Aromatic Compounds and Their Real World Applications

Aromatic compounds is any of a substantial class of unsaturated chemical compounds portrayed by one or more planar rings of molecules joined by covalent bonds of two different types. The extraordinary stable nature of these compounds is known as aromaticity. Despite the fact that the word sweet-smelling initially concerned smell, today its utilization in science is limited to compounds that have particular electronic, structural, or compound properties. Aromaticity results from specific bonding plans that cause particular π (pi) electrons inside a particle to be firmly held. Aromaticity is regularly reflected in smaller than anticipated heats of combustion and hydrogenation and is connected with low reactivity (Rooney-Varga, Anderson, Fraga, & Lovley, 1999).

Benzene is the smallest of the natural aromatic hydrocarbons. It has 6 carbon atoms that are held together by strong covalent bonds. The bonds between the atoms are alternatively single and double bonds.

Benzene (C₆H₆) is the best-known fragrant compound and the basis to which various other aromatic compounds are related. The six carbons contained in benzene are linked in a ring, forming the planar shape of a general hexagon in which the majority of the C—C bond separations are equivalent (Gesellschaft Deutscher Chemiker, 1994). The six π electrons flow in a part above and beneath the plane of the ring, every electron being imparted by each of the six carbons, which boosts the power of attraction between the cores (positive) and the electrons (negative). Similarly paramount is the amount of π electrons, which, as indicated by atomic orbital hypothesis, must be equivalent to 4n + 2, in which n = 1, 2, 3, and so forth. For benzene with six π electrons, n = 1 (Rooney-Varga, Anderson, Fraga, & Lovley, 1999).

Most aromatic compounds are formed when one or more hydrogen atoms of benzene are supplanted by some other molecule or compound, as in toluene (C₆H₅CH₃) and benzoic acid (C₆H₅CO₂H).

Benzene
Physical properties of benzene

Benzene is a colorless clear aromatic compound with a characteristic sweet odor.
It is non-explosive and highly inflammable. It is also non-polar and does not carry a major charge area and no net charge for the entire molecule. It is almost insoluble in water and mixes with polar solvents like chloroform, alcohol, acetone and carbon tetrachloride.
Burns with a sooty flame due to its high carbon content

Chemical properties of benzene

Benzene is highly stable compared to alkenes. Its stability is resultant of an electron cloud formed after the p-orbitals overlap in a benzene molecule. It reacts with other substances mainly by substitution of a hydrogen atom. This reaction often only happens in the presence of special conditions (“Aromatic compounds: Pt. B, C, D”, 1995). Benzene, therefore, behaves differently with alkenes and does not decolourise bromine water. It also fails to undergo any addition reaction.

Addition reaction usually call for the breaking of the stable system of benzene atoms. Benzene instead undergoes three main types of electrophyllic substitution namely: nitration, acylation and alkylation.

Substitution reactions

Benzene reacts through the aromatic substitution reaction when it is reacted with any of the halogens in the presence of the respective iron (III) halide. A halobenzene is formed in the process (“Aromatic compounds: Pt. B, C, D”, 1995). Benzene also reacts with nitric (V) acid in the presence of concentrated sulfuric (VI) acid to form another aromatic compound called nitrobenzene (International Society for Polycyclic Aromatic Compounds, 1990). Benzene also reacts with concentrated sulfuric (VI) acid in the presence of heat to form benzenesulfonic acid (Gesellschaft Deutscher Chemiker, 1994). Benzene reacts with haloalkanes in the presence of Lewis acid to form benzene halides

Uses of benzene

Benzene is a broadly utilized modern compound. Benzene is found in unrefined petroleum and in fuels. It’s utilized to make plastics, shoe polish, manufactured filaments, colors, cleansers, pills, elastic ointments, and pesticides (“Aromatic compounds: Pt. B, C, D”, 1995).

Its sweet smell makes it suitable for use in cleansers (Gesellschaft Deutscher Chemiker, 1994). Most household uses for benzene have however been discontinued as a result of it being found to be toxic and even found to cause cancer. Benzene had been used extensively before the se was banned based on fears that it could be cancer causing. Today, it is barely in use and where it is used, it is highly controlled.

Nitrobenzene

Physical properties

Nitrobenzene, C₆H₅NO₂, is an exceptionally toxic, combustible, pale yellow, liquid aromatic compound with a smell like that of bitter almonds. It is now and then called oil of mirbane or nitrobenzol. Nitrobenzene has a melting point of 5.85°C, boils at 210.9°C, is just marginally soluble in water due to its non-polar nature, however is exceptionally soluble in ethanol, ether, and benzene (International Society for Polycyclic Aromatic Compounds, 1990). It is prepared in science laboratories by reacting benzene with a combination of concentrated nitric and concentrated sulfuric acids; in the ensuing nitration response, one hydrogen atom in the benzene particle is supplanted with a nitro group, NO₂ (Gesellschaft Deutscher Chemiker, 1994).

