Abstract
In 1972 Erich Clar formulated his aromatic π-sextet rule that allows discussing qualitatively the aromatic character of benzenoid species. Now, 40 years later, Clar's aromatic π-sextet rule is still a source of inspiration for many chemists. This simple rule has been validated both experimentally and theoretically. In this review, we select some particular examples to highlight the achievement of Clar's aromatic π-sextet rule in many situations and we discuss two recent successful cases of its application.
Highlights
In 1972 Erich Clar formulated his aromatic π-sextet rule that allows discussing qualitatively the aromatic character of benzenoid species
One can differentiate three type of benzenoid species. Those that have only π-sextets and “empty” rings called by Clar “fully benzenoid”; second, those that have π-sextets and rings with a single double bond; and third, those that have rings with two double bonds and for which one can write more than a single Clar structure
Müllen et al (Berresheim et al, 1999; Dötz et al, 2000) have been able to synthesize via oxidative cyclodehydrogenation of oligophenylene precursors large polycyclic aromatic hydrocarbons (PAHs) that are all fully benzenoid hydrocarbons
Summary
In 1972 Erich Clar formulated his aromatic π-sextet rule that allows discussing qualitatively the aromatic character of benzenoid species. In Clar’s rule one can classify the six-membered rings (6-MRs) of benzenoid species in four types of rings, namely, aromatic sextets (for instance, the phenanthrene external ring), migrating sextets (anthracene rings), empty rings (triphenylene central ring), and rings with localized double bonds (phenanthrene central ring).
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