I Can See Clar-ly Now

It's Monday, I don't feel like doing a lit review, nor have I found any good chem-related poop jokes, so it must be time for another unsolicited advice column.
One of the stranger tenets of aromatic chemistry, when first diving into it, is that not all molecules we'd call aromatic by definition are very stable. We learn as sophomores that benzene is remarkably stable, by nearly 30 kcal/mol, due to its 6π-electron, cyclic, conjugated system, and, if you were like me as a sophomore, you were entranced by the idea that a simple formulation such as that would impart such stability into a molecule. My fascination with organic chemistry started with benzene, and suffice it to say, that fascination hasn't gone away. Just check out the background.

But after benzene, things get kind of weird with aromatic molecule with more than one ring (polycyclic aromatic hydrocarbons, or PAH's). Naphthalene isn't as stable as benzene, as it'll react with conc. sulfuric acid at room temperature. Anthracene can react as a diene in Diels-Alder reactions. But they all conform to Hückel's 4n+2 rule, so what gives? In the middle 20th century, Eric Clar developed a theory of PAHs laid out in two now-classic books: The Aromatic Sextet and Polycyclic hydrocarbons. In them, he gives an explanation for the increased reactivity of those systems, which I'll briefly introduce here. In any case, The Aromatic Sextet, from 1972, is a quick read, complete with some ass-old NMRs for your amusement. His thesis is, put simply: Hückel's 4n+2 rule doesn't necessarily apply to PAH's, and a far more important factor in the stability of PAH's is the setup of their aromatic sextets.

Benzene contains what Clar defines as an aromatic sextet: that is, the six π electrons in benzene constitute a sextet. These sextets can be written as circles. Note, however, that only sextets can be written as circles: in naphthalene, there is only one sextet, and four other electrons, so writing two circles in for the electrons is wrong. It's something we teach sophomores to do, even though a lot of teachers and grad students get it wrong too:



Of course, since the two rings have equal electron density due to resonance, that sextet isn't on the one ring alone. Clar denotes that there is only one sextet in the molecule, although it is dispersed about the ring, with arrows.



An example of reactive non-sextets is phenanthrene. Phenanthrene has two sextets, shown here on the outside rings, that are in resonance with the middle ring, which contains a double bond. Even though there is resonance, the middle ring's double bond acts like a real double bond, and the outer rings have benzene-like stability.



With linear acenes, there is only one available sextet, and the longer the system, the more reactive the rings. Anthracene is reactive, tetracene is even more reactive, and the violet pentacene is so reactive that it'll dimerize and must be stored under nitrogen and in the dark.



In general, the more sextets that can be utilized to represent the pi-system of a PAH, the more "stable" it is. Molecules that can be represented with only sextets, called "fully-benzenoid" by Clar, are extremely stable. Triphenylene, isolated from coal tar, is very unreactive. peri-hexabenzocoronene (HBC), which I've blagged about before, is so stable that you can't use a mel-temp to find the melting point, because the glass will melt before the compound does.



This lil' introduction only scratches the surface of the large, rather diverse, and exceptionally pretty-colored field of arene chemistry. Next week, I'll share my award-winning* recipe for chicken parmesan.

*My mom likes it

THE TOP PIC: According to legend, Kekulé's discovery of benzene's ring structure was sparked by a day-dream of a snake biting its own tail. In ancient cultures, a snake or dragon biting its own tail is known as Ouroboros, representing the cyclic nature of the universe, among other things.