Benzene - C6H6

What is Benzene?

Benzene is one of the most important organic compounds with the chemical formula C6H6. Benzene is the parent compound of the various aromatic compound.

Benzene is the simplest organic, aromatic hydrocarbon. Benzene is one of the elementary petrochemicals and a natural constituent of crude oil. It has a gasoline-like odour and is a colourless liquid. Benzene is highly toxic and carcinogenic in nature. It is primarily used in the production of polystyrene.

Benzene is a naturally occurring substance produced by volcanoes and forest fires and present in many plants and animals, but benzene is also a major industrial chemical made from coal and oil. As a pure chemical, benzene is a clear, colourless liquid. In industry benzene is used to make other chemicals as well as some types of plastics, detergents, and pesticides. It is also a component of gasoline.

Discovery of Benzene

The word benzene derives historically from gum benzoin, sometimes called ‘Benjamin’. Gum benzoin was known as an aromatic resin. Michael Faraday, an English scientist first discovered Benzene in illuminating gas. The name benzene was given by German Chemist Mitscherich in 1833. The cyclic structure of benzene remained a mystery until 1865 when German professor August Kekule elucidated it when he dreamt of a snake biting its own tail.

However, Kekule did not discover the presence of interactions between the double bonds. Americal professor Linus Pauling proposed that benzene exhibited a hybrid structure composed of delocalized electrons. This was the refinement of Kekule’s discovery. Benzene has a somewhat pleasant, sweet smell, however it is carcinogenic.

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Benzene Characteristics

Benzene was discovered in 1825 by the English physicist Michael Faraday, and was made available in large quantities in 1842 after it was found to contain benzene. Large amounts of benzene are now extracted from petroleum.
Benzene is a colorless liquid with a characteristic odor of formula C6H6. Benzene is a closed ring of six carbon atoms linked by bonds that alternate between single and double bonds. Each carbon atom is bound by a single hydrogen atom. Benzene melts at a temperature of 5.5 ° C, boils at 80.1°C. Benzene and its derivatives are part of an essential chemical community known as aromatic compounds. Benzene is a precursor in the production of medicines, plastics, oil, synthetic rubber and dyes.
As a result, the hydrogenation of benzene happens much more slowly than the hydrogenation of other organic compounds containing carbon-carbon double bonds, and benzene is much more difficult to oxidize than alkenes. Most of the reactions of benzene belong to a class called electrophilic aromatic substitution, which leaves the ring intact but replaces one of the hydrogens attached to it. These reactions are versatile and commonly used for the preparation of benzene derivatives.

Structure of Benzene

The structure of benzene has been of interest since its discovery. Benzene is a cyclic hydrocarbon (chemical formula: C6H6), i.e., each carbon atom in benzene is arranged in a six-membered ring and is bonded to only one hydrogen atom. According to molecular orbital theory, benzene ring involves the formation of three delocalized π – orbitals spanning all six carbon atoms, while the valence bond theory describes two stable resonance structures for the ring.

Benzene structure

Benzene structure  

Properties of Benzene

The various properties of benzene are mentioned below:

  • Benzene is immiscible in water but soluble in organic solvents.
  • It is a colourless liquid and has an aromatic odour.
  • It has a density of 0.87g cm-3. It is lighter than water.
  • Benzene has a moderate boiling point and a high melting point. (Boiling point: 80.5°C, Melting point: 5.5°C)
  • Benzene shows resonance.
  • It is highly inflammable and burns with a sooty flame.

Resonance of Benzene

The oscillating double bonds in the benzene ring are explained with the help of resonance structures as per valence bond theory. All the carbon atoms in the benzene ring are sp2 hybridized. One of the two sp2 hybridized orbitals of one atom overlaps with the sp2 orbital of adjacent carbon atom forming six C-C sigma bonds. Other left sp2 hybridized orbitals combine with s orbital of hydrogen to form six C-H sigma bonds. Remaining unhybridized p orbitals of carbon atoms form π bonds with adjacent carbon atoms by lateral overlap.

This explains an equal possibility for the formation of C1 –C2, C3 – C4, C5 – C6 π bonds or C2 – C3, C4 – C5, C6-C1 π bonds. The hybrid structure is represented by inserting a circle in the ring as shown below in the figure. Hence, it explains the formation of two resonance structures proposed by Kekule.

