Benzenoid aromatic hydrocarbons

The series of benzenoid aromatic hydrocarbons shown in Figure 6 have been studied in the present work, and should allow for an extrapolation towards an... 

HeBr87] W. C. Herndon and A. J. Bruce, Perimeter code for benzenoid aromatic hydrocarbons, in Graph Theory and Topology in Chemistry, ed. by R. B. King and D. H. Rouvray, Studies in Physical and Theoretical Chemistry 51 (1987) 491-513. 

The Diels-Alder reaction of benzenoid aromatic hydrocarbons with dienophiles was discovered more than fifty years ago [61, 62]. A classical example is the reaction of anthracene with maleic anhydride (Scheme 5). Although this type of reaction, termed endogenic or endocyclie Diels-Alder reaction, could be expected to be particularly well suited for correlating structure (topology) of benzenoid hydrocarbons with kinetic data, the problem has been systematically studied only very recently. Biermann and Schmidt in a series of publications [12, 29, 45, 63, 64] reported second-order rate constants (k2), measured under standard conditions (1,2,4-trichlorobenzene, 91.5 + 0.2 °C), for the endocyclie Diels-Alder reaction between maleic anhydride and 102 benzenoid hydrocarbons. Each rate constant was measured twice, the values usually... 

In this connection, also a paper of O. E. Polansky and I. Gutman [11] (1980) on so-called all-benzenoid aromatic hydrocarbons and a paper by B. Dzonova-Jerman-Blazic and N. Trinajstic [12] (1982) should be mentioned. 

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. 

In practice, the valence bond picture has probably exerted more influence on how chemists actually think than the HMO picture. However most early applications were primarily qualitative in nature. This qualitative VB picture can be summarized under die name of resonance theory [10]. The basic concept is that in general the more ways one has of arranging the spin pairing in the VB wave function, the more stable the molecule is likely to be. Thus, VB theory predicts that phenanthrene with 14 carbon atoms and 5 Kekule structures should be more stable than anthracene with 14 carbon atoms but just 4 Kekule structures, in complete accord with the experimental evidence. It also predicts that benzenoid hydrocarbons with no Kekule structures should be unstable and highly reactive, and in fact no such compounds are knowa Extensions of this qualitative picture appear, for example, in Clar s ideas of resonant sextets [11], which seem to be very powerful in rationalizing much of the chemistry of benzenoid aromatic hydrocarbons. The early ascendancy of HMO theory was thus largely based on the ease with which it could be used for quantitative computations rather than on any inherent superiority of its fundamental assumptions. 

The Non-Benzenoid Aromatic Hydrocarbon Pentalene. A Theoretical Discussion. 

An all carbon conjugated ladder polymer (graphite ribbon) was synthesized by a novel electrophile-induced cyclization reaction to provide fused benzenoid aromatic hydrocarbon in quantitative yield [161]. Suzuki cross-coupling of dieneyne 105 with 1,4-didode-cylbenzene-2,5-diboronic acid (106) gave rigid-rod polymers 107, which was further treated with TFA to produce 108 graphite ribbon as a yellow/orange solid. 

Herndon, W.C. and Szentpaly, L.V. (1986) Theoretical model of activation of carcinogenic polycyclic benzenoid aromatic hydrocarbons. Possible new classes of carcinogenic aromatic hydrocarbons. 

Systematic studies of the absorption spectra of benzenoid aromatic hydrocarbons, mostly done by Clar in the 1930s and 1940s,279 showed that these compounds exhibit four types of UV VIS absorption bands, which are shifted in a regular way along a homologous series such as the linear acenes (benzene, naphthalene, anthracene,. ..) (Figure 4.16). 

Figure 4.17 Correlation of the p band positions of benzenoid aromatic hydrocarbons with HMO HOMO LUMO gaps. Data taken from ref. 282, except for the outliers azulene (1) and cycl[3.3.3]azine (2) that do not belong to the same class of compounds...

The electronic structure of the lowest excited states of planar aromatic compounds is well described by MO theory (Sections 4.5 and 4.7). Benzenoid aromatic hydrocarbons exhibit three or four7t,7t absorption bands in the near-UV region, which are labelled Lb,1 La, 1 Bb... 

Members of a class of arenes called polycyclic benzenoid aromatic hydrocarbons possess substantial resonance energies because each is a collection of benzene rings fused together. 

A large number of polycyclic benzenoid aromatic hydrocarbons are known. Many have been synthesized in the laboratory, and several of the others are products of combustion. Benzo[a]pyrene, for example, is present in tobacco smoke, contaminates food cooked on barbecue grills, and collects in the soot of chimneys. Benzo[a]pyrene is a carcinogen (a cancer-causing substance). It is converted in the liver to an epoxy diol that can induce mutations leading to the uncontrolled growth of certain cells. 

