Understanding aromatic hydrocarbons

Arenes are a type of aromatic hydrocarbons with a distinct scent or aroma. They are organic compounds due to the presence of one or more benzene rings within the molecule. One of the largest arenes is benzene, which has a distinctive planar shape due to the electrical requirements of six or more electrons. Aryl substituents are another name for these aromatic units.

 Aromatic hydrocarbons are a type of organic chemical found in the atmosphere that, despite being extremely reactive, has a high emission rate. Aromatic species are also substantial contributors to total global anthropogenic non-methane organic carbon emissions, with car exhaust from gasoline-powered vehicles being the primary source.

Aromatic hydrocarbons

Benzene is the noticeable compound of the fragrant chemical circle of relatives, including a vast range of organic compounds. Benzene has only six hydrogen atoms, an unsaturated ring and a double bond between each carbon atom. Benzene has distinct configurations with alternating single and double bonds across the ring.

In benzene, all bonds are a mix of single and double bonds. The pi bonding electrons are free to journey around the ring in this configuration. Delocalised electrons are those that may transfer among three or more locations. The electrons in benzene are represented as a ring in the hexagon, which can be used to apprehend hydrogen atoms.

Straight-chain hexane is a less stable and reactive chemical than benzene. A wide range of natural compounds and biomolecules include benzene rings. The structural formulation for vanilla and naphthalene explains the similarity.

Aromatic compounds nomenclature

Aromatic compounds are benzene rings with one hydrogen atom replaced by every other. An alkyl group serves as the substituent. Ethylbenzene is the molecule depicted below.

If multiple substituents are present, their relative positions on the benzene ring can be denoted by numbering the positions. When the chlorine atom is replaced with hydrogen, the compound is known as chlorobenzene. Because the An-NH2 group is the amino group, the equivalent molecule is aminobenzene or aniline. A nitro group is an An-NO2  group. Examples: 

 The double-substituted benzene rings are named using the ortho, meta and para prefixes. The meta placement here refers to being in the 1, 3 configuration. Aside from that, the meaning of para placement is to be in a 1, 4 arrangement, with three options illustrated below, in which there are examples of ortho dimethylbenzene, meta diethyl benzene, para diethyl benzene,

As seen below, for 2-phenylbutazone belonging to the phenyl group, a hydrogen atom lacking in a benzene ring is thought to be a substituent on the lengthy chain of carbon atoms.

Aromatic hydrocarbon reactions

There are some specific reactions related to aromatic hydrocarbons, which are mentioned below;

Aromatic substitution

A nitro group can be introduced to the ring in sulfuric solid and nitric acids, where the double bond attacks the NO2 cation and then loses a proton or hydrogen cation to rearrange the system. Substitution of aromatic compounds below is an illustration.

 Electrophilic aromatic substitutions

In an electrophilic aromatic substitution process, an electrophile replaces a substituent on an aromatic ring. Aromatic nitration, aromatic halogenation, aromatic sulfonation and Friedel–Crafts acylation and alkylation are examples of these processes. A resonance-stabilised carbocation intermediate known as a sigma complex may be involved in the reactions below. The reactivity of the substituted benzene rings is shown by each other.

 An electron-donating material usually speeds up the substitution. It directs the reactivity to the ortho and para locations on the ring, whereas an electron-withdrawing material slows down the reaction. Because the intermediate positive charge is stabilised, electrophilic aromatic replacements are ortho or para to electron-donating groups like amines. Resonance structures are the four structures indicated in the middle of the diagram.

Nucleophilic aromatic substitution reaction

In a nucleophilic aromatic substitution process, a nucleophile displaces a substituent on an aromatic ring. An excellent leaving group, such as nitrogen gas or a halide ion, is usually replaced. This type of reaction can be accelerated by the presence of an electron-withdrawing group on the ring. This is chemically analogous to adding an electron-deficient, unsaturated system to a Michael acceptor, followed by an elimination process.

Coupling reactions

Coupling reactions are chemical reactions that use a metal catalyst to produce a carbon-carbon bond between two radicals.

 Conclusion 

Aromatic hydrocarbons are among the most common pollutants released by natural and artificial processes. Bioremediation is one of the methods for removing these insoluble and harmful aromatic pollutants from the environment cost-effectively, using ecofriendly microorganisms. 

They are crucial in developing genetically altered microbes for practical and efficient bioremediation, requiring a thorough understanding of the metabolic pathway and its regulation. They also contribute to creating and maintaining a clean and green environment.