Benzene - Kekule structure

Kekule structure of benzene

Aromaticity of benzene

Benzene is an aromatic compound, as the C-C bonds formed in the ring are not exactly single or double, rather they are of intermediate length. Aromatic compounds are divided into two categories: benzenoids (one containing benzene ring) and non-benzenoids (those not containing benzene ring), provided they follow Huckel rule. According to Huckel rule, for a ring to be aromatic it should have the following property:

  • Planarity
  • Complete delocalization of the π electrons in the ring
  • Presence of (4n + 2) π electrons in the ring where n is an integer (n = 0, 1, 2, . . .)

Uses of Benzene

Benzene is used in various industrial processes such as in the manufacture of lubricants, plastics, rubbers, dyes, synthetic fibres, etc. However, it has non-industrial uses too which are limited due to the reason benzene is toxic and carcinogenic. The different uses of Benzene are mentioned below.

  • Benzene is used in the preparation of phenol. It is also used to prepare aniline used in dyes and in dodecylbenzene used for the detergents.
  • In early times, benzene was used in degreasing of metal.
  • It is used for manufacturing of nylon fibres.
  • The main use of benzene is that it is used in the manufacture of other chemicals such as ethylbenzene, cyclohexane, cumene, nitrobenzene, alkylbenzene, etc.

Health Effects of Benzene

Acute exposure to benzene produces toxic effects on the central nervous system; however, in order to evaluate the chronic effects, consideration must be given to the myelotoxic and possible chromosome damaging and leukemogenic effects of benzene. The time required for expression of chlorine benzene toxicity indicates a vast difference in individual sensitivity.

Most cases of severe benzene intoxication have been reported in workers exposed to rather high concentrations of benzene under somewhat unhygenic working conditions. It is probable that all cases reported as “leukimia associated with benzene exposure” have resulted from exposure to rather high concentrations of benzene and other chemicals.

Frequently Asked Questions

What discovered the structure of benzene?

Friedrich August Kekule.

List any 3 food items that contain benzene

Pineapple Crush, Giant Light Cranberry Juice Cocktail, AquaCal Strawberry Flavored Water Beverage

What are the three important steps to produce benzene industrially?

The following three chemical processes contribute equally to the production of benzene at an industrial level:

  • Catalytic reforming
  • Toluene hydrodealkylation
  • Steam cracking

Briefly explain the shape of benzene

Benzene has a ring structure which consists of six carbon atoms that are bonded in a planar or flat hexagon ring.

Is C6H6 toxic to humans?

The Department of Health and Human Services has stated that C6H6 can cause cancer (a carcinogen).

Is benzene a solvent?

Benzene is a widely used solvent and occurs in gasoline, car emissions and cigarette smoke. Historically, high-level benzene contamination has been widespread and, because of its carcinogenic effects, benzene contamination has typically become a matter of considerable concern.

What happens if you smell benzene?

Exposure to benzene may cause bone marrow cells not to produce red blood cells or may cause your immune system’s white blood cells to fail. After smelling benzene during a leak, there’s a window of time to be able to take action or leave the area without harm, but persistent exposure can be dangerous.

What is the most characteristic reaction of benzene?

Benzene is outstandingly unreactive. This does not undergo alkene-specific add-on, oxidation, and degradation reactions. Benzene, for example, does not react with bromine, hydrogen chloride, or other reagents that usually add to double carbon – carbon bonds.

Is benzene toxic to humans?

The Department of Health and Human Services has established that benzene is a recognized carcinogen (may cause cancer). Both the International Organization for Research on Cancer and the EPA have determined that benzene is carcinogenic to humans. Exposure to benzene can be harmful to reproductive organs.

What does benzene smell like?

Benzene has a sweet, aromatic, gasoline-like smell. Most individuals may begin to smell benzene in the air at 1.5 to 4.7 ppm. The odor threshold generally provides adequate warning for acutely hazardous exposure concentrations, but is insufficient for more chronic exposures.

Why is benzene so important?

Some essential chemical compounds are derived from benzene by substituting one or more of its hydrogen atoms for another functional group. Linking benzene rings are biphenyl, C6H5-C6H5. The depletion of hydrogen results in the formation of fused aromatic hydrocarbons such as naphthalene and anthracene.

Is benzene acidic or basic?

According to Lewis’ theory of acids and bases, an acid takes a pair of electrons and a base gives a pair of electrons, so benzene is a base since it gives a pair of electrons. Benzene is a base since it gives a pair of electrons.

What is the structure of C6H6?

The chemical formula of benzene is C6H6, consisting of six carbon atoms and six hydrogen atoms. Benzene is an aromatic hydrocarbon, a chemical compound composed of carbon and hydrogen atoms with alternating double bonds forming a ring.

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