history, uses, structure and sources of benzene


Benzene is an aromatic hydrocarbon with a pleasant smell and has no color. It is highly flammable, volatile, miscible in organic solvents but slightly soluble in water. It occurs naturally through forest fires or volcanic eruptions, but a significant amount of results from human activities and industrial processes. In the United States, it is among the top 20 chemicals commonly used in industries and other purposes. Most of the aromatic substances are derived from benzene and are referred to as benzenoid compounds or benzenoids, for example, ethylbenzene, cumene, and aniline. It is also a natural constituent of cigarette smoke, gasoline, and crude oil. Despite its numerous benefits to humans, it also poses a great health danger because it is thought to be carcinogenic causing leukemia and other blood disorders (American Cancer Society, 2013). The symbol of benzene is as shown.

(Singh, 2004).

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History of Benzene

Michael Faraday first discovered and isolated benzene in 1825. He discovered an oily liquid that was produced at high pressure in coal tanks that stored illuminating gas. Faraday also discovered that the product had the same number of hydrogen and carbon, so he called it carbureted hydrogen. Laurent was a chemist who had differing thoughts about the naming of the compound. He suggested the name pheno, which means to shine because the compound was derived from a lighting gas. However, his proposal was not accepted, but the name was carried on to the present day as phenyl to represent the C6H5 compounds. In 1835, Eilhardt Mitscherlich heated benzoic acid with lime and obtained the same product as Faraday. He named the compound benzin and identified that the empirical formula of the product was C6H6. Other scientists disagreed with the name benzin because alkaloids bear the same name, which could be misleading. The name benzol was also proposed, but also rejected to avoid confusion with names of alcohols that end with –ol. Thus, the name benzene was adopted because it was used in both France and England (Singh, 2004).

Structure of Benzene

Friedrich August Kekule was the first chemist to discover that benzene has a ring structure, in 1865 while Kathleen Lonsdale, a crystallographer, confirmed the cyclic structure. Researchers found out that the carbon-carbon bonds were identical in length, unlike the conventional fact that a double bond is shorter than a single bond. The angle between the bonds is 120°. The bond length of a benzene molecule is also greater than a normal double bond but does not exceed that of a single bond. X-ray diffraction was used to examine and study the structure of the benzene molecule, deducing the aforementioned characteristics of the structure. The structure of benzene constitutes six atoms of carbon to form a planar regular hexagon. All the atoms in the benzene molecule are on the same plane; thus, the name planar. Covalent bonds join the individual carbon atoms to the other, which have sp2 hybridization. The planar shape occurs due to the sideways overlapping of the p orbitals resulting in a delocalized pi system and electrons. Two atom neighbors surround each carbon atom; thus, it utilizes two electrons from each atom. An additional electron from the carbon atom has the role of bonding the hydrogen atom attached to the carbon. The other six electrons make a right-angled revolution around the nucleus of the atom as they overlap each other. Subsequently, there is the equal sharing of electrons with three being in the upper ring and three in the lower ring, forming two clouds of electrons below and above the carbon ring plane. Therefore, the six electrons are termed as delocalized because they are not restricted to distinct carbon atoms. The resultant structure is that of a hexagon with a circle in the middle. The resonance theory was deduced to elucidate the structure of benzene and why it is less reactive than alkenes. The theory supposes that hybrid molecules composed of two or more Lewis structures exhibit greater stability than non-hybrid molecules.

(Stoker, 2011).

Thus, the stability of benzene and other aromatic molecules make them less reactive than alkenes. Delocalization of the electrons also reduces the reactivity of benzene as compared to the alkenes.

Delocalized electrons with a circular ring at the center (Singh, 2004).

Manufacturing of Benzene

Benzene is produced through catalytic reforming, which is a process used to produce aromatic compounds from various raw materials. This process accounts for about 30% of commercial benzene globally. The methods of extracting the aromatic compounds include dehydroisomerizing alkyl cyclopentanes, dehydrogenating cycloparaffins, and paraffin dehydrogenation and cyclization. Solvent extraction techniques are used to recover the benzene yielded from the reformate.  Another process through which benzene is produced is the hydrodemethylation of toluene in the presence of a catalyst or heat. The most common catalytic processes include DETOL and Hydeal while thermal processes include HDA and THD. This process contributes to about 25% of the world’s benzene production. In addition, coal tar is also used to produce benzene where the tar acids are removed with caustic soda. Distillation at the lowest boiling fraction is used to yield benzene, which is then purified through hydrodealkylation (Schwartz, 2013).

