Structures of Benzene and Naphthalene

Benzene is described as a resonance hybrid of the two extreme forms which correspond, in terms of orbital interactions, to the two possible spin-coupled pairings of adjacent p electrons structures 1 and 2. These are known as resonance contributors , or mesomeric structures , have no existence in their own right, but serve to illustrate two extremes which contribute to the real structure of benzene. Note the standard use of a double-headed arrow to inter-relate resonance contributors. Such arrows must never be confused with the use of opposing straight fish-hook arrows that are used to designate an equilibrium 

Heterocyclic Chemistry 5th Edition John Joule and Keidi Mills 2010 Blackwell Publishing Ltd 

Structure of benzene resonance contributors (mesomeric structures) 

Treating naphthalene comparably reveals three resonance contributors, 3, 4 and 5. The valence-bond treatment predicts quite well the non-equivalence of the bond lengths in naphthalene in two of the three contributing structures, C-l-C-2 is double and in one it is single, whereas C-2-C-3 is single in two and double in one. Statistically, then, the former may be looked on as 0.67 of a double bond and the latter as 

33 of a double bond the measured bond lengths confirm that there indeed is this degree of bond fixation, with values closely consistent with statistical prediction. 

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V. The Quantum-Mechanical Calculation of the Resonance Energy of Benzene and Naphthalene and the Hydrocarbon Free Radicals," J.Chem.Physics 1 (1933) 362374 Linus Pauling and J. Sherman, "The Nature of the Chemical Bond. VI. Calculation from Thermochemical Data of the Energy of Resonance of Molecules Among Several Electronic Structures," J.Chem.Physics 1 (1933) 606617 and Pauling and Sherman, "The Nature of the Chemical Bond. VH. The Calculation of Resonance Energy in Conjugated Systems," J.Chem.Physics 1 (1933) 679686. 

Formation by a radical addition reaction The presence of methyl and ethenyl groups in the products indicates that shock waves destroyed the structure of benzene and formed some lower-molecular-weight radicals (e.g., methyl and ethenyl radicals). If we assume Uiat the formation of a MeNap or PhNap molecule is a result of attack by a methyl or a phenyl radical against naphthalene molecule, respectively, the yield relations in isomers estimated from tire reactivity indices would be 1-MeNap >2-MeNap and 1-PhNap >2-PhNap. These relative amounts are inconsistent with Uiose of the shock products. Therefore, it is unreasonable to invoke a radical reaction only for the reaction of MeBip and PhNap. 

Figure 15 Labeled Kekule structure K, S3 structure of benzene, and D3j structure of naphthalene...

The relation between these two very interesting structures and belt- or tube-shaped molecules becomes clear after careful contemplation of C70 (58). One recognizes an aromatic belt consisting of benzene and naphthalene rings as a subunit which very much resembles molecule 59 proposed by Vogtle [5]. 

While borazine is analogous in shape and structure to benzene, the behavior observed here differs sharply from that of benzene. During the pyrolysis of benzene at temperatures in excess of 500 C, linear biphenyl, terphenyl and polyphenyl units are formed which do not exhibit liquid crystallinity [22]. In contrast, when naphthalene or anthracene are heated, they will enter a liquid crystalline mesophase and will produce graphitizable carbon upon continued pyrolysis (Figure 2.3). This mesophase consists of a wide range of planar, polynuclear species which have a tendency to stack, thereby forming a discotic phase. The mesophase is also observed to form when a 50 50 mixture of benzene and naphthalene is heated [23]. 

The coupling constants of ortho ( Jhh = 7 Hz), meta Jhh =1-5 Hz) and para protons CJhh I Hz) in benzene and naphthalene ring systems are especially useful in structure elucidation (Table 2.5). With naphthalene and other condensed (hetero-) aromatics, a knowledge of zig zag coupling = 0.8 Hz) is helpful in deducing substitution patterns. 

