Aromatic Hydrocarbon

3.4 Aromatic Hydrocarbons

Aromatic hydrocarbons (also called arenes), despite their name, often (but not necessarily always) have distinctive aromas (odors). In fact, the name is a traditional one, and these organic compounds are characterized by the presence of one or more benzene rings in the molecule. Benzene, and all the larger arenes, has a characteristic planar structure forced on them by the electronic requirements of the six (or more) π electrons. When named as substituents on other structural units, the aromatic units are referred to as aryl substituents.

The aromatic hydrocarbon group contains not only benzene but also derivatives such as toluene (C₆H₅CH₃), the isomeric dimethyl benzene derivatives (CH₃C₆H₄CH₃), which are used as solvents, as well as in the synthesis of drugs, dyes, and plastics.

The isomeric xylene derivatives: the line projections for the rings indicate the relative positions of the two methyl groups on each benzene ring.

One of the more famous (or infamous) products in the single-ring aromatic hydrocarbon group is trinitrotoluene (or TNT):

Naphthalene, the fused two-ring condensed aromatic hydrocarbon, is derived from coal tar and used in the synthesis of other compounds. A crystalline solid with a powerful odor is found in mothballs and various deodorant-disinfectants. Benzo(a)pyrene, an aromatic hydrocarbon produced in small amounts by the combustion of organic substances, contains five fused benzene rings. Like several other polycyclic aromatic hydrocarbons, it is carcinogenic. Aromatic compounds are widely distributed in nature. Benzaldehyde, anisole, and vanillin, for example, have pleasant aromas.

Introduction

Aromatic hydrocarbons are an important class of organic compounds found in the atmosphere, which are highly reactive and have large emission rates. It has been estimated that aromatic species contribute about 10% to the total global anthropogenic nonmethane organic carbon (NMOC) emissions, the major source being car exhaust from gasoline-powered vehicles, with a significant contribution also from solvent usage. The observation that biomass burning represents a significant source of aromatics has attracted special attention in recent years, since such processes occur on a global scale. Natural, minor sources of aromatics such as emission from soils and plants have also been identified. The emission of aromatics will therefore have an impact on tropospheric processes on local, regional, and global scales.

In many urban areas, emissions of volatile organic compounds (VOCs) and NO_x, when combined under meteorological conditions that favor smog formation, can lead to the formation of ground-level ozone and particulate matter (PM) at harmful levels. Aromatic hydrocarbons make a significant contribution to the formation of ozone and other photooxidants in urban atmospheres. It has been estimated that the percentage contribution of aromatic hydrocarbons to ozone production can be between 30 and 40%. Such a high contribution would make the aromatics the most important class of hydrocarbons with regard to photochemical ozone formation. However, it should be borne in mind that the oxidation mechanisms of aromatic hydrocarbons are still imprecisely known, and verification of such estimates is necessary. In addition to their high atmospheric photochemical reactivity and their consequent major influence on the formation of tropospheric ozone and on the oxidizing capacity of the atmosphere, it is now firmly established that the photochemistry of aromatic compounds leads to the formation of secondary organic aerosols (SOAs), which are known to be harmful to human health, reduce visibility, and can contribute to climate change. In fact, studies have shown that the atmospheric organic aerosol formation potential of whole gasoline vapor can be accounted for solely in terms of the aromatic fraction of the fuel. As emissions of aromatics are concentrated in urban areas, where many people live and work, the formation of SOAs becomes a more acute problem. With regard to health effects, benzene, a ubiquitous aromatic environmental contaminant, is hematotoxic, clastogenic, and leukemogenic in humans and produces bone marrow toxicity and various types of cancer in animals. The reason for the carcinogenesis of benzene is still not entirely clear and it has been classified as a Group-A human carcinogen by the US Environmental Protection Agency. Laboratory studies have also shown that the photooxidation of other aromatic hydrocarbons leads to the formation of mutagenic products.

Presented here is a brief overview of the atmospheric composition and gas-phase chemistry of aromatic hydrocarbons concentrating mainly on benzene, toluene, the xylene isomers, and ethyl benzene, since these rank highly among the most important aromatic hydrocarbons emitted to the atmosphere. This group of aromatic hydrocarbons is often referred to as BTEX in the literature. Since the subject is complex, the reader is encouraged to consult the recent articles and books listed in the reading material for coverage in depth on particular aspects of aromatic hydrocarbon chemistry covered here.

