Aromatic Hydrocarbons

Introduction to Aromatic Hydrocarbons

Aromatic hydrocarbons are organic molecules with a circular structure, sigma bonds and delocalised pi electrons. Arenes or aryl hydrocarbons are other names for them. These are unsaturated hydrocarbons containing a single, or more planar 6-carbon rings termed benzene rings. A benzene ring can be found in several aromatic hydrocarbons. Further, a resonance stabilises the benzene ring, and the pi electrons in the structure of the ring get delocalised.

Heteroarenes are aromatic hydrocarbons that don’t have a benzene ring in their structure. A minimum of one carbon is substituted by nitrogen, oxygen, or sulfur in these compounds. Huckel’s rule is followed by all of these heteroarenes. Furan and pyridine are two examples of heteroarenes.

Aromatic Hydrocarbon: Properties

The aromatic hydrocarbon benzene was the first to be categorized. It is among the most difficult aryl hydrocarbons to grasp. Every small atom of carbon in the benzene ring consists of two carbon-carbon sigma bonds, out of which one is a carbon-hydrogen sigma bond, and the other one is a delocalized pi-electron double bond with a neighbouring carbon.

A circle inside the hexagon indicates the process of delocalization of benzene molecules’ pi electrons. All C-C bonds in this molecule are thought to have a bond order of 1.5. and the benzene’s resonance structures can be used to explain this equivalency.

The following are some general characteristics of aromatic hydrocarbons: 

• Aromaticity is present
• The carbon-to-hydrogen atom ratio is comparatively high
• When aromatic hydrocarbons are burned, they produce a yellow flame that is intense and sooty

Types of Aromatic Hydrocarbons

Aromatic hydrocarbon is a closed-chain hydrocarbon that contains a benzene ring or has the basic properties of an aromatic molecule. The three types of aromatic hydrocarbons are mono-cyclic aromatic hydrocarbons, polycyclic aromatic hydrocarbons, and non-benzene aromatic hydrocarbons. Toluene, xylene, ethylbenzene, styrene, benzene, and acetylene are monocyclic aromatic hydrocarbons with only one benzene ring in their molecule.

Two or more benzene rings make up a polycyclic aromatic hydrocarbon. It can be divided into:

1. polybenzo-aliphatic hydrocarbon, such as diphenylmethane, triphenylmethane, stilbene, styrene, and so on, based on the interconnected ways of the benzene ring; 

2. polybenzo-aliphatic hydrocarbons, such as diphenylmethane, triphenylmethane, stilbene, styrene, and so on; 

3. polybenzo-Biphenyl and polybenzene compounds, such as biphenyl, terphenyl, and tetraphenyl, among others;

4. Polycyclic, aromatic hydrocarbons, such as naphthalene, anthracene, phenanthrene, indene, fluorene, acenaphthene, pyrene, and coronene. 

5. Non-aromatic hydrocarbons, such as azulene and annulene. These are hydrocarbons with certain aromatic properties but no benzene ring. It is simple to have a substitution reaction, but it is difficult to have a different reaction because the ring is difficult to break.

Polyaromatic Hydrocarbons: Examples

Aromatic substances with two or more fused benzene rings in their structural configurations are polycyclic aromatic hydrocarbons (PAHs). The ring system of PAHs is devoid of heteroatoms and substituents. PAHs with up to four rings are referred to as light PAHs, whereas those with more than four rings are heavy PAHs. Light PAHs are less stable and hazardous than heavy PAHs. The hydrophobicity and electrochemical stability of a PAH molecule increase as the size and angularity of the molecule increase. In PAHs, ring linkage patterns can emerge in which the tertiary carbon atoms are the foci of two or three interconnected rings. Here is a list of some PAHs: 

• Naphthalene
• Anthracene
• phenanthrene
• Acenaphthylene
• Acenaphthene
• Fluorene
• Fluoranthene
• Pyrene
• Benz[a]anthracene
• Chrysene
• Benzo[b]fluoranthene and other PAHs 

PAHs are hazardous compounds that can be formed naturally or anthropogenically. Because of the health risks associated with their exposure, PAHs must be controlled via air quality management. The refinery industry is mandated to monitor and control its discharges. There is an urgent need for significant research efforts to degrade and monitor potentially harmful compounds to control forecasts and avoid the detrimental impacts of PAH contamination.

Types of Polynuclear Hydrocarbons

A hydrocarbon made up of fused aromatic ring molecules is a polynuclear aromatic hydrocarbon. PAHs can also be considered molecules formed by fusing two or more benzene rings. These rings have delocalized electrons on one or more sides. Only carbon and hydrogen atoms make up polynuclear aromatic hydrocarbon molecules.

PAHs are also now considered carcinogens. Seven types of  polynuclear aromatic hydrocarbons have been designated by the Environmental Protection Agency as potential human carcinogens or leukaemia agents:

• Benzo[a]pyrene
• Anthracene
• Benzo[b]pyrene
• Fluoranthene
• Anthracene

Although the focus is on preventing PAH exposure, these compounds are important in producing medications, polymers, dyes, and insecticides.

Uses of Aromatic Hydrocarbons

Aromatic hydrocarbons are used extensively in both synthetic and biological processes. The following are some of the many applications of aromatic hydrocarbons: 

• Chlorophyll is a greenish pigmentation that is found in plants made up of aromatic hydrocarbons and is necessary for the generation of food in plants
• Aromatic hydrocarbons are found in the nucleic and amino acids of the human body
• Methylbenzene, an aromatic hydrocarbon, is utilised as a solvent in model glues
• The chemical naphthalene is required for the manufacturing of mothballs
• Phenanthrene is an aryl hydrocarbon used to make colours, medicines, and explosives
• TNT, or trinitrotoluene, is an important aromatic hydrocarbon commonly employed as an explosive
• Aromatic hydrocarbons are also widely used in the petrochemical and plastic industries

Conclusion

Aromatic hydrocarbons are vital for more than merely providing pleasant fragrances, despite their original name. Note that Aromatic hydrocarbons have high stability as one of the qualities that make them useful to humans.