Benzenoid skeletons

Example 14.3 Triphenylene. The same situation arises with triphenylene (33), which is significantly nonplanar [274]. The bonds joinning the external rings to one another were thus calculated at sec = 88.45kcal/mol for R = 1.46 A, following the approach used for biphenyl. It is clear that some advance knowledge is necessary in our calculations, namely, regarding planarity (or lack of it) of the benzenoid skeleton. 

A graph with an odd number of points is non-Kekulean by definition. No benzenoid hydrocarbon molecule or radical corresponding to non-Kekulean graph has ever been synthesized. The phenalene skeleton, XHIa, is the smallest non-Kekulean benzenoid. However, it is regrettably true that even phenalenyl radical is... 

Carbon skeletons of some polycyclic aromatic compounds of the (a) benzenoid type and (b) non-benzenoid type. 

Many aromatic compounds have considerable resonance stabilization but do not possess a benzene nucleus, or in the case of a fused polycyclic system, the molecular skeleton contains at least one ring that is not a benzene ring. The cyclopentadienyl anion C5HJ, the cycloheptatrienyl cation C7H+, the aromatic annulenes (except for [6]annulene, which is benzene), azulene, biphenylene and acenaphthylene (see Fig. 14.2.2(b)) are common examples of non-benzenoid aromatic hydrocarbons. The cyclic oxocarbon dianions C Of (n = 3,4,5,6) constitute a class of non-benzenoid aromatic compounds stabilized by two delocalized n electrons. Further details are given in Section 20.4.4. 

The mathematical chemist will recognize a as one of the Kekule valence-bond structures of the hydrocarbon, while is the corres nd-ing molecular graph Model is called an inner dual or dualist [2], is a caterpillar tree [3], and is called a Clar graph [4]. The latter two models are apparently quite different from the original skeleton, however, as it will turn out later, the topological properties of this benzenoid system are best modeled by either d or e-... 

To date, NMR has been used to study (a) the structures of indoles, substituted in both heterocyclic as well as the benzenoid ring (recently, there have been a number of publications wherein NMR is used to elucidate the complicated structures of biologically important antibiotics95,110-112 and alkaloids113-117 containing the indole skeleton) (b) tautomeric structures and equilibria 96, 118-120 (c) the stereochemistry of side-chain substituents and (d) the protonation of indoles. It is hoped that in the future many significant applications of NMR in the indole field, such as a study of intermolecular and intramolecular reactions and the rates of fast reactions, will be found. 

Arrange the skeleton of the benzenoid hydrocarbon so that some of its edges are vertical... 

Fig. 12. An unbranched benzenoid hydrocarbon, the corresponding caterpillar tree T, alkane hydrocarbon skeleton and Young diagram. T is a non-caterpillar tree...

The thermal automerization and rearrangement reactions of PAHs have been widely investigated during the past two decades (for examples see refs. [31 e, g, 62-64]). The main objective was to understand the processes of formation of aromatic hydrocarbons in fuel rich flames and the mechanisms of transformation of the PAHs that have been observed at these elevated temperatures. In most cases, thermally initiated rearrangement reactions in the carbon skeletons of PAHs require high enthalpies of activation resulting in low product selectivities and poor overall yields. Because the expected products are often more effectively prepared by conventional routes, this approach has been used as a synthetic tool only in a few cases, e.g. the synthesis of azulenes [65] and the rearrangement of bifluorenylidenes to benzenoid hydrocarbons [38]. 

In 1983, Vogtle [5] proposed the synthesis of 1, a molecular belt with a completely aromatic carbon skeleton. The benzenoid nuclei are fused laterally in a polyacene-like manner. The compound which could be called superacene [5] is a representative of the [njcyclacenes 2 which gained more and more interest in the following years. 

It will be noted that in a formal sense, cinchonamine (CCXXV at CCX) may be converted to a substance having the normal cinchona skeleton (CCXXVI) through cleavage of the N.1-C.2 bond, followed by formation of a new link between N.l and C.2, after rotation of the benzenoid ring. This relationship indicates that cinchonamine repre-... 

We will use in this report the term polyhexes for benzenoids to emphasize that we deal only with the carbon skeleton of benzenoids schematically presented as ensembles of regular hexagons. Polyhexes are mathematical structures which can be obtained by any combination of regular hexagons such that any two of its hexagons have exactly one common edge or are disjoint (e.g., Trinajstic 1990,... 

Fig. 9.10 The Wiswesser codes for cata-condesed polyhexes representing carbon skeletons of stable benzenoids with five six-membered rings C22H14...

Phenylpropenes are derived from cinnamic acid and share the first steps of lignin/lignan biosynthesis. Shortening of the cinnamic acid side chain by two carbons leads to compounds with a C -C skeleton called benzenoids. Other volatile... 

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