Benzene: Do you know why “Vaseline” is known as Petroleum Jelly? Do you know the moisturising cream with such a beautiful fragrance is made up of fossilised organic materials? Benzene is an elementary petrochemical naturally found in crude oil.

Benzene, discovered by Michael Faraday, is a colorless liquid with a gasoline like odour. It is the simplest organic hydrocarbon. It is a cyclic hydrocarbon in which each Carbon atom is arranged in a six-membered ring and is bonded to only one atom of Hydrogen. Benzene is carcinogenic in nature. It is used to produce Polystyrene. Benzene is produced by volcanoes, coal, oil and forest fires. Let us explore this article to understand Benzene better.

What is Benzene?

Benzene is a sweet-smelling, colourless chemical derived from natural gas, crude oil, or coal.

Discovery of Benzene

Benzene derives historically from gum benzoin- an aromatic resin known to European pharmacists and perfumers since the \({\rm{1}}{{\rm{5}}^{{\rm{th}}}}\) century. An acidic material named ‘flowers of benzoin or benzoic acid was derived from benzoin by sublimation.

A hydrocarbon derived from benzoic acid acquired the name benzol, Benzin or Benzene. In \(1825,\) Michael Faraday, an English scientist, first isolated and identified Benzene from an oily residue obtained during the production of illuminating gas, giving it the name bicarburet of hydrogen. In \(1833,\) Eilhard Mitscherlich produced it by distilaating benzoic acid (from gum benzoin) and lime.

In \(1865,\) German professor August Kekule elucidated the cyclic structure of Benzene when he dreamt of a snake biting its tail.

However, Kekule failed to explain the interactions between the double bonds. American professor Linus Pauling proposed that Benzene exhibited a hybrid structure composed of delocalised electrons.

Structure of Benzene

Molecular Formula

The molecular formula of Benzene is \({{\rm{C}}_6}{{\rm{H}}_6}.\) Molecular weight determination and elemental analysis have proved that Benzene is a highly unsaturated compound.

Linear Chain Structure Not Possible

No change was observed when Benzene was treated with Bromine water (bromine in carbon tetrachloride or acidified \({\rm{KMn}}{{\rm{O}}_{\rm{4}}}.\) No decolourisation was observed. Benzene did not react with water in the presence of acid. Hence, a straight-chain or ring compound is not feasible. Benzene did not exhibit the properties of alkenes or alkynes.

Evidence of Cyclic Structure

(1) Substitution of Benzene

Benzene, when it reacts with bromine in the presence of \({\rm{AlC}}{{\rm{l}}_{\rm{3}}},\) forms only one mono bromobenzene. It indicates the presence of identical hydrogen atoms. It is possible only if it has a cyclic structure of six carbons, each containing one hydrogen.

(2) Addition of Hydrogen

The presence of three carbon-carbon double bonds is confirmed when Benzene forms Cyclohexane by adding three moles of hydrogen in the presence of nickel catalyst. This ensures the cyclic structure of Benzene

Kekule’s Structure of Benzene

In \(1865,\) August Kekule proposed that Benzene consists of a cyclic planar structure of six carbon with alternate single and double bonds.
There were two objections:

i) Kekule’s structure predicts two ortho-di substituted products as shown below, whereas Benzene forms only one ortho disubstituted product.

ii) Benzene, like other alkenes, did not give additional reactions. To overcome this contradiction, Kekule suggested that Benzene was a mixture of two forms \(1\) and \(2\) which are in rapid equilibrium with each other.

Resonance Description of Benzene

1. Resonance is the phenomenon in which the actual normal state of a molecule is represented by a combination of several alternative distinct structures.
2. A resonance hybrid has energy lower than the energy possessed by its alternative structures. Hence, such a molecule is said to be resonance stabilised.
3. Resonance energy is the energy difference between the resonance hybrid and its alternative structures.
4. This property also explains the stability of the benzene molecule. The benzene molecule is a resonance hybrid of two main contributing structures \(\left( {1,2} \right),\) as shown below.
5. Due to resonance, the carbon-carbon bonds in a benzene molecule acquire an intermediate character of carbon-carbon single and double bonds.
6. In Benzene, Kekule’s structures \(1\) and \(2\) represented the resonance structure, and structure \(3\) is the resonance hybrid of structure \(1\) and \(2\)

