Clar's rule

Some such structures may contain aromatic π-sextets, namely groups of six π-electrons localized in a benzene-like moiety and separated by adjacent rings through C–C bonds.

[1][2] In 1984, Glidewell and Lloyd provided an extension of Clar's rule to polycyclic aromatic hydrocarbons containing rings of any size.

Following rule 4 above, anthracene is better described by a superposition of these three equivalent structures, and an arrow is drawn to indicate the presence of a migrating π-sextet.

Following the same line of reasoning, one can find migrating π-sextets in other molecules of the acene series, such as tetracene, pentacene, and hexacene.

The dependence of the color and reactivity of some small polycyclic aromatic hydrocarbons on the number of π-sextets in their structures was reported by Clar himself in his seminal contribution.

[5] Clar's rule has also been supported by experimental results about the distribution of π-electrons in polycyclic aromatic hydrocarbons,[7] valence bond calculations,[8] and nucleus-independent chemical shift studies.

[10] Aromatic π-sextets play an important part in the determination of the ground state of open shell biradical-type structures.,[4] Clar's rule can rationalize the observed decrease in the bandgap of holey graphenes with increasing size.

Two representations of the same resonance structure of anthracene . Above, each covalent bond between carbon atoms is represented by one or two segments. Below, the aromatic π-sextet is put in evidence by means of a circle.
Two resonance structures of phenanthrene : above, one with only one circle; below, one with two circles, which is also a Clar's structure. Clar's rule states that the latter structure contributes the most to the properties of phenanthrene.
Representation of the anthracene molecule: above, three equivalent resonance structures; below, its Clar structure, with the arrow denoting a migrating π-sextet.
Three Clar structures with an increasing number of π-sextets: anthracene (on the left), phenanthrene (in the middle), and triphenylene (on the right). The chemical stability of these compounds increases from left to right due to the increase in the number of π-sextets.