Benzene - Physical and Chemical Properties

Dhristi JEE 2022-24

Benzene

The word ‘Benzene’ is derived from the name of Gum Benzoin, which is an Aromatic form of Resin. Michael Faraday first discovered Benzene in some illuminating gas, and it was named so by Mitscherich, a German Chemist. Benzene is an Organic Compound with the Chemical formula of C6H6. It is also widely known as the father of the Aromatic Compounds. Benzene is the simplest HydroCarbon and has a very sweet Aroma.


Benzene Chemical Structure and Detailed Description

C₆H₆ is the Chemical formula of Benzene. It is a form of Cyclic HydroCarbon, i.e., each of its Carbon atoms is arranged in a ring of 6 members, and is only bonded with 1 Hydrogen atom. There are two resonance structures available in Benzene.


It is an Aromatic PetroChemical and crude oil’s natural element. Having an odour just like Gasoline, the Liquid is colourless, Highly Toxic, and Carcinogenic. It naturally occurs in the Environment and forms in volcanic eruptions and forest fires. It is also produced in industries using coal and oil.


Benzene Preparation and Properties

For the preparation of Benzene, the following methods are carried out.

1. Benzene Preparation from Alkynes

With the help of Cyclic polymerization, Benzene can be prepared from Ethyne. In this method, Ethyne passes from a tube of red-hot iron at 873K, the molecules of Ethyne then goes through Cyclic polymerization for the formation of Benzene.


2. Benzene Preparation from Aromatic Acids

Benzene can also be prepared from the Aromatic acids by a Decarboxylation Reaction. In this method, the Sodium Salt of Benzoic Acid heats with Soda Lime for the formation of Benzene and Sodium Carbonate.


3. Benzene Preparation from Phenol

Benzene can be prepared from the Reduction of Phenols. In this method, the vapours of Phenol pass over the heated dust of Zinc. The dust then reduces the vapour for the formation of Benzene.


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4. Benzene Preparation from Sulphonic Acids

Benzene can be prepared through the Hydrolysis of Sulphonic Acids. In this method, the acid is exposed to heated steam, and this leads to the formation of Benzene.


\[ C_{6} H_{5} + SO_{3}H + H_{2}O \rightarrow C_{6}H_{6} + H_{2}SO_{4}\]


Benzene is a naturally occurring substance that has the capability of Chemical production. It is also used in the industries for serving various needs. Here, we will describe the properties of Benzene.


Now, let us discuss Benzene physical and Chemical properties.


Physical Properties of Benzene:

  1. Benzene is a colourless Compound, and the physical state of Benzene is liquid.

  2. Benzene melts at 5.5 °C, and it boils at 80.1 °C.

  3. Benzene is not miscible in water and is soluble in organic solvents.

  4. It has an Aromatic odour.

  5. The density of Benzene is 0.87 gm/cm3 and is lighter than water.

  6. Benzene exhibits resonance.

  7. It is inflammable, and its combustion produces sooty flames.


Chemical Properties of Benzene and its Derivatives:

The Chemical composition of Benzene is C6H6 , i.e., it is made of 6 Carbon atoms and 6 Hydrogen atoms.


1. Nitration of Benzene

At 323-333K, Benzene reacts with nitric acid in the presence of sulphuric acid for the formation of nitroBenzene. 


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2. Sulfonation of Benzene

It is a process in which Benzene is heated with fuming sulphuric acid, i.e. H2SO4 + SO3 for the formation of Benzene sulphuric acid. It is a reversible reaction.


3. Halogenation of Benzene

In the presence of Lewis acids (FeCl2, FeBr2), Benzene reacts with the halogens for forming the aryl halides.


4. Friedel Craft’s Alkylation Reaction

Benzene gets treated with an alkyl halide in the presence of any Lewis acid for the formation of alkylBenzene.


5. Friedel Craft’s Acylation Reaction

Benzene is treated with an acyl halide in the presence of any Lewis acid for the formation of acyl Benzene.


6. Addition Reaction

Adding chlorine to Benzene in the presence of UV rays leads to the formation of Benzene hexachloride, also known as gammaxene.