Introduction

Aromatic hydrocarbons are an important class of organic compounds found in the atmosphere. It has been estimated that aromatic species contribute about 10% to the total global anthropogenic nonmethane organic carbon (NMOC) emissions, the major source being car exhaust from gasoline-powered vehicles, with a significant contribution from solvent usage. The observation that biomass burning represents a significant source of aromatics has attracted special attention in recent years, since such processes occur on a global scale. Natural, minor sources of aromatics such as emission from soils and plants have also been identified. The emission of aromatics will therefore have an impact on tropospheric processes on local, regional and global scales.

In many urban areas, emissions of volatile organic compounds (VOCs) and NO_x, when combined under meteorological conditions that favor smog formation, can lead to the formation of ground-level ozone and PM at harmful levels. Aromatic hydrocarbons make a significant contribution to the formation of ozone and other photooxidants in urban atmospheres. It has been estimated that the percentage contribution of aromatic hydrocarbons to ozone production can be between 30 and 40%. Such a high contribution would make the aromatics the most important class of hydrocarbons with regard to photochemical ozone formation. However, it should be borne in mind that the oxidation mechanisms of aromatic hydrocarbons are still imprecisely known, and verification of such estimates is necessary. In addition to their high atmospheric photochemical reactivity and their consequent major influence on the formation of tropospheric ozone and on the oxidizing capacity of the atmosphere, there is now strong evidence that the photochemistry of aromatic compounds can lead to the formation of secondary organic aerosols (SOA), which are known to be harmful to human and ecosystem health. In fact, studies have shown that the atmospheric organic aerosol formation potential of whole gasoline vapor can be accounted for solely in terms of the aromatic fraction of the fuel. As emissions of aromatics are concentrated in urban areas, where many people live and work, the formation of secondary organic aerosols becomes a more acute problem. With regard to health effects, benzene, a ubiquitous aromatic environmental contaminant, is hematotoxic, clastogenic, and leukemogenic in humans and produces bone marrow toxicity and various types of cancer in animals. The reason for the carcinogenesis of benzene is still not entirely clear. Laboratory studies have also shown that the photooxidation of other aromatic hydrocarbons leads to the formation of mutagenic products.

Presented here is a brief overview of the atmospheric composition and gas-phase chemistry of aromatic hydrocarbons concentrating mainly on benzene, toluene, and the xylene isomers, since these rank highly among the most important aromatic hydrocarbons emitted to the atmosphere. Since the subject is complex, the reader is encouraged to consult the recent articles and books listed in the reading material for coverage in depth on particular aspects of aromatic hydrocarbon chemistry covered here.

4.2.1.3 Aromatic hydrocarbons

Aromatic hydrocarbons are cyclic, planar compounds that resemble benzene in electronic configuration and chemical behavior. Benzene has the molecular formula C₆H₆ and is the simplest aromatic hydrocarbon. The carbon atoms in benzene are linked by six equivalent σ bonds and six π bonds. The six p electrons are shared equally or delocalized among the donating carbon atoms, forming a continuous ring of orbitals above and below the plane of the carbon atoms. These delocalized π bonds are more stable than isolated double bonds. All the carbon-carbon bonds in benzene are the same length (1.397 Å), and all of the bond angles are 120°. A wide range of aromatic compounds has benzene rings located in ortho positions. These are called condensed or fused rings. Aromatic compounds with two or more fused aromatic benzene rings are called polycyclic aromatic hydrocarbons (PAH) and they have the general formula C_4r+2H_2r+4 for rings without substituents, where r = number of rings.

Aromatics in petroleum include the mono-aromatic hydrocarbons such as BTEX (the collective name for benzene, toluene, ethylbenzene, and o-, m-, and p-xylene isomers) and other alkyl-substituted benzene compounds (C_n-benzenes), and PAHs, including oil-characteristic alkylated C₀–C₄-PAH (naphthalene, phenanthrene, dibenzothiophene, fluorene, and chrysene) homologous series and US EPA priority PAHs. Benzene is the simplest one-ring aromatic compound. The commonly analyzed PAH compounds range from 2-ring PAHs (i.e., naphthalene) up through 6-ring PAHs (e.g., benzo[g,h,i]perylene). BTEX and PAHs are of concern because of their acute toxic and carcinogenic potential.

