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Clar's rule states that for a benzenoid polycyclic aromatic hydrocarbon (i.e. one with only hexagonal rings), the resonance structure with the largest number of disjoint aromatic π-sextets is the most important to characterize its chemical and physical properties.
According to Clar's rule, [20] the resonance structure of a PAH that has the largest number of disjoint aromatic pi sextets—i.e. benzene-like moieties—is the most important for the characterization of the properties of that PAH. [21] Benzene-substructure resonance analysis for Clar's rule
Two different resonance forms of benzene (top) combine to produce an average structure (bottom). In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected by the stabilization of conjugation alone.
Erich Clar (23 August 1902 – 27 March 1987) was an Austrian organic chemist, born in Hřensko, who studied polycyclic aromatic hydrocarbon chemistry. He is considered as the father of that field. [ 1 ]
Triangulene (also known as Clar's hydrocarbon) is the smallest triplet-ground-state polybenzenoid. [1] It exists as a biradical with the chemical formula C 22 H 12. [2] It was first hypothesized by Czech chemist Erich Clar in 1953. [3]
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Diagrams such as this are always artificial representations -bonds as connecting lines don't exist anyway. Drawing the different resonance forms is a way to illustrate to the student why the electron density is in fact the same everywhere, but of course, no bond-flipping actually takes place -the electrons simply distribute equally.
A diagram of an aromatic ring current. B 0 is the applied magnetic field, the red arrow indicating its direction. The orange ring shows the direction of the ring current, and the purple rings show the direction of the induced magnetic field.