Compounds polycyclic aromatic

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

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. 

FIGURE 9.7 An orbital picture and electrostatic potential map of naphthalene, showing that the ten TT electrons are fully delocalized throughout both rings. 

Just as there are heterocyclic analogs of benzene, there are also many heterocyclic analogs of naphthalene. Among the most common are quinoline, isoquinoline, indole, and purine. Quinoline, isoquinoline, and purine all contain pyridine-like nitrogens that are part of a double bond and contribute one electron to the aromatic tt system. Indole and purine both contain pyrrole-like nitrogens that contribute two -it electrons. 

Among the many biological molecules that contain polycyclic aromatic rings, the amino acid tryptophan contains an indole ring and the anti-malarial drug quinine contains a quinoline ring. Adenine and guanine, two 

Thiamin, or vitamin Bi, contains a positively charged five-membered nitrogen-sulfur heterocycle called a thiazolium ring. Explain why the thiazolium ring is aromatic. 

Problem 15.10 I Show the relative energy levels of the seven 77 molecular orbitals of the cvclohepta-trienyl system. Tel) which of the seven orbitals are filled in the cation, radical, and anion, and account for the aromaticity of the cycloheptatrienyl cation. 

The Huckel rule is strictly applicable only to monocyclic compounds, but the genera] concept of aromaticity can be extended beyond simple monocyclic compounds to include polycyclic aromatic compounds. Naphthalene, with two 

The concept of aromaticity extends to compounds that contain two or more fused rings, so called because two carbon atoms are common to two rings. Compounds of this type, called polycyclic aromatic hydrocarbons, have a 2p orbital on every carbon atom. Examples of polycyclic aromatic hydrocarbons include anthracene and phenanthrene. All carbon atoms in naphthalene, anthracene, and phenanthrene are sp hybridized. All atoms in the rings, as well as those directly attached to the rings, are coplanar. Naphthalene, with two fused rings, is the simplest polycyclic aromatic molecule. Note that all the carbon atoms except those at the points of fusion have a bond to a hydrogen atom. 

The 10 71 electrons of naphthalene are delocalized over both rings. Three resonance forms can be written using localized double bonds. 

Draw the four resonance forms of anthracene and determine which one(s) make the greatest contribution to the resonance hybrid. 

Start with the two possible Lewis structures for the ring at the left. Then place alternating single and double bonds in the other two rings. Two structures result. Neither structure has a Lewis structure for a benzene ring immediately to the right. 

We can write two more structures with the same method, starting with the central ring. These contributing structures can also be obtained by rotating the original two structures by 180°. 

A rather simple derivative of 1-indanone itself has been reported to possess analgesic activity. This is particularly noteworthy in that this agent, drindene ( ), 

Further investigation on the chemistry of the very potent diuretic drug ethacrinic acid (8) led to a compound 

Fig ure 15.10 Energy levels of the six benzene - molecular orbitals. There is a single, lowest-energy orbital, above which the orbitals come in degenerate pairs. 

The results reported in Table 4 were obtained without performing optimization experiments with the most reactive substrates, yields can probably be further increased by using the same experimental expedients employed with strongly 

Aromatic substrate N-chloroamine Orientation (%) Yield (%) Refs. 

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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... 

TABLE 5.3 The nitration of polycyclic aromatic compounds in solutions of acetyl nitrate in acetic anhydride... 

Under different conditions [PdfOAcj2, K2CO3, flu4NBr, NMP], the 1 3 coupling product 86 with 4-aryl-9,10-dihydrophenanthrene units was obtained. The product 86 was transformed into a variety of polycyclic aromatic compounds such as 87 and 88[83], The polycyclic heteroarene-annulated cyclopen-tadicnc 90 is prepared by the coupling of 3-iodopyridine and dicyclopentadiene (89), followed by retro-Diels Alder reaction on thermolysis[84]. 

Non-halogenated plastics Polycyclic aromatic compounds Aliphatics Substituted benzenes... 

Fluorescence scanning of chromatograms of polycyclic aromatic compounds is a vivid example of their employment. A careful choice of the wavelengths of exci-... 

Polycyclic Aromatic Compounds and Their Reduction Products... 

One example of normal-phase liquid chromatography coupled to gas chromatography is the determination of alkylated, oxygenated and nitrated polycyclic aromatic compounds (PACs) in urban air particulate extracts (97). Since such extracts are very complex, LC-GC is the best possible separation technique. A quartz microfibre filter retains the particulate material and supercritical fluid extraction (SPE) with CO2 and a toluene modifier extracts the organic components from the dust particles. The final extract is then dissolved in -hexane and analysed by NPLC. The transfer at 100 p.1 min of different fractions to the GC system by an on-column interface enabled many PACs to be detected by an ion-trap detector. A flame ionization detector (PID) and a 350 p.1 loop interface was used to quantify the identified compounds. The experimental conditions employed are shown in Table 13.2. 

Alkylated, oxygenated and nitrated polycyclic aromatic compounds... 

Figure 13.16 LC separation of urban air particulate exrtact (a), along with the GC/FID cliro-matogram (b) of an oxy-PAC fraction (transfeired via a loop-type interface). Reprinted from Environmental Science and Technology, 29, A. C. Lewis et al., On-line coupled LC-GC-ITD/MS for the identification of alkylated, oxygenated and nirtated polycyclic aromatic compounds in urban air particulate exti acts , pp. 1977-1981, copyright 1995, with permission from the American Chemical Society.

A. C. Eewis, R. E. Robinson, K. D. Bartle and M. J. Pilling, On-line coupled EC-GC-ITD/MS for the identification of alkylated, oxygenated and nitr-ated polycyclic aromatic compounds in urban ah particulate extr acts . Environ. Sci. Technol. 29 1977-1981 (1995). 

Another interesting, but rather complex system, which couples flow injection analysis, EC and GC has been recently reported (47). This system allows the determination of the total amount of potentially carcinogenic polycyclic aromatic compounds (PACs) in bitumen and bitumen fumes. This system could also be used for the analysis of specific PACs in other residual products. 

I. L. Davies, K. D. Battle, P. T. Williams and G. E. Andrews, On-line fractionation and identification of diesel fuel polycyclic aromatic compounds by two-dimensional microbore liigh-peiformance liquid-cliromatography/capillary gas-cliiomatography . Anal. Chem. 60 204-209 (1988). 

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