The procedure used in the preceding sections for cyclopropane serves equally well in the analytic determination of the numbers of structure and stereoisomeric compounds which obtain when essentially different radicals of valence one or alkyl radicals are substituted in the basic compound. We have to assume, however, that there is enough information on the basic compound to determine the three groups discussed in Sec. 56. This is certainly the case for the most important basic compounds, benzene and naphthalene. I omit the formulation of rules which are obvious in the preceding example. 

Although the observed products are consistent with initial formation of 4a, Untch proposed that they might arise via the intermediacy of cyclic allene 3a.23 Subsequent experiments by W.M. Jones and co-workers supported the initial formation of 3a in the dehydrohalogenation reactions, but left open the possibility of a rapid equilibrium between cycloheptatetraene (3a) and cycloheptatrienylidene (4a, Scheme 9).22,24 It was also found that fusion of benzene or naphthalene rings to different positions on the seven-membered ring strongly influenced whether a carbenic or allenic structure predominates.25... 

Compounds 1 and 2 were identified by FTIR and 13C-NMR. The 13C proton decoupled spectra for 1 and 2 are dominated by signals ranging from 62 to 195 ppm. The 13C chemical shift assignments were made based on comparisons with 4,4 -(hexafluoroisopropylidene)diphenol and from calculations based on substituted benzenes and naphthalenes.15 The 13C-NMR spectrum clearly showed that the Friedel-Crafts acylation of 1 by 4-fluorobenzoyl chloride yielded the 1,4-addition product exclusively. The 13C chemical shifts for 2 are listed in Table 8.1. The key structural features in the FTIR spectrum of2 include the following absorptions aromatic C-H, 3074 cnr1, ketone C=0, 1658 cm-1, aromatic ether Ar—0—Ar, 1245 cm-1, and C—F, 1175 cm-1. 

Nitrogen. Pyridine is one of the most important heterocycles. The aromaticity of pyridine was intensively connected to structural considerations and chemical behavior. The relative difference between the aromaticity of benzene and pyridine is controversial generally calculations give similar orders of magnitude and differences depend on the criterion of aromaticity considered and the mode of calculation used. A comprehensive review on the theoretical aspects in connection with the aromaticity of pyridine was published.191 Pyridine is about as aromatic as benzene according to theoretical calculations and to experimental data, while quinoline is about as aromatic as naphthalene and more aromatic than isoquinoline.192193 The degrees of aromaticity of pyridine derivatives strongly depend on their substituents. 

The fact that most alkylated benzenes show the same tendency to soot is also consistent with a mechanism that requires the presence of phenyl radicals, concentrations of acetylene that arise from the pyrolysis of the ring, and the formation of a fused-ring structure. As mentioned, acetylene is a major pyrolysis product of benzene and all alkylated aromatics. The observation that 1-methylnaphthalene is one of the most prolific sooting compounds is likely explained by the immediate presence of the naphthalene radical during pyrolysis (see Fig. 8.23). 

Benzene and naphthalene sulfonate moieties are present in the structures of many dyes that can be found in large amounts in wastewaters from textile and food industries. Even if wastes are decolored before the final discharge, not enough attention is nowadays devoted to the identification of possible uncolored degradation products, potentially toxic, that form during the decolorization process and are discharged into the aquatic systems. Besides sulfonate derivatives, aromatic amines have also been reported as possible degradation products of dyes [109],... 

These simple molecular orbital pictures provide useful descriptions of the structures and spectroscopic properties of planar conjugated molecules such as benzene and naphthalene, and heterocychc species such as pyridine. Heats of combustion or hydrogenation reflect the resonance stabilization of the ground states of these systems. Spectroscopic properties in the visible and near-ultraviolet depend on the nature and distribution of low-lying excited electronic states. The success of the simple molecular orbital description in rationalizing these experimental data speaks for the importance of symmetry in determining the basic characteristics of the molecular energy levels. 

