Naphthalene A Polycyclic Aromatic Compound

The Huckel rule is strictly applicable only to monocyclic compounds, but the general concept of aromaticity can be extended beyond simple mono-cyclic compounds to include polycyclic aromatic compounds. Naphthalene, with two benzene-like rings fused together, anthracene, 1,2-benz-pyrene, and coronene are all well-known compounds, BenzoLa]pyrene is particularly interesting because it is one of the cancer-causing substances that has been isolated from tobacco smoke. 

All polycyclic aromatic hydrocarbons can be represented by a number of different resonance forms. Naphthalene, for instance, has three  

As was true for benzene with its two equivalent resonance forms, no individual structure is an accurate representation of naphthalene. The true structure of naphthalene is a hybrid of the three resonance forms. 

Naphthalene and other polycyclic aromatic hydrocarbons show many of the chemical properties associated with aromaticity. Thus, heat of hydrogenation measurements show an aromatic stabilization energy of approximately 250 kJ/mol (60 kcal/mol). Furthermore, naphthalene reacts slowly with electrophiles such as Br2 to give substitution products rather than double-bond addition products. 

Problem 15.11 Azulene, a beautiful blue hydrocarbon, is an isomer of naphthalene. Is azulene aromatic Draw a second resonance form of azulene in addition to that shown. 

An orbital picture of naphthalene, showing that the ten w electrons are fully delocalized throughout both rings. 

---

Dewar and his co-workers, as mentioned above, investigated the reactivities of a number of polycyclic aromatic compounds because such compounds could provide data especially suitable for comparison with theoretical predictions ( 7.2.3). This work was extended to include some compounds related to biphenyl. The results were obtained by successively compounding pairs of results from competitive nitrations to obtain a scale of reactivities relative to that of benzene. Because the compounds studied were very reactive, the concentrations of nitric acid used were relatively small, being o-i8 mol 1 in the comparison of benzene with naphthalene, 5 x io mol 1 when naphthalene and anthanthrene were compared, and 3 x io mol 1 in the experiments with diphenylamine and carbazole. The observed partial rate factors are collected in table 5.3. Use of the competitive method in these experiments makes them of little value as sources of information about the mechanisms of the substitutions which occurred this shortcoming is important because in the experiments fuming nitric acid was used, rather than nitric acid free of nitrous acid, and with the most reactive compounds this leads to a... 

A set of n = 209 polycyclic aromatic compounds (PAC) was used in this example. The chemical structures have been drawn manually by a structure editor software approximate 3D-structures including all H-atoms have been made by software Corina (Corina 2004), and software Dragon, version 5.3 (Dragon 2004), has been applied to compute 1630 molecular descriptors. These descriptors cover a great diversity of chemical structures and therefore many descriptors are irrelevant for a selected class of compounds as the PACs in this example. By a simple variable selection, descriptors which are constant or almost constant (all but a maximum of five values constant), and descriptors with a correlation coefficient >0.95 to another descriptor have been eliminated. The resulting m = 467 descriptors have been used as x-variables. The y-variable to be modeled is the Lee retention index (Lee et al. 1979) which is based on the reference values 200, 300, 400, and 500 for the compounds naphthalene, phenanthrene, chrysene, and picene, respectively. 

The study of vapour-deposited organic films has been applied to a large number of molecular systems. However, most of the work has concentrated on the study of the growth and optoelectronic characteristics of planar stacking molecules such as the Pc and polycyclic aromatic compounds based on naphthalene, and perylene. In particular, PTCDA has become extensively studied. 

Lane, D. A., and H. Tang, Photochemical Degradation of Polycyclic Aromatic Compounds. I. Naphthalene, Polycyclic Aromat. Compd., 5, 131-138 (1994). 