Three studies on radical cations discuss the characterization of polynuclear aromatic radical cation salts as organic metals (8), the reactions of cation radicals with neutral radicals (9), and the magnetic-electrical properties of perfluoroaromatic radical-cation salts (10). Chapters on polynuclear aromatic compounds in nonvolatile petroleum products (II) and in coal-based materials (12) present reviews of the subject and new findings. The remaining chapters in this book discuss the thermal conversion of polynuclear aromatic compounds to carbon (13), the nitration of pyrene by mixtures of N02 and N204 (14), the spectra, structures, and chromatographic retention times of large polycyclic aromatic hydrocarbons (15), the desulfurization of polynuclear thiophenes correlated with tt electron densities (16) and simple theoretical methods to predict and correlate polynuclear benzenoid aromatic hydrocarbon reactivities (IT). 

Condensed polycyclic benzenoid aromatic hydrocarbons are customarily regarded as planar molecular structures because of the geometrical constraints of carbon atoms in a state of sp2 hybridization. A well-known exception is the class of compounds called the helicenes (18) for which the nonbonded overlap of two terminal benzenoid rings in a cata-condensed structure, as in structure 1, forces a molecule into a nonplanar helical structure. A second exceptional class of compounds is related to corannulene (2) and other an-nulenes of this type (19, 20). In corannulene, strain associated with the pericondensed five- and six-membered rings requires adoption of a bowlshaped structure (20, 21). For both structures 1 and 2 the aromatic character of the benzenoid rings is retained to an appreciable extent. 

Theoretical methods to predict chemical reactivity properties of polycyclic benzenoid aromatic hydrocarbons are reviewed. These methods include the usual molecular orbital (MO) quantum chemical calculations, as well as pencil-and-paper MO and valence-bond procedures to derive indexes related to the rates of chemical reactions. Justification for the pencil-and-paper procedure termed the pertur-bational molecular orbitahfree-electron method (PMO F) is presented, and the modifications (PMO.Fw) of this procedure necessary to handle the differing reactivity patterns with neutral and ionic intermediates are also given. Examples of correlations of experimental results are used to illustrate these modifications. 

The QUANTUM theoretical characterization of the molecular structure of polycyclic benzenoid aromatic hydrocarbons (PAHs) and the relationships of structure to the physical and chemical properties of PAHs are problems that have been of concern to theoreticians (and experimentalists) for more than 50 years. In general, quantum chemical procedures can be used successfully to correlate kinetic and thermodynamic data for PAHs. These procedures are usually restricted to the it systems of the PAHs and normally seem to yield very good results because (1) the it system properties are described accurately by quantum mechanical calculations and (2) the energetics of a given type of reaction in a group of related PAHs is mainly... 

These ideas were applied for comparing aliphatic alcohols, carcinogenic polycyclic benzenoid aromatic hydrocarbons, and binding constants between human corticosteroid binding globulin and a set of 47 steroids. The last set of compounds was also investigated by comparative molecular field analysis (CoMFA) with comparable results. A similar approach was used by Klopman and Raychaudhury on the basis of the Wiswesser Line Notation system. 

FIGURE 14.14 Benzenoid aromatic hydrocarbons. Some poiycyciic aromatic hydrocarbons (PAHs), such as dibenzo[a,/]pyrene, are carcinogenic. (See important, but hidden, epoxides at the end of Chapter 11.)... 

Jerry Dias, a chemist at the University of Missouri—Kansas City, has devised a periodic classification of a class of organic molecules called benzenoid aromatic hydrocarbons, of which naphthalene, Cj Hg, is the simplest example (figure 1.10). By analogy with Johann Dobereiner s triads of elements, described in chapter 2, these molecules can be sorted into groups of three in which the central molecule has a total number of carbon and hydrogen atoms that is the mean of the flanking entries, both downward and across the table. This periodic scheme has been apphed to making a systematic study of the properties of benzenoid aromatic hydrocarbons, which has led to the predictions of the stabihty and reactivity of many of their isomers. 

FIGURE 1.10 Dias s periodic classification of benzenoid aromatic hydrocarbons.. Dias, Setting Benzenoids to Order, Chemistry in Britain, 30, 384—386,1994, p. 384 (by permission). 

Herndon WC (1990) On Enumeration and Classification of Condensed Polycyclic Benzenoid Aromatic Hydrocarbons. J Am Chem Soc 112 4546... 

A large number of polycyclic benzenoid aromatic hydrocarbons are known. One of these, benz[tobacco smoke. From the literature, locate and then draw the structure of this hydrocarbon. Can you suggest other sources where this material might be expected to be present ... 

A completely planar, delocalized 14 n-electron system which does not contain cyclically conjugated substructures characterizes the title compound 2. A productive synthesis of the l,2-bis(ferf-butylcyclopentadienyl)ethane 1 enabled the construction of the novel non-benzenoid aromatic hydrocarbon 2 which also contains one of the few planar eight-membered rings. 

The structure of ir-cyclopentadienyl hexatrifluoromethylbenzene rhodium, 7.18, is of some interest. Normally benzenoid aromatic hydrocarbons... 

---