Bharat Kumar, Ruth Viboh, Margel Bonifacio, William Thompson, Jonathan Buttrick, Babe Westlake, Min-Soo Kim, Robert Zoellner, Sergey Varganov, Philipp Mçrschel, Jaroslav Teteruk, Martin Schmidt, and Benjamin T. King
Angew. Chem. Int. Ed.
Both Kekulé and Clar representations of aromatic systems are commonplace in organic chemistry notation. The use of one or the other is usually dependent upon convention or preference. In most cases, neither one is wrong. Very rarely do the differing conventions predict differing physical properties of a molecule.
Kekulene is a well known cyclic polyaromatic hydrocarbon (PAH) consisting of 12 fused benzene rings. It’s structure can be described consistently by both Kekulé and Clar representations. However, not all PAHs exhibit the same consistency.
King and coworkers report a new PAH, dubbed septulene, which they synthesise, characterise and compare to the properties of the related kekulene molecule.
Unlike kekulene, septulene has an odd number of carbon atoms in both the inner and outer carbon rings. Because of this, the only way to resolve the Kekulé structure of septulene is to include a radial π-bond. This π-bond is fundamentally different from all the other bonds in the molecule and breaks the symmetry present in the structure described by the Clar representation. As such, if the Kekulé representation is consistent with reality, septulene will present very different physical properties to kekulene.
The authors studies do, in fact, show that septulene has very similar properties to those of kekulene. The NMR chemical shifts of the inner protons are particularly characteristic. The crystal structure also reveals that there are only 6 unique bonds in the molecule, matching kekulene, highlighting the symmetry of the system. The authors conclude that the similarity in properties between septulene and kekulene cannot follow from the Kekulé representation of the molecule and that only the Clar representation is correct in describing septulene’s, and, indeed, all PAHs’, structure.
Of course, the use of Clar representation for all molecules is not prescribed, the authors suggest that chemists understand the subtle differences between the two representations and use the most appropriate to describe each molecule.
[For anyone up for a challenge, can you ChemDraw the structure of septulene less haphazardly than me?]