However, it should be noted that even where kerosene heaters do not contribute significantly to particle mass concentrations, they may still be important in terms of health effects. This is because of the composition of the particles emitted, which include polycyclic aromatic compounds and other mutagenic species, as well as sulfate (Leaderer et al., 1990 Traynor et al., 1990). For example, Traynor et al. (1990) studied the emissions from unvented kerosene space heaters and identified a number of PAHs (naphthalene, phenanthrene, fluoranthene, anthracene, chrysene, and indeno[c,rf]pyrene) and nitro-PAHs (1-nitronaphtha-lene, 9-nitroanthracene, 3-nitrofluoranthene, and 1-nit-ropyrene), in addition to a host of other gaseous species. Baek et al. (1997) also reported increased levels of a number of gases indoors in homes and offices in Korea due to the use of kerosene heaters. 

The intercalation of polycyclic aromatic compounds into duplex DNA structures was used to develop nucleic acid-based electrochemical sensors.66 For example, the bis-ferrocene-tethered naphthalene diimide (16) was used as a redox-active intercalator to probe DNA hybridization.67 The thiolated probe was assembled on a Au electrode, and the formation of the duplex DNA with the complementary analyte nucleic acid was probed by the intercalation of (16) into the double-stranded nucleic acid structure and by following the voltammetric response of the ferrocene units (Fig. 12.17a). The method enabled the analysis of the target DNA with a sensitivity that corresponded to ca. 1 x 10-20mol. 

Isoprenoid structures for carotenoids, phytol, and other terpenes start biosynthetically from acetyl coenzyme A (89) with successive additions giving mevalonate, isopentyl pyrophosphate, geranyl pyrophosphate, farnesyl pyrophosphate (from which squalene and steroids arise), with further build-up to geranyl geranyl pyrophosphate, ultimately to a- and /3-carotenes, lutein, and violaxanthin and related compounds. Aromatic hydrocarbon nuclei are biosynthesized in many instances by the shikimic acid pathway (90). More complex polycyclic aromatic compounds are synthesized by other pathways in which naphthalene dimerization is an important step (91). 

As shown in Figure 1.14, some aromatic compounds, such as naphthalene and the polycyclic aromatic compound, benzo(a)pyrene, contain fused rings. 

Polycyclic aromatic compounds also undergo electrophilic aromatic substitution reactions. Because the aromatic resonance energy that is lost in forming the arenium ion is lower, these compounds tend to be more reactive than benzene. For example, the brotni-nation of naphthalene, like that of other reactive aromatic compounds, does not require a Lewis acid catalyst ... 

All possible interactions between the K and L groups were taken into account and Akk = 0. The definition contains a minimum number of groups and is satisfactory for most of the binary systems studied. However, it cannot take into account the structural differences which exist between position isomers. This is the case of polycyclic aromatic compounds presenting cycle position isomers or substitute position isomers. Structural differences of this type determine the gaps between the values of certain thermophysical properties of isomers, such as, for example, the fusion temperature or sublimation enthalpy. The further the temperature falls, the more these differences are accentuated. The representation of the solid-fluid (low temperature) equilibria is consequently more difficult and the model must take into account the existing structural differences. We came across this problem in the compounds such as anthracene, phenanthrene, pyrene, methylated naphthalenes, hexamethylbenzene and triphenylmethane. As it was out of the question to increase the number of groups because... 

The pyrolytic studies on meteorites are commonly done at different temperatures. A preheating step is utilized to insure that any possible adsorbed gases on the surface of the meteorite from the terrestrial environment are eliminated. Several organic compounds are monitored in pyrolysates such as polycyclic aromatic compounds. As an example, the results on naphthalene production upon pyrolysis from several carbonaceous chondrites, normalized by the total carbon content before pyrolysis, are shown in Figure 17.2.1 [76],... 

