Polycyclic benzenoid

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 benzenoids These aromatic compounds have two or more benzene rings fused together, e.g. naphthalene and anthracene. 

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 other compounds also exhibit aromatic characteristics. Some of the most common have two or more benzene rings fused together. Such compounds are called polycyclic benzenoid aromatic compounds. A typical example of this type of molecule is naphthalene, C10H8. Structurally, naphthalene looks like this ... 

Polansky and Derflinger proposed a useful concept of benzene character which is the projection of the ocupied it-MO s in a given hexagon L of a polycyclic benzenoid hydrocarbon onto the three occupied MO s of a benzene molecule located on that position [27]. This quantity is shown to be expressed as a linear combination of the Coulson bond order for the component six bonds and for the three para-bonds in L. Their original definition of the benzene character for L can be transformed into the normalized benzene character as [28]... 

Early in 1963 Culbertson and Pettit6 reported the base strengths of ten simple polycyclic benzenoid aldehydes and ketones and attempted to relate the pKa values to molecular structure. They were able to show the existence of a linear correlation between -pKa and the protonation it energy, that is, the gain in n energy of the protonated over the neutral compound. In their treatment... 

Balaban and Artemi wrote very recently (1990) [63]. The enumeration of polycyclic benzenoid hydrocarbons (polyhexes, or benzenoids) continues to be a challenging problem. The field has been flourishing, not least during the very last few years, as documented by the present list of references. It contains 16 relevant publications from 1989, which can be supplemented by a few more... 

The concept of "resonant rings" follows from Clar s representations of benzenoids [48] in which one identifies "pi-sextets", that coincide with six pi-electrons of various individual benzene rings in polycyclic-benzenoids- If a pi-sextet is represented by an inscribed circle we can Immediately see that, for example, rings 1 and 4 in the benzenoid considered are "resonant", i-e-, both rings can simultaneously be "sextet" rings- However, rings 2 and 4 are not resonant, because we cannot... 

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 ... 

Aihara, J. (1996). Bond resonance energies of polycyclic benzenoid and non-benzenoid... 

Balaban, A.T. (1988a). Chemical Graphs. Part 49. Open Problems in the Area of Condensed Polycyclic Benzenoids Topological Stereoisomers of Coronoids and Congeners. Rev.Roum.-Chim.,33,699-7(n. 

E C omaticity in Polycyclic Benzenoid Hydrocarbons, William C. Herndon and Paul C. Nowak. The Dewar Furan Story, Ronald N. Warrener. Author Index. Subject Index. Volume 3,1995, 316 pp. 97.50 ISBN 1-55938-698-3... 

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. 

Compounds having an -OH group attached to a polycyclic benzenoid ring are also phenols. They are known as naphtols or phenantrols. 

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. 

Polycyclic benzenoid hydrocarbons are compounds that contain two or more fused benzene rings. Fused rings share two adjacent carbons—naphthalene has two fused rings, anthracene and phenanthrene have three fused rings, and tetracene, triphenyl-ene, pyrene, and chrysene have four fused rings. There are many polycyclic benzenoid hydrocarbons with more than four fused rings. 

Like benzene, all the larger polycyclic benzenoid hydrocarbons undergo electrophilic substitution reactions. Some of these compounds are well-known carcinogens. The chemical reactions responsible for causing cancer and how you can predict which compounds are carcinogenic were discussed in Section 12.8. 

Polycyclic benzenoid hydrocarbons contain two or more fused benzene rings fused rings share two adjacent carbons. Polycychc benzenoid hydrocarbons undergo electrophilic aromatic substitution reactions. Naphthalene undergoes irreversible substitution predominantly at the 1-position and reversible substitution predominantly at the 2-position. The nature of the substituent determines which ring of a substi-tuted-naphthalene undergoes electrophilic substitution. 

Of the numerous aromatic polycyclic benzenoid hydrocarbons discussed by Kry-gowski and Cyranski in their chapter [50], there are enthalpy-of-formatibn data for fewer than half of them. Despite the paucity of compounds for which there are data, there is no scarcity of data itself for naphthalene alone there are at least 10 determinations of the enthalpy of formation of the solid and 15 of its enthalpy of sublimation All but a few of the solid substances have more than one reference for an enthalpy of formation and usually several references for the enthalpy of sublimation. Benzene, a liquid, seems the sole compound for which there are relatively few measurements and consistent results. Our favorite compendium of data [4] does not include some of the more recent values so we begin by recommending gaseous... 

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... 

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