7. Structures \(1\) and \(2\) are theoretical. The actual structure of a benzene molecule is the hybrid of two hypothetical resonance structures depicted as in \(\left( 3 \right).\)

Spectroscopic Measurements

1. Elemental analysis and spectroscopic measurements show that Benzene is planar.
2. All the carbon-carbon bonds of a benzene molecule are of equal length \({\rm{1}}{\rm{.40}}\mathop {\rm{A}}\limits^{\rm{o}} .\) This value lies between carbon-carbon single bond length \({\rm{1}}{\rm{.54}}\mathop {\rm{A}}\limits^{\rm{o}} \) and carbon-carbon double bond length \({\rm{1}}{\rm{.34}}\mathop {\rm{A}}\limits^{\rm{o}} .\)

Molecular Orbital Structure

1. The \(6\) carbon atoms of a benzene molecule are \({\rm{sp2}}\) hybridised.
2. Carbon in Benzene which is \({\rm{sp2}}\) hybridised, linearly overlaps with six \(1\) s-orbitals of hydrogen atoms. This leads to the formation of six \({\text{C-H}}\) sigma bonds—the remaining \({\rm{sp2}}\) hybridized orbitals of carbon overlap with each other forming \(6\) carbon-carbon \(\sigma \) bonds.
3. In a benzene molecule, all of the \(\sigma \)  bonds have a bond angle of \({\rm{120}}^\circ .\) All these sigma bonds lie in one plane.
4. Each carbon atom in Benzene possesses an unhybridised p-orbital. This unhybridized p-orbital contains one electron which overlaps laterally to produce \({\rm{3\pi }}\) – bond
5. These six electrons of the p-orbitals are said to be delocalized.
6. A strong \({\rm{\pi }}\) -bond is formed due to the delocalization of the electrons. This makes the molecule stable. Hence, Benzene undergoes substitution reactions rather than addition reactions under normal conditions. This is in contradiction to alkenes and alkynes.

Representation of Benzene

Hence, the benzene molecule is represented as follows:

Preparation of Benzene

1. Benzene is obtained from coal tar and petroleum.
2. It is also prepared in the laboratory by using some simple aliphatic compounds.

1. Industrial Preparation of Benzene

(a) From Coal Tar

Pyrolisis of coal produces coal tar. Volatile compounds like Benzene, toluene, xylene are produced when coal tar is heated through fractional distillation. This happens in the temperature range of  \(350\) to \(443\,{\rm{K}}.\) The collection of these vapours takes place at the upper part of the fractionating column.

(b) From Acetylene

When acetylene is passed through a red-hot tube, it trimerizes to give Benzene.

2. Laboratory Preparation of Benzene

(a) Decarboxylation of Aromatic Acid

Benzene vapours distil over when sodium benzoate is heated with soda lime.

(b) From Phenol

When phenol reacts with zinc dust, Benzene is formed.

Properties of Benzene

Physical Properties

1. Benzene is an aromatic hydrocarbon with six carbon atoms forming a perfectly regular hexagon. All the carbon-carbon bond lengths are equal- in between single and double bonds.
2. There are electrons delocalized above and below the plane of the ring.
3. Each carbon atom in a benzene molecule is joined to each of its neighbours by a one-and-half bond. Each bond in the benzene ring has the same number of electrons and is of the same length.
4. Benzene has a molecular weight of \(78.1\,{\rm{g/mol}}.\)
5.  It occurs as a clear or colourless to light-yellow liquid with a gasoline-like odour.
6. The boiling point of Benzene is \(80.1^\circ \,{\rm{C}}.\)
7. The density of Benzene is \(0.87\,{\rm{g}}\,{\rm{c}}{{\rm{m}}^{ – 3}}\) at \(20^\circ \,{\rm{C}}.\)
8. It has a high vapour pressure \(\left( {9.95\,{\rm{kPa}}\,{\rm{at}}\,20^\circ \,{\rm{C}}} \right)\) and evaporates easily.
9. It is slightly soluble in water \(\left( {1.8\,{\rm{g/L}}\,{\rm{at}}\,25^\circ \,{\rm{C}}} \right)\) and miscible with alcohol, ether, chloroform, acetone, carbon tetrachloride, carbon disulphide, oils, and glacial acetic acid.

Chemical Properties

Benzene act as an electro rich centre due to the presence of delocalized \({\rm{\pi }}\) electrons. So, an electrophilic substitution reaction occurs in Benzene.