Polycyclic aromatic compounds (PAC) are a significant environmental chemical group, with an associated health effect. PACs includes PAH (polycyclic aromatic hydrocarbon), PASH (polycyclic aromatic sulfur hydrocarbon), and PANH (polycyclic aromatic nitrogen hydrocarbon). The main PAHs are naphthalene, fluorene, phenanthrene, and their alkylated homologs PASHs are benzothiophene and diben-zothiophene PANHs are indole, carbazole, quinoline, and their alkylated derivatives (see Table 1). 

ABA triblock copolymers cannot be produced by monofiinctional initiation when the A anion is not sufficiently basic to initiate polymerization of B monomers. In such cases the bifunctional initiators like alkali metal complexes of polycyclic aromatic compounds (e.g., naphthalene and biphenyl) can be used to produce ABA triblock copolymers. In these cases polymerization would be started with monomer B to produce a polymeric dianion which would initiate polymerization of the A monomer that is added later. The process is illustrated below for the commercially important styrene-butadiene-styrene (SBS) triblock copolymer ... 

A number of polycyclic aromatic compounds have been investigated by coherent INS of the perdeuterated compounds including naphthalene [41] and anthracene [42]. Perylene was used as a model compound for the aromatic regions of coal [43]. 

Catalytic hydrogenations over CojfCOjg (using Hj and CO) or with stoichiometric quantities of preformed hydridocarbonyl complex CoH(CO)4 are useful for the partial selective reductions of polycyclic aromatic compounds. Isolated benzene rings are not affected. Naphthalene is reduced to tetralin, at 200°C under a pressure of 20 X 10 kPa and anthracene to 9,10-dihydroanthracene (99%). The substituted phenanthrene nucleus is stable under these conditions as illustrated by hydrogenation of perylene 1 and pyrene 2. ... 

Many of the organic contaminants which were found in Lippe river water were also present in the source samples (see Table 3). The sewage effluent sample and the Seseke river showed the best accordance with the compound spectrum of the Lippe river. However, also in the two tributaries from the rural upper reaches of the river, numerous specific contaminants like 9-methylacridine (No. 8), alkyl phosphates (Nos. 31, 32) and chlorinated alkyl phosphates (Nos. 34, 36) appeared. In the effluent of a pharmaceutical plant, only a few Lippe river contaminants like n-alkanes (No. 1), naphthalene (No. 3), TXIB (No. 21) and caffeine (No. 67) were detected (see Table 3). Therein, mainly structural relatives of androstanone like 3p-hydroxy-5p-androstan-17-one, 3a-hydroxy-5p-androstan-17-one and androstan-50-3,17-dione were present. These compounds are probably by-products of the synthesis of hormone preparations. Some polycyclic aromatic compounds, halogenated compounds and terpenoids were not detected in the source samples (see the underlined compounds in Table 3) and probably have another origin. Representative sampling of various input sources have to be carried out to prove the origin of these compounds. Hexachlorobutadiene (No. 38) and bis(chloropropyl)ethers (No. 44) appear exclusively at the lower reaches of the Lippe river (see Table 1), downstream the chemical plants in Marl. They are attributed to inputs of the chlorochemical industry (see section 3.1). Hence, this suggests their input by an industrial point source. 

Polycyclic aromatic compounds such as naphthalene, methoxynaph-thalenes, dimethylnaphthalenes, and anthracene undergo efficient acylation with acyl chlorides and anhydrides in the presence of the SZ produced from zirconium(lV) oxychloride by a methodology similar to that previously mentioned. ... 

The criteria for determining whether a monocyclic hydrocarbon compound is aromatic can also be used to determine whether a polycyclic hydrocarbon compound is aromatic. Naphthalene (five pairs of tt electrons), phenanthrene (seven pairs of tt electrons), and chrysene (nine pairs of tt electrons) are aromatic. 

All aromatic compounds discussed to this point contain a single benzene ring. There are also substances called polycyclic aromatic compounds, which contain two or more benzene rings sharing a common side, or fused together. The simplest of these compounds, naphthalene, is the original active ingredient in mothballs ... 

---