ABSTRACT
The Clar structure is defined as a structure, which presents the maximum number of benzenoid rings with inscribed circles with the property that there is no circle in adjacent rings. The aromatic sextet theory proposed by Clar (1972) can be considered a method to estimate the degree of cyclic conjugation in benzenoid hydrocarbons in a semi-quantitative manner. In Clar model, the aromatic sextet formula is used to describe the p-electrons arrangement, the hexagon in which an aromatic sextet is present is assuming to have a higher degree of cyclic conjugation, while the hexagons without circle are assuming to be less conjugated. A novel approach on Clar structures is represented by the work of Randic and Plavsic (2011) where they propose a new classification of these structures in three types: Clar Structural formulas, Algebraic Clar formulas, and Canonical Clar formulas.
