What Are Aromatic Compounds?

Aromatic compounds are a class of compounds with a benzene ring structure. They are stable in structure, difficult to decompose, and highly toxic, causing serious pollution to the environment and great harm to the human body. Historically, a class of substances with aromatic odors obtained from plant gums was called aromatic compounds. Aromatic compounds in high school generally refer to hydrocarbon molecules containing at least one benzene ring with a delocalized bond, but there are examples of modern aromatic compounds that do not contain benzene rings. Aromatic compounds are all "aromatic".

Modern aromatics refer to hydrocarbon molecules that contain at least one

General term for all hydrocarbons with aromatic benzene or heterocyclic rings. Can be divided into two categories: benzene or monobenzene aromatics, compounds with a benzene ring and its derivatives. Such as benzene, phenol, halogenated benzene, toluene, etc .; Polycyclic aromatic hydrocarbons (PAHs), polycyclic hydrocarbons with benzene rings or heterocyclic rings. Such as naphthalene, anthracene, pyrene, pyrene, benzopyrene and the like. For example, a polycyclic compound formed by two or more benzene rings and heterocyclic rings with a common ring edge is called a benzene condensed heterocyclic compound, such as indole, quinoline, and fluorene. Emissions from the industries of coking, petrochemicals, dyes, pharmaceuticals, pesticides, paints and fossil fuels are the main man-made sources of aromatic hydrocarbons in the environment. Some plants and bacteria in nature can also produce such compounds, such as eugenol and wintergreen oil. Many aromatic hydrocarbons are harmful substances in the environment, especially the pollution of polycyclic aromatic hydrocarbons can cause mutagenesis and carcinogenicity, which has attracted worldwide attention. ^[2]

Monocyclic aromatic hydrocarbons

Degradation of benzene

Studies on the degradation of benzene have been very successful 30 years ago. When benzene is degraded, there are two branch pathways, as shown in Figure 1 (a). The benzene ring was initially attacked by benzene dioxygenase to form catechol, and catechol further produced muconic acid semialdehyde or muconic acid through the action of meta or ortho dioxygenase.

Degradation of substituted benzene

The presence of a substituent group makes the degradation of the benzene ring possible in two ways: the benzene ring is degraded first or the side chain is degraded first. The general route for a monohydrocarbyl-substituted benzene containing 2 to 7 carbon atoms is shown in Figure 1 (b). When C> 7, the hydrocarbyl chain is replaced by , oxidative degradation, and then the benzene ring is degraded. The long hydrocarbyl side chain is oxidized enough to provide microorganisms with energy for growth, so that the microorganisms will not degrade the benzene ring.

Degradation of biphenyl

The biodegradable biphenyl pathway is shown in Figure 1 (c). There are two pathways for the degradation of oxygenated biphenyls: 1, 2 and 3, 4 oxygenation. The former is mostly, and biphenyls undergo two-step dioxygenase action. It then forms 2-6-keto-6-phenyl-6-phenyl-2,3-hexadiene acid (HOPDA), which is then further degraded into benzoic acid (BA). Biphenyls and low-substituted biphenyls can also be degraded by microorganisms, and the products of degradation are single- and double-base compounds.

Polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (Polycyclic Aromatic Hydrocarbons, PAH) are a class of compounds containing two or more fused aromatic rings produced by incomplete combustion of organic matter. The pathway of microbial degradation of Tsai is shown in Figure 1 (d). As with the degradation of other aromatic compounds, in the first step the dioxygenous bacteria attack ring forms 1, 2-via Gizai, and then breaks between the 1st and 9th carbon atoms. ^[4]

Aromatic compound oxygenase

The benzene ring compound is difficult to decompose because of its benzene ring structure. To decompose it under normal temperature and pressure, it must rely on the participation of enzymes. The oxidases involved in the metabolism of benzene ring compounds can be divided into two categories: one is benzene ring hydroxylated oxygenase; the other is benzene ring cleaved oxygenase ¨3'Hj. The benzene ring hydroxylation oxygenase is to add two hydroxyl groups to the benzene ring through the oxygen molecule and NADH or NADPH to provide electrons, such as toluene catalyzed by toluene dioxygenase to form cis toluene dihydrodiol with oxygen molecules. The benzene ring cleavage oxygenase is the epoxidation and ring opening of benzene by oxygen molecules. For example, catechol forms muconic acid or muconic acid semialdehyde under the action of oxygen molecules and enzymes. The earliest benzene ring hydroxylated oxygenase was isolated from Gibson et al. From P. putida. All enzymes belong to the complex enzyme system and are composed of 2 to 3 proteins, but in the composition of subunits. There are quite a few differences. Batie et al. Divided them into 3 categories: ClassI, Class, and ClassUl. Clam is composed of 2 components, while ClasslI and ClassHI are composed of 3 components. Benzene ring cleavage oxygenase can be divided into two forms when performing ring-opening reaction: one is cleavage inside the dihydroxyl group, which can also be called ortho-position cleavage; , Also known as meta-cutting. ^[3]