Sources of Benzene in the Environment

Benzene finds its way to the atmosphere through various processes, both directly and indirectly. It is directed emitted to the environment through exhausts of gasoline vehicles. Industrial processes utilizing or manufacturing benzene also release benzene to the air. They include oil refineries, shoe manufacturers, chemical plants, rubber industry, and gasoline-related plants. Products that also use benzene as an intermediary such as dyes, pesticides, detergents, drugs, and lubricants also emit the benzene vapor to the environment. In addition, water released from benzene-related industrial processes and practices in the form of effluents and spills drains into water bodies causing contamination. Smoking cigarette and other tobacco products release benzene into the air through the smoke particles. Coke ovens, production of nonferrous metal, coal and ore mining, textile manufacture and processing of wood and wooden products indirectly emit benzene to the environment. The benzene released to the soil may leach through the ground into the underground reservoirs and aquifers; thus, contaminating underground water. The benzene release to the air is in the vapor phase, which dissolves in atmospheric water and rain may wash it from the environment (Schwartz, 2013).

Effects of Contamination with Benzene

Benzene compounds can enter the body through inhaling contaminated air especially in poorly ventilated areas and living near gasoline stations, and consumption of contaminated water or food. Contamination through the skin can also occur when one makes contact with benzene-based products such as gasoline, but this is rare since benzene is volatile. Benzene is thought to cause cancer and other terminal illnesses such as leukemia and other disorders relating to blood cells. Firefighters, printers, steel workers, and lab technicians are the groups most susceptible to contamination with benzene because they work in industries that involve benzene in their production process. In a study conducted, the aforementioned group of workers had higher levels of benzene and the rates of leukemia were frequent in the group. Various forms of cancer such as such as acute lymphocytic leukemia, chronic lymphocytic leukemia and acute myeloid leukemia were detected in people exposed to high levels of benzene. Severe anemia and lymphoma were also common in the susceptible group because benzene adversely affects blood cells. Other health effects associated with benzene include drowsiness, confusion, tremors, dizziness, unconsciousness, and headaches. Problems associated with consumption of benzene-contaminated food or water include stomach upset, convulsions, vomiting, and rapid breathing. Benzene also causes low blood cell count leading to weakened immunity and excessive bleeding (American Cancer Society, 2013).

However, the government, through the Occupational Safety & Health Administration (OSHA) controls the levels of exposure to benzene at the work place. The level of benzene in the air should not exceed 1 part per million and a maximum of 5 parts per million in every 15 minutes. The federal agency also requires workers in high-risk zones to have personal protective equipment, for example, respirators, to minimize the level of exposure. Individuals and workers can also limit their subjection to benzene by taking appropriate precautionary measures. Cessation of smoking is one of the ways to limit benzene exposure because cigarette smoke has high levels of benzene. Wearing gas masks while at work and minimizing exposure to products containing benzene such as paints and solvents are also vital in reducing exposure to benzene fumes (Singh, 2004).

Physical Properties of Benzene

The chemical formula of benzene is C6H6 and a molecular weight of 78.11 g/mol. Its vapor pressure is 100 mm Hg at 26.1 C, and its solubility is 1.8 g/L of water at 25° C. It has a melting point of 5.5°c, a boiling point of 80.5°c and a density of 0.87g cm-3, making it slightly lighter than water. It increases with the increasing molecular mass in the homologous series because it has strong Vander Waal’s forces. Benzene burns with a sooty flame because of the many number of carbon atoms in its structure, and it is highly flammable. It is a stable compound because of the positioning of the double bonds, which gives it resonance and the ability to exist in various forms. Therefore, benzene undergoes only substitution reactions instead of additional reactions with other compounds (Stoker, 2011).

Chemical Properties

Benzene reacts vigorously with alkyl halides such as alkyl chloride together with the catalysts ethyl aluminum dichloride. It does not react with oxidizing agents like nitric acid. It ignites when in contact with powdered chromic anhydride and other interhalogens such as bromine pentafluoride. It does not have any area of charge either electrical or otherwise in the molecule, making it nonpolar. However, it can react with compounds with an opposite charge such as carbon tetrachloride, alcohol, chloroform, and acetone. Highly polarized compounds such as bromine react with benzene in substitution reactions to yield brominated benzene. There is the production of hydrochloric acid as a byproduct when benzene reacts with cyanogen halides. Benzene undergoes substitution reactions although it is highly unsaturated, and this characteristic is referred to as aromaticity. Its resonance enables it to undergo electrophilic substitution reactions as opposed to addition reactions in order to maintain its aromatic nature. Two or more benzene rings can join to form various hydrocarbons, which have enhanced reactivity and aromaticity than benzene, for example, naphthalene (Stoker, 2011). A diagram of naphthalene is as shown.

