Carbocyclic Aromatic Systems

Benzene is described as a resonance hybrid of the two extreme forms which correspond, in terms of orbital interactions, to the two possible spin-coupled pairings 

Sometimes, benzenoid compounds are represented using a circle inside a hexagon although this emphasises their delocalised nature and the close similarity of the ring bond lengths (all exactly identical only in benzene itself), it is not helpful in interpreting reactions, and we do not use this method here. 

Treating naphthalene comparably reveals three canonical structures, 3, 4, and 5. Note the standard use of a double-headed arrow to interelate resonance contributors. This must never be confused with the use of opposing straight fish-hook arrows which are used to designate an equilibrium between two species resonance contributors have no separate existence they are not in equilibrium one with the other. 

This valence bond treatment predicts quite well the non-equivalence of the bond lengths in naphthalene in two of the three contributing structures, C-l-C-2 is double and in one it is single, whereas C-2-C-3 is single in two and double in one. Statistically, then, the former may be looked on as 0.67 of a double bond and the latter as 0.33 of a double bond the measured bond lengths confirm that there indeed is this degree of bond fixation, with values closely consistent with statistical prediction. 

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The discovery of the metal-like properties of conducting polymers has once again focused attention on the oxidation and reduction characteristics of aromatic systems. It turns out that most of these conducting materials consist of chainlike connected carbocyclic or heterocyclic aromatics [94-97]. 

Chia and Simmons388 calculated the resonance energies (ER)20 of four mono- and dibenzotetraazapentalenes (Scheme 24). Values are comparable with those of o-condensed aromatic systems (naphthacene, ER = 110 kcal mol-1 chrysene, ER = 116.5 kcal mol-1), and, like these carbocyclic systems, angularly-shaped molecules are more stable than linear ones. HMO calculations of delocalization energies (DE) show that the tetraazapentalene structure 15 is more stable than the tetra-azacyclooctatetraene valence isomer 324 (Scheme 14, Section IV,B,2) whether 324 is planar or tub-shaped. Calculations of electrophilic reactivity (Section IV,C,4,d), electronic spectra (by the PPP method employing all singly excited configurations), and bond orders have been carried out, and they confirm the aromatic nature of these systems. 

Scheme 1 Three possible types of heteroatoms and the relationship between carbocyclic and heterocyclic aromatic systems...

When the carbocyclic aromatic core in the starting material is replaced by a heteroaromatic ring, systems such as 123 arise and Mori elimination/dimerization provides the dioxa- and dithia[6.6]cyclophanes 125 (Scheme 27) [74]. 

Scheme 1 The relationship between carbocyclic and heterocyclic aromatic systems.

The first example of this type of complex was the molecule Fe(C5H5)2, now known as ferrocene, in which the 6jr-electron system of the ion C5H5 is bound to the metal. Other aromatic systems with the magic numbers of 2, 6, and 10 for the aromatic electronic configuration are the carbocycles ... 

Because of their aromatic character, pyrones undergo annulation reactions and the introduction of substituents less readily than do most cyclic a,B-unsaturated carbonyl compounds. Nevertheless, some suitable modifications have been developed that allow these compounds to be used effectively in annulation reactions. Some reviews about synthesis of 2-pyrones with 3,4-fused carbocyclic ring systems are currently available (82S337 86G109 92T9111). 

Although hydrogenation of aromatic systems requires more energetic conditions than do those of olefinic double bonds, carbocyclic aromatics are readily hydrogenated to the fully saturated product. Controlling hydrogenolysis, selectivity and stereochemistry are major problems connected with arene hydrogenations. The substituents play an important role, and their effects are not the same for all catalysts. ... 

Allcycllc compounds - Aliphatic compounds having a carbocyclic ring structure which may be saturated or unsaturated, but may not be a benzenoid or other aromatic system. [5]... 

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