Halogenated aromatic hydrocarbons (HAHs) are ubiquitous environmental pollutants and they include polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), and biphenyls (PCBs). Most humans accumulate these chemicals over a lifetime via a diet of fish, meat, poultry, and dairy products. A large number of human epidemiology studies have been conducted in an effort to elucidate the degree to which exposure to PCDDs, PCDFs, and PCBs may increase the morbidity and/or mortality from cardiovascular disease. Additionally, numerous laboratory-based cell culture and experimental animal studies have investigated the cardiovascular toxicity of HAH exposure and provide evidence for the biological plausibility and potential mechanism of action of HAH-induced cardiovascular disease. This chapter will review both the human epidemiology data and laboratory-based experimental studies that have been conducted. Cardiovascular disease etiology is multifactorial and heterogeneous, and gene–environment interactions are important risk factors for disease pathogenesis. The data from both human epidemiology and laboratory animal studies strongly suggest that sustained activation of the aryl hydrocarbon receptor (AHR) by exposure to HAHs represents an important gene–environment interaction that has the potential to contribute to the burden of human cardiovascular disease morbidity and mortality.

4.3 Phyllosphere fungi in bioremediation

Aromatic hydrocarbons (AH) are hazardous air pollutants due to their high genotoxicity and carcinogenicity to all living organisms (Rengarajan et al., 2015). They settle mainly on leaf surfaces and soil through the deposition. It was reported that various microorganisms that inhabit the phyllosphere tend to break down these aromatic hydrocarbons (Waight, Pinyakong, & Luepromchai, 2007), where lignolytic and nonlignolytic fungi, in particular, are capable of degrading AHs into nontoxic substances using various types of enzymes such as oxidases and peroxidases. Undugoda, Kannangara, and Sirisena (2016) reported on the in vitro ability of the phyllosphere non lignolytic fungi; Colletrotrichum siamense, Penicillium oxalicum, Aspergillus oryzea and Aspergilllus aculeatus to degrade xylene, toluene, phenanthrene and naphthalene.

Some fungi can break down formaldehyde, as Yu, Song, Song, Wang, and Guo (2015) demonstrated the ability of Aspergillus sydowii HUA to grow in the presence of 2400 mgL^−   1 formaldehyde. They recorded high formaldehyde dehydrogenase and formate dehydrogenase; 5.02 and 1.06 U mg^−   1, respectively, indicating that this fungal strain is an excellent candidate for removing formaldehyde. Moreover, some fungi that inhabit roots can also colonize leaves and remediate formaldehyde found in the air (Khaksar, Treesubsuntorn, & Thiravetyan, 2016). Recently, Kucharska, Wachowska, and Czaplicki (2020) found that some fungicide-resistant yeast isolates from the phyllosphere were also able to degrade propiconazole. These strains are not only capable of protecting plants against the toxic effects of fungicides but also can accelerate the degradation of fungicides.

Aromatic hydrocarbons

Aromatic hydrocarbons are a special class of unsaturated hydrocarbon based on a six-carbon ring moiety called benzene. The saturated hydrocarbon cyclohexane is transformed into the aromatic hydrocarbon benzene by adding three alternating carbon–carbon double bonds, as shown in Fig. 1.11. The benzene structure can have two arrangements of these double bonds, shown by the pair of benzene molecules in the middle of Fig. 1.11. In nature, these arrangements of bonds rapidly alternate in the benzene structure. Because of this rapid changing or resonating of the position of the three carbon–carbon double bonds, benzene is usually represented as a hexagon with a circle in the center, as shown on the right side of Fig. 1.11. As discussed earlier, carbon–carbon double bonds are usually considered less stable than carbon–carbon single bonds. However, the resonating alternating double bonds in the benzene distribute the electron sharing over all six carbons and impart more stability to the molecule.

Figure 1.11. The benzene structure.