Electrophilic Substitution Reactions

Benzene is easily attacked by electrophiles and gives substituted products because it readily undergoes an electrophilic substitution reaction. Due to the presence of delocalized π electron, it is considered to be an electron-rich system.

Mechanism

Step 1: Electrophile generation.

Step 2: A carbocation intermediate is formed when the electrophile attacks the aromatic ring. This carbocation is stabilized by resonance.

Step 3: Loss of proton gives the substitution product.

Following are some of the reactions which Benzene undergoes to form substituted products:

(a) Nitration

In nitration, Benzene is heated at \({\rm{330}}\,{\rm{K}}\) with a nitrating mixture (Con. \({\rm{HN}}{{\rm{O}}_3} + {\rm{Con}}{\rm{.}}{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}\).

Nitronium ion \({\rm{N}}{{\rm{O}}^{2 + }}\) acts as the electrophile.

Nitro benzene is formed when a hydrogen atom of the Benzene is replaced by a nitronium ion.

The addition of concentrated \({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}\) is done to produce nitronium ion \({\rm{N}}{{\rm{O}}^{2 + }}.\)

(b) Halogenation

Halo benzene is formed when Benzene reacts with halogens \(\left( {{{\rm{X}}_2} = {\rm{C}}{{\rm{l}}_2},{\rm{B}}{{\rm{r}}_2}} \right)\) in the presence of Lewis acids such as \({\rm{FeC}}{{\rm{l}}_3},\,{\rm{FeB}}{{\rm{r}}_3}\) or \({\rm{AlC}}{{\rm{l}}_3}.\)

In the absence of catalyst, fluorine reacts vigorously with Benzene. However, iodine is quite stable even in the presence of a catalyst.

(c) Sulphonation

Benzene sulphonic acid is formed when Benzene reacts with fuming sulphuric acid \(\left( {{\rm{Conc. }}{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4} + {\rm{S}}{{\rm{O}}_3}} \right).\)

Fuming sulphuric acid releases \({\rm{S}}{{\rm{O}}_3},\) which acts as the electrophile.

Although the \({\rm{S}}{{\rm{O}}_3}\) molecule does not have a positive charge, it still acts as a strong electrophile.

The octet of the sulphur atom is not reached. Hence \({\rm{S}}{{\rm{O}}_3}\) acts as an electrophile. This reaction is reversible. Desulphonation occurs readily in an aqueous medium.

(d) Friedel Craft’s Alkylation: (Methylation)

Alkylbenzene is formed when Benzene is treated with an alkyl halide in the presence of the only \({\rm{AlC}}{{\rm{l}}_3}.\)

(e) Friedel Craft’s Acylation: Acetylation

When Benzene is treated with acetyl chloride in the presence of \({\rm{AlC}}{{\rm{l}}_3},\) acyl benzene is formed.

The benzene ring is stable due to the presence of delocalized \({\rm{\pi }}\) electrons. Benzene exhibits addition and oxidation reactions under specific conditions.

Chemical Reactions of Benzene

1. Hydrogenation of Benzene

Hydrogenation of Benzene takes place to yield Cyclohexane. This happens when Benzene reacts with hydrogen in the presence of Platinum or Palladium.

2. Chlorination of Benzene

Gammaxene or Lindane is a powerful insecticide. It is produced when Benzene reacts with three molecules of \({\rm{C}}{{\rm{l}}_2}\) in the presence of sunlight or UV light.  

Gammaxene is chemically known as Benzene Hexa Chloride \(\left( {{\rm{BHC}}} \right)\) \({{\rm{C}}_6}{{\rm{H}}_6}{\rm{C}}{{\rm{l}}_6}.\)

Oxidation Reactions

(i) Vapour – Phase Oxidation Reaction

Despite being very stable to strong oxidizing agents, Benzene quickly undergoes oxidation reaction. Vapour phase oxidation takes place by passing benzene vapour mixed with oxygen over \({{\rm{V}}_2}{{\rm{O}}_5}\) at \(773\,{\rm{K}}.\) The ring breaks to give maleic anhydride.

(ii) Birch Reduction

Benzene on treating with \({\rm{Na}}\) or \({\rm{Li}}\) in a mixture of liquid ammonia and alcohol can be reduced to \(1,{\rm{ }}4\)-cyclohexadiene. Cyclic dienes are prepared through this method.

