Polynuclear Aromatic-Hydrocarbons

Polynuclear aromatic compounds, like the mononuclear aromatics, show characteristic absorption in three regions of the spectrum. 

In the spectra of a-substituted naphthalenes the bands for the isolated hydrogen and the two adjacent hydrogen atoms of /3-naphthalenes are replaced by a band for three adjacent hydrogen atoms. This band is near 810-785 cm1. 

Additional bands may appear because of ring bending vibrations. The position of absorption bands for more highly substituted naphthalenes and other polynuclear aromatics are summarized by Colthup et al. (1990) and by Conley (1972). 

The aromatic C—H stretching and the skeletal vibrations absorb in the same regions as observed for the mononuclear aromatics. The most characteristic 

In the spectra of a-substituted naphthalenes, the bands for the isolated hydrogen and the two adjacent hydrogen atoms of /3-naphthalenes are replaced by a band for three adjacent hydrogen atoms. This band is near 810-785 cm 1. Additional bands may appear because of ring bending vibrations (see Table 2.3). The position of absorption bands for more highly substituted naphthalenes and other polynuclear aromatics are summarized by Colthup et al. (1990) and by Conley (1972). 

The polynuclear aromatic hydrocarbons (abbreviated PAHs or PNAs) are composed of two or more fused benzene rings. Fused rings share two carbon atoms and the bond between them. 

Naphthalene Naphthalene (C oHg) is the simplest fused aromatic compound, consisting of two fused benzene rings. We represent naphthalene by using one of the three Kekuld resonance structures or using the circle notation for the aromatic rings. 

The two aromatic rings in naphthalene contain a total of 10 pi electrons. Two isolated aromatic rings would contain 6 pi electrons in each aromatic system, for a total of 12. The smaller amount of electron density gives naphthalene less than twice the resonance energy of benzene 252 kJ/mol (60 kcal/mol), or 126 kJ (30 kcal), per aromatic ring, compared with benzene s resonance energy of 151 kJ/mol (36 kcal/mol). 

Because they are not as strongly stabilized as benzene, anthracene and phenanthrene can undergo addition reactions that are more characteristic of their nonaromatic polyene relatives. Anthracene undergoes 1,4-addition at the 9- and 10-positions to 

The black material in diesel exhaust consists of small particles that are rich in polynuclear aromatic hydrocarbons. 

Large numbers of polynuclear aromatic compounds occur in the tar from hard coal distillation, in products of pyrolysis and in a variety of residues from combustion. These compounds have been separated by TLC on silica gel [46, 47], alumina [46, 59] and acetyl-cellulose [1, 78]. 

Further improvements in separation have been achieved using two-dimensional TLC on impregnated layers (Berg and Lam [3]) on mixed layers (Kohler et al. [42]) and using multiple development (Petrowitz [67, 68]). Suitable solvents for use with silica gel and alumina layers are hexane, heptane or carbon tetrachloride, also with small additions of a polar solvent, as in the mixtures listed in Table 152. 

The substances are detected by inspection of the layer in UV light or by spraying with reagents such as antimony(V)chloride (No. 18), tetracyanoethylene (No. 243) or formaldehyde-sulphuric acid (No. 125). 

As both Petrowitz [68] and Matsushita and co-workers [52] have been able to show, a linear relation exists for polynuclear aromatic 

Sawicki and co-workers [78] have separated in addition to the compounds in Table 152, a series of aza compounds of structures resembling those of polynuclear aromatic compoxmds they ui dimethyl-formamide-water (35 + 65) and ethanol-water (30 + 70) on cellulose layers and pentane-ether (95 -f 5) on alumina layers. 

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Outside of carbon monoxide for which the toxicity is already well-known, five types of organic chemical compounds capable of being emitted by vehicles will be the focus of our particular attention these are benzene, 1-3 butadiene, formaldehyde, acetaldehyde and polynuclear aromatic hydrocarbons, PNA, taken as a whole. Among the latter, two, like benzo [a] pyrene, are viewed as carcinogens. Benzene is considered here not as a motor fuel component emitted by evaporation, but because of its presence in exhaust gas (see Figure 5.25). 