The basic benzene ring structure can be used to build much large molecules by attaching saturated hydrocarbon chains (either straight or branched) or by building multiple ring structures, similar to the naphthenes. These ring structures, as illustrated in Fig. 1.12, may consist of all aromatic rings, pure aromatic compounds, or a mix of aromatic and saturated rings, naphthenoaromatic compounds. In the pure aromatic structures, the resonating stabilization of the benzene unit is extended to the entire structure. And just like in the naphthenes, one or more side chains can be added to any of the carbons in any of the aromatic ring structures.

Figure 1.12. Some examples of basic pure aromatic and naphthenoaromatic structures.

11.6 Conclusions

Aromatic hydrocarbons are one of the major contaminants introduced from the natural as well as through anthropogenic activities. The fate of these aromatics is of major concern due to its toxic, mutagenic, and carcinogenic properties. Bioremediation is one of the alternative environmental-friendly, economical, and cost-effective methods to reduce/remove the load of these recalcitrant and toxic aromatic pollutants from the environment with the help of microorganisms. There is a need to understand the myriad of factors such as environmental factors (temperature, pH, growth conditions, soil properties, etc.), genetic pool, and potential that influences the growth of the microorganism and rate of degradation for an efficient removal of these pollutants from the contaminated sites. Understanding of metabolic pathway and its regulation is also necessary to develop genetically engineered microbe for the effective and efficient bioremediation. These approaches would help us to create and sustain the clean and green environment.

What Are Oil-Related Environmental Pollutants?

Aromatic hydrocarbons including benzene, toluene, ethyl benzene, xylene (BTEX) are retrieved during fossil fuel extraction and used as solvents in consumer and industrial products, as gasoline additives, and as intermediates in the synthesis of organic compounds for many consumer products. Unfortunately, BTEX are persistent organic pollutants, released into the environment mainly by exploration activities of petroleum industry. BTEX, are especially dangerous because of their (1) multiple sources of contamination in the environment (e.g., oil production, oil refrigeration, oil transportation, the production of petroleum, solvents, coal-derived products, traffic, tobacco smoking, voluntary inhalation as drugs of abuse, cosmetics etc.), (2) relatively high solubility in water and air and therefore easy migration and distribution in the environment and easy transport into the cells, (3) highly solubility in lipids and thus readily absorbed from environment in gastrointestinal tract (4) low physico-chemical and biological degradation in ecosystems, and (5) multiple toxic influences and (5) accumulation and low degradation in organisms.

6.11.2 Traditional Approaches to the Study of Aromatic Hydrocarbon Metabolic Pathways

Biodegradation of aromatic hydrocarbons has been extensively studied [4], [5]. Numerous bacterial strains have been isolated for the ability to aerobically degrade a variety of aromatic hydrocarbons. Microbial cells cultivated on aromatic hydrocarbons often exhibit the induction of enzymes involved in metabolic pathways [4]. The bacterial degradation of aromatic hydrocarbons consists of many reaction steps, which have often been broadly separated into peripheral and central pathways [4]. Peripheral pathways convert a large proportion of different aromatic hydrocarbons into a limited number of key central intermediates, such as catechol and protocatechuate. They are normally initiated by dioxygenases, often called Rieske nonheme iron ring-hydroxylating oxygenases (RHOs), catalyzing the introduction of two atoms of oxygen into aromatic hydrocarbons to form cis-dihydrodiols [6]. In the central pathway, these dihydroxylated intermediates are metabolized by enzymes that cleave the aromatic rings via ortho and meta pathways, which are then further degraded into TCA cycle intermediates.

Traditionally, aromatic hydrocarbon degradation pathways have been investigated by straightforward techniques that include experimental identification of metabolic intermediates based on analytical chemistry, biochemical characterization of enzymes, and molecular genetic methods, such as gene cloning, sequencing, and gene knockout mutagenesis. These efforts, based on ‘step-by-step’ or ‘one-gene-at-a-time’ approaches, have greatly contributed to the understanding of a large number of individual metabolic reactions. They have provided fundamental information about metabolic processes and mechanisms involved in the degradation of aromatic hydrocarbons. However, much still remains to be answered. For example, global cellular responses or interactions between pathways of aromatic hydrocarbon metabolism cannot be elucidated by piecewise descriptions of the individual reactions. In particular, some of the components in the metabolic pathways for PAHs containing more than three benzene rings, such as missing metabolites, genes, and enzymes, remain to be determined.