Aromatic Allotropes of Carbon

What do you get when you make an extremely large polynuclear aromatic hydrocarbon, tvith millions or billions of benzene rings joined together You get graphite, one of the oldest-known forms of pure elemental carbon. Let s consider how aromaticity plays a role in the stability of both the old and the new forms of carbon. 

We don t normally think of elemental carbon as an organic compound. Historically, carbon was known to exist as three allotropes (elemental forms with different properties) amorphous carbon, diamond, and graphite. 

Structures of diamond and graphite. Diamond is a lattice of tetrahedral carbon atoms linked in a rigid three-dimensional array. Graphite consists of planar layers of fused aromatic rings. 

We picture each layer of graphite as a nearly infinite lattice of fused aromatic rings. All the valences are satisfied (except at the edges), and no bonds are needed between layers. Only van der Waals forces hold the layers together, consistent with their ability to slide easily over one another. The pi electrons within a layer can conduct electrical currents parallel to the layer, but electrons cannot easily jump between layers, so graphite is resistive perpendicular to the layers. 

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Polynuclear Aromatic Hydrocarbons 735 16-11 Aromatic Allotropes of Carbon 737 16-12 Fused Heterocyclic Compounds 739 16-13 Nomenclature of Benzene Derivatives 740... 

Polycyclic aromatic hydrocarbons, which are built of fused benzene rings, include naphthalene, anthracene, and phenanthrene. Some, such as benzo[a]pyrene, are carcinogens. Graphite, a common allotrope of carbon, consists of layers of planar,... 

Chapter 16 A section has been added that discusses the aromaticity of Fullerenes and their relationship to other allotropes of carbon. 

The 1996 Nobel Prize in chemistry was awarded to three American scientists for their identification of a new allotrope of aromatic carbon molecules called fullerenes. These unusual carbon molecules form a closed-cage structure of joined carbon atoms. The original soccer ball-shaped carbon molecule called buckminsterfullerene contained 60 carbon atoms and was nicknamed Bucky Ball in honor of Buckminster Fuller (1859—1983), who used similar shapes in some of his architectural structures. Since then additional organic pentagon structures beyond the original icosahedral fullerene (C ) have been developed, all with an even... 

Graphite, which is one of the allotropes in carbon, has unique features characterized by two-dimensionality and amphoteric nature. This is based on the 2D layered structure with the Ji-electronic structure extended in the constituent graphene sheets. The electronic structure of graphene, which is described in terms of the extrapolation of condensed polycyclic aromatic molecules to infinite size,... 

This article summarizes efforts undertaken towards the synthesis of the cyclo[ ]carbons, the first molecular carbon allotropes for which a rational preparative access has been worked out. Subsequently, a diversity of perethynylated molecules will be reviewed together, they compose a large molecular construction kit for acetylenic molecular scaffolding in one, two and three dimensions. Finally, progress in the construction and properties of oligomers and polymers with a poly(triacetylene) backbone, the third linearly conjugated, non-aromatic all-carbon backbone, will be reviewed. 

The past decade has led to the detection of new carbon allotropes such as fullerenes26 and carbon nanotubes,27 28 in which the presence of five-mem-bered rings allows planar polycyclic aromatic hydrocarbons to fold into bent structures. One notes at the same time that these structures are not objects of controlled chemical synthesis but result from unse-lective physical processes such as laser ablation or discharge in a light arc.29 It should be noted, on the other hand, that, e.g., pyrolytic graphitization processes, incomplete combustion of hydrocarbon precursors yielding carbon black, and carbon fibers30 are all related to mechanisms of benzene formation and fusion to polycyclic aromatic hydrocarbons. 

The new carbon allotropes, fullerenes [14] and nanotubes [15], can accept a large number of electrons to their -system however, the neutral compounds are hardly soluble, especially in THF. Because of the low solubility of fullerenes, their complete reduction to hexaanions with Li is possible only under extreme conditions, such as the use of an ultrasound bath [16], When the reduction of fullerenes is performed in the presence of a small amount of 2, moderate conditions are required, e.g., low temperature and without sonication [17]. Corannulene, which is a fragment of fullerenes, is highly soluble in THF and is easily reduced. The resulting corannulene anion serves as an efficient electron shuttle, which assists in complete reduction of the practically insoluble fullerenes. In a similar way, single walled nanotubes were reduced by charge transfer by the use of small aromatic compounds, such as naphthalene, fluorenone and anthraquinone [18]. 

The discovery of fullerenes offered a new challenge in the field of the study of the aromaticity of Ti-conjugated systems. These carbon allotropes embody completely conjugated spheroidal 7r-systems, so the carbon skeletons are boundaryless and thus large numbers of Kekule structures can be drawn. 

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