(Stoker, 2011).

Naphthalene has a higher reactivity than benzene and some of the reduction and oxidation reactions involving naphthalene are elaborated diagrammatically.

(Stoker, 2011)

Electrophilic substitution reactions occur in various stages. The first step involves a reaction between co-reagents and catalysts yielding a strong electrophilic species. An interaction of benzene with electrophiles results in the formation of cyclohexadienyl cation, which is referred to as the arenium ion, the σ complex or the Wheland complex. Step 2 is the reaction of the complex with a base to yield a substituted product, a process known as deprotonation.

(Singh, 2004).

The table below shows various electrophilic substitution reactions, conditions for the reactions, the products and by-products.



An example of electrophilic substitution reaction is a reaction between bromine or chlorine and benzene to form halobenzene in the presence of aluminium salts of corresponding halogen or Lewis acid.

(Singh, 2004).

Benzene also undergoes addition reactions such as those involving alkynes and alkenes under special conditions yielding stable additional products. For example, hydrogenation is an additional reaction involving benzene and nickel or palladium catalyst to yield cyclohexane at a temperature of 475-500K.

(Singh, 2004).

Halogenation between chlorine and benzene is also an example of an additional reaction, which yields benzene hexachloride, an insecticide, in the presence of sunlight sans a catalyst.

(Stoker, 2011).

The third chemical property of benzene is the ability to yield water and carbon dioxide where it is oxidized through combustion.

2C6H6 + 15O2 → 2CO2 + 6H2O + heat

Oxidation of benzene also occurs in the presence of vanadium pentaoxide catalyst at a temperature of 725 K to yield maleic anhydride.

(Singh, 2004).

Uses of Benzene

Benzene is used as a key component in various products such as adhesives, paints, rubber, detergents, and coatings. It is also important in dry cleaning, lithography and printing, graphic designing where ink is needed, rectification, extraction, and in the shoe and tire industries. It is used to attach soles of shoes, clean the components of the fuel system, brakes, and hydraulic systems, and reducing knocking of engines in gasoline vehicles. In addition, benzene is an essential intermediary in the synthesis of nylon, phenolic resins, synthetic rubber, polyester resins, detergents, aniline, chlorobenzenes and polystyrene plastics (Schwartz, 2013). It is also necessary during the production of insecticides, explosives, lacquers, drugs, aniline, varnishes, plastics and dyes. Benzene also serves as an essential solvent for inks, waxes, oils, and fats especially in the extraction of oil from nuts and seeds. Benzenoids are mainly incorporated in the synthesis of polymers and resins (American Cancer Society, 2013). The table below shows various benzoids and the products derived from them.    (Schwartz, 2013)


Benzene is a commonly used organic compound in industries and other institutions globally. It has numerous industrial uses including being a significant constituent of gasoline, a solvent for many processes, a cleaner for major parts of machinery and equipment, and manufacturing of plastics, polyester resins, and polymers. Benzene undergoes mainly substitution reactions, but there are also additional reactions with alkenes and alkynes under special conditions. The substitution reactions, which include sulphonation, nitration, and halogenation, are essential in maintaining the sweet smell (Singh, 2004). Subjection to high benzene levels has adverse health effects because the compound is associated with leukemia and other blood disorders. Therefore, the government has put regulatory measures to help minimize exposure to benzene at workplaces


American Cancer Society. (2013). Benzene. Retrieved on 8 Dec. 2013 from http://www.cancer.org/cancer/cancercauses/othercarcinogens/intheworkplace/benzene

Schwartz, A. (2013). Uses of benzene: Unsafe products cause cancer and leukemia. Anapol Schwartz: Personal Injury Lawyers. Retrieved on 8 Dec. 2013 from http://www.anapolschwartz.com/practices/benzene/benzene-uses.asp

Singh. (2004). Advanced organic chemistry: Reactions and mechanisms. New Delhi: Pearson Education India.

Stoker, S.H. (2011). General, organic, and biological chemistry, 6th  ed. Boston MA: Cengage Learning.


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