The purpose of this eornpuLer project is Lo examine several polynuclear aromatic hydrocarbons and to relate their electron density patterns to their carcinogenic activity. If nucleophilic binding to DN.A is a significant step in blocking the normal transcription process of DN.A, electron density in the hydrocarbon should be positively correlated to its carcinogenic potency. To begin with, we shall rely on clinical evidence that benzene, naphthalene, and phenanthrene... 

Troost and Olavesen investigated the application of an internal standardization to the quantitative analysis of polynuclear aromatic hydrocarbons. The following results were obtained for the analysis of the analyte phenanthrene using isotopically labeled phenanthrene as an internal standard... 

Methanol use would also reduce pubHc exposure to toxic hydrocarbons associated with gasoline and diesel fuel, including ben2ene, 1,3-butadiene, diesel particulates, and polynuclear aromatic hydrocarbons. Although pubHc formaldehyde exposures might increase from methanol use in garages and tunnels, methanol use is expected to reduce overall pubHc exposure to toxic air contaminants. 

M. Castegnaro, M. Coombs, M. A. PhiUipson, M. C. Bourgade, andj. Michelon, the 7th Polynuclear Aromatic Hydrocarbons, International Symposium, BateUe Press, Columbus, Ohio, 1982, pp. 257—268. 

The fused 3+ ring aromatics in petroleum include both cata- and peri-condensed stmctures (see Table 4, Fig. 8). The cata-condensed species are those stmctures where only one face is shared between rings, the peri-condensed molecules are those that share more than one face. The fused ring aromatics form the class of compounds known as polynuclear aromatic hydrocarbons (PAH) which includes a number of recognized carcinogens in the 4+ ring family (33). Because of the potential health and environmental impact of PAH, these compounds have been studied extensively in petroleum. 

Table 4. Fused-Ring Polynuclear Aromatic Hydrocarbons Found in Petroleum ...

Fig. 8. Stmctures of fused-ring polynuclear aromatic hydrocarbons. See Table 4.

Cancerous skin lesions of workers exposed to pitch dust undoubtedly support the behef that these lesions are caused by polynuclear aromatic hydrocarbons, although it had not been possible to demonstrate their carcinogenic action in animals more closely related to humans, such as monkeys. 

Aerobic, Anaerobic, and Combined Systems. The vast majority of in situ bioremediations ate conducted under aerobic conditions because most organics can be degraded aerobically and more rapidly than under anaerobic conditions. Some synthetic chemicals are highly resistant to aerobic biodegradation, such as highly oxidized, chlorinated hydrocarbons and polynuclear aromatic hydrocarbons (PAHs). Examples of such compounds are tetrachloroethylene, TCE, benzo(a)pyrene [50-32-8] PCBs, and pesticides. 

Benzene monoxide-oxepin and its sulfur analog are treated elsewhere (Chapter 5.1.7) (67AG(E)385). However, we point out here that electron-withdrawing substituents often favor the benzene oxide tautomer. The first study on oxides of the environmentally hazardous polychloro- and polybromo-biphenyls shows that they exist mainly in the benzene oxide form (81JOC3721). Oxides of polynuclear aromatic hydrocarbons (PAH) also exist mainly in the fused-ring oxirane form. 

Fig. 12. A, Schematic representation of parallel arrays of polynuclear aromatic hydrocarbon molecules in a mesophase sphere. B, a) isolated mesophasc spheres in an isotropic fluid pitch matrix b) coalescence of mesophase c) structure of semi-coke after phase inversion and solidification.

The Determination of 6 Specific Polynuclear Aromatic Hydrocarbons in Waters [Using High-Performance Liquid Chromatography,Thin-layer Chromatography], 1985... 

Silica gel, per se, is not so frequently used in LC as the reversed phases or the bonded phases, because silica separates substances largely by polar interactions with the silanol groups on the silica surface. In contrast, the reversed and bonded phases separate material largely by interactions with the dispersive components of the solute. As the dispersive character of substances, in general, vary more subtly than does their polar character, the reversed and bonded phases are usually preferred. In addition, silica has a significant solubility in many solvents, particularly aqueous solvents and, thus, silica columns can be less stable than those packed with bonded phases. The analytical procedure can be a little more complex and costly with silica gel columns as, in general, a wider variety of more expensive solvents are required. Reversed and bonded phases utilize blended solvents such as hexane/ethanol, methanol/water or acetonitrile/water mixtures as the mobile phase and, consequently, are considerably more economical. Nevertheless, silica gel has certain areas of application for which it is particularly useful and is very effective for separating polarizable substances such as the polynuclear aromatic hydrocarbons and substances... 