Chemical properties

Nitrobenzene is reduced to phenylammine by treating it with conc hydrochloric acid and tin and heating. Hydrogen may be used but must be combined with a catalyst

C₆H₅NO₂ + 6[H] à C₆H₅NH₂ + 2H₂O

Uses of nitrobenzene

The significant use of nitrobenzene is in the creation of aniline, economically the most critical amine; nitrobenzene is warmed with iron and hydrochloric acid, and the resultant anilinium chloride is reacted with sodium carbonate to discharge aniline. In the pharmaceutical business nitrobenzene is utilized as a part of the creation of the pain relieving acetaminophen, or paracetamol (International Society for Polycyclic Aromatic Compounds, 1990). Nitrobenzene is additionally utilized as a part of shoe and floor polishes, cowhide dressings, and paint solvents to cover disagreeable smells. Such uses have however been minimized and controlled since, like benzene, nitrobenzene is feared to be a carcinogen.

Naphthalene

Naphthalene is the simplest of the condensed ring hydrocarbon compounds made out of two benzene rings offering two adjoining carbon atoms; its chemical equation is, C₁₀H₈. It is a vital hydrocarbon crude material that offers ascent to a group of substitution items utilized within the assembling of dyestuffs and manufactured gums (International Society for Polycyclic Aromatic Compounds, 1990). Naphthalene is the most plentiful single constituent of coal tar, an unpredictable item from the dangerous refining of coal, and is additionally shaped in current methodologies for the high-temperature cracking of petroleum. It is economically produced by crystallization from the intermediate portion of consolidated coal tar and the heavier fraction of split petroleum (Rais, 1976). The substance solidifies into radiant white plates, melting at 80.1° C and boiling at 218° C. It is just slightly soluble in water.

Chemical properties

Naphthalene behaves like benzene due to the presence of the benzene rings. It undergoes alkylation, sulfonation and nitration. It is possible to diazotize its amino

Uses

Naphthalene is profoundly unstable and has a trademark smell. Due to this smell, it has been widely utilized as moth repellent (Rooney-Varga, Anderson, Fraga, & Lovley, 1999). Naphthalene is also used in the soil as a fumigant due to its toxic nature. It is also used in attics to repel animals
Pyridine

Physical properties

Pyridine is a fundamental heterocyclic natural compound with the chemical formula C₅H₅N. It is structurally identified with benzene, with one methine group (=CH-) supplanted by a nitrogen molecule. The pyridine ring exists in numerous chemical compounds, like the vitamin niacin, azines and pyridoxal (Zollinger, 1973).
It is a colourless, highly inflamable, slightly basic, water-solvent liquid with a characteristic, upsetting fish-like smell (Gesellschaft Deutscher Chemiker, 1994).

Chemical properties

Pyridine is weakly basic. It is reduced by hydrogen in the presence of nickel and  to form piperadine. It may also be reduced using a weak reducing agent known as sodium borohydrant.

Uses of pyridine

Pyridine is utilized as a precursor to agrochemicals and pharmaceuticals and is likewise an important solvent and reagent. Pyridine is mixed with ethanol to poison it and make it unsuitable for consumption in the preparation of methylated spirit (Gesellschaft Deutscher Chemiker, 1994). Its smell is likely to discourage drinking and also distinguish genuine alcohol from methylated spirit. It is used in vitro synthesis of DNA, in combination with sulfapyridine (a pill against bacterial and viral contaminations), antihistaminic pills tripelennamine and mepyramine, and also water repellents, bactericides, and herbicides. Some compounds, in spite of the fact that they are not prepared from pyridine, hold its ring structure (Zollinger, 1973). They incorporate B vitamins niacin and pyridoxal, the opposition to tuberculosis drug isoniazid, nicotine and other nitrogen-holding plant products. Historically, pyridine was delivered from coal tar and as a by-product of the coal gasification. On the other hand, expanded interest for pyridine brought about the advancement of more practical systems for manufactured products from acetaldehyde and alkali, and more than 20,000 tons every year are fabricated around the world (Rooney-Varga, Anderson, Fraga, & Lovley, 1999).

Conclusion

In conclusion, aromatic hydrocarbons are characterized by their possession of a benzene ring. Most of them have been classified as toxic, cancer causing or both. This has necessitated their discontinuation from most of their uses. Various industrial uses however remain. The discovery of their negative effects to both the environment and to health should be viewed as a positive step in development. Aromatic compounds are characterised by different qualities depending on their formulation. Most of them are characterised by stability due to the strength of the bonds holding the molecules together.

References

Aromatic compounds: Pt. B, C, D. (1995). Amsterdam: Elsevier Science Pub. Co.

Gesellschaft Deutscher Chemiker (1994). Nitrobenzene. Stuttgart: S. Hirzel.

International Society for Polycyclic Aromatic Compounds (1990). Polycyclic aromatic compounds: PAC : an international and interdisciplinary journal ; the journal of the International Society for Polycyclic Aromatic Compounds. London [u.a.: Francis & Taylor.

Rais, J. (1976). Extraction of alkali metals into nitrobenzene in the presence of univalent polyhedral borate anions. Journal of Inorganic and Nuclear Chemistry.

Rooney-Varga, J. N., Anderson, R. T., Fraga, J. L., & Lovley, D. R. (1999). Microbial Communities Associated with Anaerobic Benzene Degradation in a Petroleum-Contaminated Aquifer.

Zollinger, H. (1973). Aromatic compounds. London: Butterworths.