Aromaticity of Benzene

1. As proposed by Huckel, the aromaticity of a compound is a function of its electronic structure. In order to be aromatic, a compound must obey the following rules:

i. The molecule must be co-planar
ii. Complete delocalization of \({\rm{\pi }}\) electron in the ring
iii. Presence of \(\left( {4{\rm{n}} + 2} \right){\rm{\pi }}\) electrons in the ring where \(n\) is an integer \(\left( {{\rm{n = 0,1}}{\rm{.}}\,{\rm{2}}…} \right)\)

2. This is known as Huckel’s rule.

What is Benzene Used for in Everyday Life?

1. Benzene acts as a solvent in the chemical and pharmaceutical industries.
2. It acts as a starting material and intermediate in the synthesis of numerous chemicals and in gasoline.
3. Benzene is the building block used to make ethylbenzene, which is used to make styrene.
4. Benzene also is used to make some types of lubricants, rubbers, dyes, detergents, drugs, explosives and pesticides.

Health Effects of Benzene

1. Continuous inhalation of certain levels of Benzene causes blood disorders in human beings. It primarily affects bone marrow (the tissues that produce blood cells).
2. It severely damages the immune system (by changes in blood levels of antibodies and loss of white blood cells).
3. Adverse effects on the foetus have been observed where pregnant animals were exposed to Benzene by inhalation. This also included low birth weight, delayed bone formation, and bone marrow damage.
4. Benzene causes chromosomal aberrations in humans.
5. In the atmosphere, Benzene can react with other chemicals to create smog. This could break down naturally, but it might also attach to rain and snow and be carried to the ground to contaminate water and soil.

Summary

As a building block chemical, Benzene is used to produce a variety of materials materials and, ultimately, consumer goods. From soft drinks to detergents, from hand sanitisers to food preservatives, all have this extremely regular hexagonal, cyclic compound-BENZENE.

Frequently Asked Questions on Benzene

Q. Why is Benzene banned?
Ans: Benzene is banned because it causes leukaemia and other blood disorders. It enters the body primarily by inhalation. It can cause a decrease in the number or size of red blood cells (anaemia). It can also cause a reduction in white blood cells and platelets and harm the immune system. 

Q. Does Coke contain Benzene?
Ans: Coke contains sodium benzoate. Beverages contain benzoate salts and ascorbic acid (vitamin CC). Exposure to heat and light can stimulate the formation of Benzene in these beverages. Sodium or potassium benzoate is added to beverages to inhibit the growth of bacteria, yeasts, and moulds. 

Q. What foods contain Benzene?
Ans: Benzene is found in non-alcoholic beverages, including soft drinks, at very low levels.

Q. What is Benzene used for in everyday life?
Ans: Benzene is used to make resins, synthetic fibres, plastics, rubber lubricants, dyes, detergents, drugs, and pesticides. Benzene is produced naturally by volcanoes and forest fires. In homes, Benzene may be found in glues, adhesives, cleaning products, paint strippers, tobacco smoke and gasoline.

Q. Do Cigarettes contain Benzene?
Ans: Tobacco smoke contains Benzene at approximately 3535 to 80μg80μg per cigarette. This could equate to up to 3mg/day3mg/day of Benzene for a 4040 per day cigarette smoker.

Q. Can Benzene be called Cyclohexene? If not, why?
Ans: No, Benzene and Cyclohexene are two different compounds.
a) The molecular formula of Benzene is: C₆H₆  and the molecular formula of Cyclohexene is: C₆H₁₀
b) There are three alternate double bonds in Benzene and one double bond in Cyclohexene. Benzene.
c) Benzene is an aromatic compound while Cyclohexene is a cyclic alkene.

Q. In which reaction does Toluene get converted to Benzaldehyde by Chromyl Chloride?
Ans: In Etard reaction, Toluene gets converted to Benzaldehyde by Chromyl Chloride.

Q. Which reaction yields Phenyl isocyanide?
Ans: Carbylamine reaction will yield Phenyl isocyanide.

Q. What is the name of the reaction in which Phenol is converted to salicylaldehyde when treated with chloroform and aq. KOH?
Ans: In Reimer-Tiemann reaction, Phenol is converted to salicylaldehyde when treated with chloroform and aq. KOH.