In the presence of certain ethers such as Me20, Me0CH2CH20Me or tetrahydrofuran, Na forms deep-green highly reactive paramagnetic adducts with polynuclear aromatic hydrocarbons such as naphthalene, phenanthrene, anthracene, etc. ... 

Polynuclear aromatic hydrocarbons. These consist of a variety of complex structures made up of aromatic rings alone, or combinations of aliphatic rings, aromatic rings, and aliphatic chains, etc. One such class of compounds is biphenyl and its derivatives, in which two benzene rings are connected by a single C — C linkage. The structural formula of biphenyl (or phenylbenzene) is... 

Binuclear aromatic hydrocarbons are found in heavier fractions than naphtha. Trinuclear and polynuclear aromatic hydrocarbons, in combination with heterocyclic compounds, are major constituents of heavy crudes and crude residues. Asphaltenes are a complex mixture of aromatic and heterocyclic compounds. The nature and structure of some of these compounds have been investigated. The following are representative examples of some aromatic compounds found in crude oils ... 

Unsubstituted polynuclear aromatic hydrocarbons show intense molecular ions. The aklylated polynuclear aromatics and the alkylated benzenes fragment similarly ... 

Interatomic Distances in Polynuclear Aromatic Hydrocarbons.—As a further example of the use of the interatomic distance function we shall discuss the polynuclear aromatic hydrocarbons. 

More recently it has become apparent that proton equilibria and hence pH can be equally important in aprotic and other non-aqueous solvents. For example, the addition of a proton donor, such as phenol or water, to dimethylformamide has a marked effect on the i-E curve for the reduction of a polynuclear aromatic hydrocarbon (Peover, 1967). In the absence of a proton donor the curve shows two one-electron reduction waves. The first electron addition is reversible and leads to the formation of the anion radical while the second wave is irreversible owing to rapid abstraction of protons from the solvent by the dicarbanion. 

Standardization. Standardization in analytical chemistry, in which standards are used to relate the instrument signal to compound concentration, is the critical function for determining the relative concentrations of species In a wide variety of matrices. Environmental Standard Reference Materials (SRM s) have been developed for various polynuclear aromatic hydrocarbons (PAH s). Information on SRM s can be obtained from the Office of Standard Reference Materials, National Bureau of Standards, Gaithersburg, MD 20899. Summarized in Table VII, these SRM s range from "pure compounds" in aqueous and organic solvents to "natural" matrices such as shale oil and urban and diesel particulate materials. 

Another useful standard Is SRM 1647, priority pollutant polynuclear aromatic hydrocarbons (in acetonitrile). It can be used to calibrate liquid chromatographic Instruments (retention times. Instrument response), to determine percent recoveries, and to fortify aqueous samples with known PAH concentrations. Figure 8 Illustrates the HPLC separation and UV detection (fluorescence is also used extensively) for the 16 priority pollutants. 

Figure 8. Reversed-phase HPLC separation of SRM 1647, priority pollutant polynuclear aromatic hydrocarbons (In acetonitrile), using UV detection.

The theory and development of a solvent-extraction scheme for polynuclear aromatic hydrocarbons (PAHs) is described. The use of y-cyclodextrin (CDx) as an aqueous phase modifier makes this scheme unique since it allows for the extraction of PAHs from ether to the aqueous phase. Generally, the extraction of PAHS into water is not feasible due to the low solubility of these compounds in aqueous media. Water-soluble cyclodextrins, which act as hosts in the formation of inclusion complexes, promote this type of extraction by partitioning PAHs into the aqueous phase through the formation of complexes. The stereoselective nature of CDx inclusion-complex formation enhances the separation of different sized PAH molecules present in a mixture. For example, perylene is extracted into the aqueous phase from an organic phase anthracene-perylene mixture in the presence of CDx modifier. Extraction results for a variety of PAHs are presented, and the potential of this method for separation of more complex mixtures is discussed. 

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