Kekulenes, cycloarenes, and heterocycloarenes have attracted much attention though the years, largely due to their electronic structure. The synthesis and characterization of these interesting molecules showed that their π electrons remained delocalized in individual benzenoid-type rings rather than globally delocalized in an annulenoid fashion. This discovery further suggested that the Clar bonding model, not the Kekulé model, is the best representation for depicting the bonding of large macrocyclic aromatic compounds. A plethora of computational studies suggest that cycloarenes gain little, if any, energetic stabilization from global delocalization, obviating the need for the concept of superaromaticity. More recently, cycloarenes have been suggested to serve as models for defects in graphene. Further study into this interesting set of compounds will continue to provide insights into fundamental questions about how aromatic compounds behave.
Nuclear magnetic resonance (NMR)
spectroscopy has become a cornerstone
tool for studying a variety of molecular features in advanced chemistry
laboratories. In this experiment, students prepare a pyrrole-β-amide
via a one-pot, three-step synthesis and use NMR spectroscopy to confirm
its molecular structure. In addition, variable temperature (VT) 1H NMR spectroscopy is used to study the dynamic amide bond,
a functional group with restricted rotation about the C–N bond.
The VT NMR data allow the students to determine the coalescence temperature
associated with the slow equilibrium at room temperature and the corresponding
energy barrier (ΔG
⧧) for
the restricted rotation about the C–N bond.
Das siebenseitige Homologe von Kekulen, Septulen, hat Eigenschaften, die denen von Kekulen verblüffend ähneln. Die Clar‐Strukturen beschreiben das richtig, indem sie die Ähnlichkeiten zwischen Septulen und Kekulen betonen, während die Kekulé‐Strukturen es falsch sehen, weil sie die Unterschiede zwischen Septulen und Kekulen unterstreichen. In der Zuschrift auf beschreiben B. T. King et al. die Synthesen und Eigenschaften von Septulen und diskutieren die Eignung dieses Moleküls für die Unterscheidung der Clar‐ und Kekulé‐Strukturen.
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