It is interesting that benzene and naphthalene form monolayer surface structures on the Pt(l 11) crystal face at 300 K and higher temperatures while monolayer surface structures form only at low temperatures ( 200 K) on the Ag(l 11) crystal face While these aromatic molecules are held by strong chemical bonds to the platinum, their heats of adsorption must not be greater than the heats of sublimation... 

The data shown in Table 2 illustrate the general paucity of comparative toxicity data within an isosteric series of chemicals. In this Table a variety of toxic end-points observed for benzene and naphthalene have been compared with those of their simple heterocyclic analogues, and it is clear that it is almost impossible to derive chemical structure-biological activity relationships from the published literature for even such a simple series of compounds. Even basic estimates of mammalian toxicity such as LD50 values cannot be accurately compared due either to the absence of relevant data or the noncomparability of those available. Thus in a field where there are little comparative data on the relative toxicity to mammals of pyrrole, thiophene and furan for example, it is difficult to relate chemical structure to biological activity in historical heterocyclic poisons such as strychnine (3) and hemlock [active agent coniine (4)]. 

The chemisorption of hydrocarbons, ethylene, cyclohexene, n-heptane, benzene and naphthalene at room temperature and above were studied on both the Au(l 11) and Au[6(l 11) x (100)] stepped surfaces (29). The difference in the adsorption characteristics of hydrocarbons on gold surfaces and on platinum surfaces is striking. The various light hydrocarbons studied (ethylene, cyclohexene, n-heptane, and benzene) chemisorb readily on the Pt(lll) surface. These molecules, on the other hand, do not adsorb on the Au(lll) surface under identical experimental conditions as far as can be judged by changes that occur in the Auger spectra. Naphthalene, which forms an ordered surface structure on the Pt(lll) face, forms a disordered layer on adsorption on the Au(l 11)surface. 

The crystal structure of benzene itself is even more difficult, and it is only quite recently that accurate results have been obtained. The early work on naphthalene and anthracene was also inconclusive and did not at first lend any support to the idea of strictly planar molecules. The first really conclusive results were obtained for the molecule of hexamethylbenzene (Lonsdale, 1929). In the triclinic crystal structure the atoms occupy general positions, but a careful study of the intensities of the reflections, particularly those from the pronounced cleavage plane in which the molecule is found to lie, established that the molecule was planar to within narrow limits, and also that the benzene ring was a regular hexagon. Soon afterwards the more difficult structures of naphthalene and anthracene were fully analysed with the aid of absolute intensity measurements and the use of Fourier methods of analysis (Robertson, 1933a), and it was shown that the atoms were coplanar to within a few hundredths of an Angstrom unit. 

The spin-coupled method has now been applied to a large number of aromatic systems benzene and naphthalene azobenzenes, such as pyridine, pyridazine, pyrimidine and pyrazine five-membered rings, such as furan, pyrrole, thiophen, and thiazole and inorganic heterocycles, such as borazine ( inorganic benzene ) and boroxine, for which we find little evidence of aromaticity. Structural formulae are collected in Fig. 1. For all of these molecules we have included the effects of electron correlation for the Jt electrons but not for the a framework. This a-n separation is an approximation whose utility rests upon the chemistry of aromatic systems — to abandon it would be to ignore this entire body of experience. Furthermore, very extensive calculations [4] have demonstrated that rc-electron only correlation affords an excellent description of ground and excited states of benzene. 

These dyes are the backbone of most commercial dye ranges. Based entirely on benzene and naphthalene derivatives, they provide yellow, red, blue, green, and black colors for all the major substrates such as polyester, cellulose, nylon, polyacrylonitrile, and leather. Typical structures are (79)—(84). 

Figure 7 A displays interference between fine structure due to vibrational coupling and a broad structure which is almost identical for the three types of monolayers. The question that arises is why such vastly different size molecules show similar broad transmission of electrons. Studies of electron scattering from these molecules in the gas phase indicated that the two first electronic resonances for benzene and naphthalene are at 1.12, 4.8 eV and...

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