Benzenoid polycyclic aromatic hydrocarbons

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. 

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. 

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. 

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 ... 

Two or more benzene rings fused together form a number of polycyclic benzenoid aromatic compounds, naphthalene, anthracene and phenanthrene, and their derivatives. All these hydrocarbons are obtained from coal tar. Naphthalene is the most abundant (5%) of all constituents of coal tar. 

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. 

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). 

Figure 14.14 Benzenoid aromatic hydrocarbons. Some polycyclic aromatic hydrocarbons (PAHs), such as dibenzo[a,Qpyrene, are carcinogenic. (See "The Chemistry of. . . Epoxides, Carcinogens, and Biological Oxidation" in Section 11.14.)...

For books on this subject, see Gutman, I. Cyvin, S.J. Introduction to the Theory of Benzenoid Hydrocarbons, Springer NY, 1989, Dias, J.R. Handbook of Polycyclic Hydrocarbons, Part A Benzenoid Hydrocarbons, Elsevier NY, 1987, Clar, E. Polycyclic Hydrocarbons, 2 vols. Academic Press NY, 1964. For a periodic table that systematizes fused aromatic hydrocarbons, see Dias, J.R. Acc. Chem. Res., 1985, 18, 241 Top. Curr. Chem., 1990, 253, 123 J. Phys. Org. Chem., 1990, 3, 765. 

CONTENTS List of Contributors. Introduction to the Series An Editor s Forward, Albert Padwa. Preface, Randolph P. Thummel. Cyclooctatetraenes Conformational and ii-Elec-tronic Dynamics Within Polyolefinic [8] Annulene Frameworks, Leo A. Paquette. A Compilation and Analysis of Structural Data of Distorted Bridgehead Olefins and Amides, Timothy G. Lease and Kenneth J. Shea. Nonplanarity and Aromaticity in Polycyclic Benzenoid Hydrocarbons, William C. Herndon and Paul C. Nowak. The Dewar Furan Story, Ronald N. Warrener. Author Index. Subject Index. 

Polycyclic aromatic hydrocarbons are moderately reactive as the diene component of Diels-Alder reactions. Anthracene forms adducts with a number of reactive dienophiles. The addition occurs at the center ring. There is no net loss of resonance stabilization, because the anthracene ring (resonance energy = 1.60 eV) is replaced by two benzenoid rings (total resonance energy = 2 x 0.87 = 1.74 eV).48 49... 

Table PAH6. Formula periodic table for benzenoid polycyclic aromatic hydrocarbons (PAH6)... 

In the light of these long traditions, extensive enumerations of the isomers of benzenoid hydrocarbons is a very new area. A systematic investigation can be dated to 1982 with the first paper of Dias [7] (but see also below). He published an article series in ten parts [7-16] entitled A Periodic Table for Polycyclic Aromatic Hydrocarbons and more recent works [17, 18]. With the invention of the periodic table, Dias created orderness in the chaotic myriads of chemical formulas for benzenoid hydrocarbons, which may be written. He has also written a monograph [19] with relevance to this topic and some other reviews [20-22], Two years before Dias, Elk [23] published a paper on benzenoids, which contains explicitly the enumeration of isomers up to h = 5. It seems that the work of Elk has largely been overlooked in the context of benzenoid isomer enumeration. 

In their analysis of the topological dependency of the aromatic sextet in polycyclic benzenoid hydrocarbons Ohkami and Hosoya [37] arbitrarily define two types of ring perfect matchings viz , orooet and improper as shown below ... 

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