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  Fused Ring Compounds Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Fused Ring Compounds

To give the larger polycyclic aromatic compounds benzene rings may be joined together (fused). Some instances are displayed below together with the approved numbering scheme for substituted derivatives. Peripheral carbon atoms are all bonded to hydrogen atoms. In these fused ring aromatics, all C-C bond lengths are not similar and there is some localization of the pi-electrons that is unlike the benzene.
In the coronene six benzene rings are fused in a planar ring; where six rings in the hexahelicene are not joined in a larger ring but assume a helical turn, because of the crowding together of the terminal ring atoms. This helical configuration cause to be hexahelicene molecule chiral and it has been resolved into stable enantiomers having purticular rotations of 3700º. 

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Like these comprehensive aromatic compounds become larger, ratio of the hydrogen to carbon decreases. For an instance, symmetrical hexacyclic compound coronene have a H/C ratio =1/2 compared with 1 for benzene. If we were to imagine the fused ring systems of this type to be further extended in space, the ratio of H/C would approach zero and resultant compound would be a form of carbon. This type of a carbon allotrope exists and is called graphite. Other than graphite a well-characterized carbon allotrope is diamond. For these two forms of carbon, Structures are very distinct and shown below in the diagram. An extended array of sp3 hybridized carbon atoms is Diamond; whereas the graphite contains overlapping sheets of sp2 hybridized carbon atoms that are arranged in a hexagonal pattern. 

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A comparison of corannulene and coronene models reveals an interesting dissimilarity in their shapes. The Coronene is completely flat and aside from the peripheral hydrogen, resembles the layer of graphite. The high melting point of it reflects this resemblance. The Corannulene, on the other hand is a little curved resulting in a bowl-like shape.

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By adding similar cycles of five benzene rings if we expanded the structure of the corannulene, curvature of the resulting molecule should increase and eventually close into a sphere of carbon atoms. Archetypical compound of this kind (C60) has been named the buckminsterfullerene due to its resemblance to the geodesic structures that is created by Buckminster Fuller. It is a member of the family of similar carbon structures that are called fullerenes. The third class of carbon allotropes are represented by these materials. Alternating views of C60 fullerene structure are shown in the diagram. Together with a soccer ball-like representation of 12 five and 20 six-membered rings composing its surface. By Atomic Force Microscopy (AFM) Precise measurement has shown that the C-C bond lengths of the six-membered rings are not all equivalent and depend on whether the ring is fused to a five or six-membered beighbor. Even though the C60 is composed of fused benzene rings its chemical reactivity resembles that of the cycloalkenes more than benzene. In reality to cage opened products, exposure to oxygen and light slowly degrade fullerenes. Various reactions thus far reported for C60 includes addition to, rather than the substitution of the core structure. These reactions involve hydroxylation hydrogenation and bromination. Strain that is introduced by curvature of the surface may be responsible for the enhanced reactivity of C60.

 Larger fullerenes, like C70, C76, C82 & C84 have the distorted or ellipsoidal spherical structures and fullerene-such as assemblies up to C240 have been detected. Fascinating aspect of these structures is the space in the carbon cage may hold small molecules atoms or ions. this type of species are called endohedral fullerenes. Cavity of C60 is comparatively small, but encapsulated the lithium, helium, and atomic nitrogen compounds have been observed.The larger fullerenes are found to encapsulate lanthanide metal atoms.

Interest in fullerenes has led to the innovation of a related group of carbon structures referred to as nanotubes. A nanotube can be viewed as a rolled up segments of graphite as displayed in the diagram. The major structural components are six-membered rings, but changes in the tube diameter, branching into the side tubes and the capping of tube ends is accomplished by fusion with five and seven-membered rings. Several interesting applications of these unusual structures have been proposed.

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