the other kids. The same is true in the aromatic compound, save for the fact that as far as we know, electrons don’t “have fun” or play games.
