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Topology of Polyene Ribbons

In this chapter we shall resume discussing the structure of aromatic compounds. Admittedly, our narration cannot lay claim to any comprehensive analysis of the problem, which is feasible only in a special monograph. Our objective is more modest. We are going to discuss the basic aspects and difficulties of the modern theories of aromaticity and to show to what extent the concepts of chemical topology can be employed in investigating cyclic conjugated systems. [Pg.68]

In search of aromatic annulenes. The detailed analysis shows that the notion of aromaticity is far from being simple and unambiguous as it may seem at first glance. We shall list here some complications. [Pg.68]

As to the [iVjannulenes with n 18, they are not planar due to the repulsion of hydrogen atoms located inside the ring. In the case of [14]annulene the planar conformation is stable only at = — 60°C. [Pg.69]

However, hydrogen atoms are not present inside the cycles such as the one shown below  [Pg.69]

In this and similar compounds the acetylene bond is supposed to donate only two jt-electrons to the conjugated system while the other jt-bond is located in the plane of the molecule and does not participate in the conjugation. Consequently, this compound satisfies the Hiickel rule for = 4. It indeed possesses aromatic properties. Anti-aromaticism. When a cyclic polyene system is studied it is important to know whether this system is nonaromatic, i.e.not stabilized by conjugation and sufficiently reactive due to the internal tension and other causes, or destabilized by conjugation, i.e. the cyclic delocalization increases the total energy of the system. In the latter case the molecule is called anti-aromatic. Here are typical examples of anti-aromatic systems cyclobutadiene, a cyclopropenyl anion, a cyclopentadi-enyl cation, and others. [Pg.69]


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