Polycyclic Benzenoid Structures

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. 

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 ... 

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... 

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. 

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... 

L.V. Szentpaly Theoretical model of activation of carcinogenic polycyclic benzenoid aromatic hydrocarbons. Possible new classes of carcinogenic aromatic hydrocarbons J. Molecul. Structure (Theochem) 148 (1986) 141-152 Structure-property relationships in polycyclic aromatics Symposium on Chem. Polynuclear Aromatic Hydrocarbons, Am. Chem. Soc. (1986) 849-855. 

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 ... 

When several benzene rings are fused together to give more extended tt systems, the molecules are called polycyclic benzenoid or polycyclic aromatic hydrocarbons (PAHs). In these structures, two or more benzene rings share two or more carbon atoms. Do these compounds also enjoy the special stability of benzene The next two sections will show that they largely do. 

In Summary The physical properties of naphthalene are typical of an aromatic system. Its UV spectrum reveals extensive delocalization of all rr electrons, its molecular structure shows bond lengths and bond angles very similar to those in benzene, and its NMR spectrum reveals deshielded ring hydrogens indicative of an aromatic ring current. Other polycyclic benzenoid hydrocarbons have similar properties and are considered aromatic. 

This chapter provides a historical perspective on the discovery of and classical approaches to carbon-rich molecules prior to 1970. Carbon-rich is defined here as a ratio of at least one C atom per H atom. Due to the tremendous amount of research in this field, this chapter focuses on the pioneering synthetic efforts and key successes thereafter and will not attempt to serve as an inclusive document on the subject. Several excellent references have been compiled which can provide further information [1], Additionally, the overwhelming plethora of substituted variants, while important in relation to derivatization and molecular diversity, will not be addressed unless unique in application or properties. Examples include the numerous alkyl-substituted or partially reduced derivatives of polycyclic aromatic hydrocarbons described in detail by Clar [la]. Finally, fused benzenoid structures are described with classical numerical nomenclature instead of the alphabetical lUPAC methodology used by Harvey [Icj. 

One can construct other indices for benzene character of benzenoid hydrocarbons, which we will designate as B in order to differentiate them from the index B. For example, one can consider the Kekule index, which assigns to individual Kekule valence structures an index derived from local properties of molecular orbitals, and take the average over all Kekule structures. Even though for many Kekule structures this index is bigger than the Kekule index of benzene, the average Kekule index appears smaller for polycyclic benzenoid hydrocarbons than it is for benzene. However, for an index to reflect benzene character, one expects certain trends among structurally related benzenoids to be satisfied, such as... 

Definition Inscribe in non-adjacent benzene rings of a polycyclic benzenoid hydrocarbon the maximal number of circles (jt-sextets) so that for bonds not involved in jt-sextets one can complete the Kekule valence structure. 

Definition A generalized Clar structure of a polycyclic benzenoid hydrocarbon is a valence formula in which one or more aromatic Ji-sextets are inscribed in non-adjacent benzene rings such that the remaining part of the structure, obtained by deletion of benzene rings with sextets, must have a Kekule valence structure. 

Fortunately, there is now a comprehensive body of knowledge on the metabolic reactions that produce reactive (toxic) intermediates, so the drug designer can be aware of what might occur, and take steps to circumvent the possibility. Nelson (1982) has reviewed the classes and structures of drugs whose toxicities have been linked to metabolic activation. Problem classes include aromatic and some heteroaromatic nitro compounds (which may be reduced to a reactive toxin), and aromatic amines and their N-acylated derivatives (which may be oxidized, before or after hydrolysis, to a toxic hydroxylamine or iminoquinone). These are the most common classes, but others are hydrazines and acyl-hydrazines, haloalkanes, thiols and thioureas, quinones, many alkenes and alkynes, benzenoid aromatics, fused polycyclic aromatic compounds, and electron-rich heteroaromatics such as furans, thiophenes and pyrroles. 

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