Ring Benzenoid Hydrocarbons

Mayneord and Roe32 have studied the absorption spectra of several angular condensed ring hydrocarbons. The absorption spectra of these compounds are more complex than those of the members of the linear series. Many of the compounds of the angular series have carcinogenic action and they can be detected in minute quantities spectrophotometrically by virtue of their high intensity absorption bands. 

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In alternant hydrocarbons, carbon atoms can be divided into two sets (starred and non-starred) such that an atom of one set is connected only to atoms of the other set. The necessary and sufficient condition for a hydrocarbon to be alternant is to have only even-membered rings. Benzenoid hydrocarbons have only six-membered rings and are therefore alternant or bipartite. 

Reduction of benzenoid hydrocarbons with solvated electrons generated by the solution of an alkali metal in liquid ammonia, the Birch reaction [34], involves homogeneous electron addition to the lowest unoccupied 7t-molecular orbital. Protonation of the radical-anion leads to a radical intermediate, which accepts a further electron. Protonation of the delocalised carbanion then occurs at the point of highest charge density and a non-conjugated cyclohexadiene 6 is formed by reduction of the benzene ring. An alcohol is usually added to the reaction mixture and acts as a proton source. The non-conjugated cyclohexadiene is stable in the presence of... 

A variety of substituted benzenes are known that have one or more of the hydrogen atoms of the ring replaced with other atoms or groups. In almost all of these compounds the special properties associated with the benzene nucleus are retained. A few examples of benzenoid hydrocarbons follow, and it will be noticed that the hydrocarbon substituents include alkyl, alkenyl, and alkynyl groups. Many have trivial names indicated in parentheses ... 

The theory of the HMO total 7r-electron energy (E) of benzenoid hydrocarbons is surveyed with particular emphasis on the research of its dependence on molecular structure. Identities, bounds and approximate formulas for E are considered. The dependence of E on the size of the molecule and on the number of Kekule structures is discussed in detail. The effect of cycles on E, and six-membered rings in particular, is considered within the framework of the theory of cyclic conjugation. 

Since the theory of cyclic conjugation is mainly concerned with non-benzenoid polycyclic conjugated systems (containing rings of sizes other than six), we shall skip its complete presentation and focus our attention to only those details which are needed in the study of benzenoid hydrocarbons. The readers interested in furhter aspects of this theory should consult the papers [34-40] and the references quoted therein. 

Extensive calculations of the ef-values of benzenoid hydrocarbons were recently reported [39, 40, 64]. In what follows we consider only the case when Z is a hexagon. As customary, we then call Z a ring. 

Rule 1. The effect of empty rings on the total ir-electron energy of fully benzenoid hydrocarbons is nearly the same. The corresponding ef-values vary in the narrow interval (0.02, 0.03). 

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 ... 

The electronic spectra of cyclic conjugated n systems depend inherently on the number of n electrons. Closed-ring systems with AN+l n electrons in the perimeter are aromatic compounds, of which benzene is the most important representative. Benzenoid hydrocarbons constitute a class of compounds whose UV spectra have been investigated most extensively both experimentally and theoretically. The fact that the spectra of aromatic compounds are so characteristic meant that formerly they were of considerable importance in the structure determination of organic compounds. However, these spectra cannot be explained in terms of the simple HMO model. If one seeks a theoretical basis for an understanding, one has the choice between the perimeter model and the Pariser-Parr-Pople or a more complicated numerical method. Before discussing these theoretical models, some empirical relations will be presented. Finally, cyclic systems derived from a perimeter of 4N Jt electrons will be considered. 

Figure 2.8 shows how the experimentally observed wavelengths for two series of cata-condensed benzenoid hydrocarbons, the linearly annelated acenes and the angularly annelated phenes, change with increasing number of benzene rings. The shifts in the Lb and Bb positions are parallel to one another in both series, whereas the bathochromic shift of the Lj, band of acenes upon annelation is so pronounced that even in anthracene it masks the Lb band. 

Krygowski, T.M., Ciesielski, A., Bird, C.W. and Kotschy, A. (1995b). Aromatic Character of the Benzene Ring Present in Various Topological Envirorunents in Benzenoid Hydrocarbons. Nonequivalence of Indices of Aromaticity. J.Chem.Inf.Comput.Sci, 35,203-210. 

Krygowski, T.M., Cyranski, M., Ciesielski, A., Swirska, B. and Leszczynski, P. (1996). Separation of the Energetic and Geometric Contributions to Aromaticity. 2. Analysis of the Aromatic Character of Benzene Rings in their Various Topological Environments in the Benzenoid Hydrocarbons. Crystal and Molecular Structure of Coronene. J.Chem.Inf.Comput.ScL, 36, 1135-1141. 

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