7. Combustion of Benzene

During the combustion of Benzene, it burns with a sooty flame and evolves CO2.


\[ C_{6}H_{6} + O_{2} \rightarrow CO_{2} + H_{2}O\]


Benzene Uses

Benzene serves many industrial needs, like the manufacturing of lubricants, rubbers, plastics, dyes, etc. Other than these, Benzene also has some other uses in non-industrial matters. However, the toxic nature of Benzene limits its usage, and there are only a few uses are listed below.

  1. It is used for preparing Phenol. It also helps in the preparation of Aniline, which is used in dyes. Also, it is used in the manufacture of detergents.

  2. Earlier, Benzene also helped in the degreasing activity of metals.

  3. It is used for the manufacture of nylon fibers.

  4. Benzene is used for effective formation of other Chemicals, like cumene, alkylBenzene, ethylBenzene, nitroBenzene, etc.

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FAQs (Frequently Asked Questions)

1. What are the Properties of Benzene?

Some properties of Benzene are listed below.

  1. It is colourless, has an Aromatic odour, and is toxic in nature.

  2. It is available in the liquid state.

  3. It does not mix with water and is miscible with the organic solvents.

  4. It exhibits resonance.

  5. It burns with a sooty flame.

  6. It reacts with chlorine in the presence of UV rays for forming Gammaxene (Benzene hexachloride).

  7. It reacts with fuming sulphuric acid for forming Benzene sulphuric acid.

  8. It reacts with nitric acid in the presence of sulphuric acid for the formation of nitroBenzene.

2. What are the Common Uses of Benzene?

The common uses of Benzene are as follows.

  1. It is used for industrial purposes for the formation of plastics, rubber, dyes, etc.

  2. It is effective in the formation of Phenol, Aniline (for dyes), and detergents.

  3. It is highly useful in the process of degreasing metals.

  4. It is effectively used in the formation of nylon fibers.

  5. It helps with the formation of many other Chemicals, like ethylBenzene, cumene, nitroBenzene, Benzene sulphuric acid, etc.

  6. Benzene is used in the formation of Gammaxene through chlorine.

3. What are the Chemical properties of Benzene?

Chemical properties of Benzene are as follows:


1. Delocalized π-electrons in Benzene cause the ring to operate as an electro rich centre.


1. Electrophilic Substitution Reaction


(a) Nitration:

Nitro Benzene is generated by replacing the Hydrogen atom with the nitronium ion NO2+ (electrophile) when Benzene is heated at 330K with a nitrating mixture (Con. HNO3 + Con. H2SO4). Concentrated H2SO4 is added to produce nitronium ion NO2+.


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To form the nitronium ion NO2+, concentrated H2SO4 is added.


(b) Halogenation:

In the presence of Lewis acid, such as FeCl3, FeBr3, or AlCl3, Benzene combines with halogens (X2=Cl2, Br2) to produce halo Benzene. Fluorine reacts aggressively with Benzene in the absence of a catalyst. Even in the presence of a catalyst, however, iodine is inactive.


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(c) Sulphonation:

Benzene sulphonic acid is formed when Benzene combines with fuming sulphuric acid (Con H2SO4 + SO3). SO3 is a Chemical that acts as an electrophile. It is a powerful electrophile despite not having a positive charge. This is because the sulphur atom's octet of electrons is not attained. Desulphonation happens rapidly in aqueous media, and the process is reversible.


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


Alkyl Benzene is generated when Benzene is reacted with an alkyl halide in the presence of just AlCl3.


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(e) Friedel Craft’s Acylation : Acetylation


Acyl Benzene is generated when Benzene is treated with acetylchloride in the presence of AlCl3.


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2. Delocalized electrons stabilise the Benzene ring. It performs addition and oxidation reactions under certain circumstances, despite its excellent stability.


a. Hydrogenation of Benzene:

Cyclohexane is formed when Benzene interacts with Hydrogen in the presence of Platinum or Palladium. Hydrogenation is the term for this process.


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b. Chlorination of Benzene:

In the presence of sunlight or UV radiation, Benzene combines with three molecules of Cl2 to form Benzene Hexa Chloride (BHC) C6H6Cl6. Gammaxane, often known as Lindane, is a strong pesticide.


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(iii) Oxidation:


a. Vapour – Phase Oxidation:-

Although Benzene is resistant to strong oxidising agents, it is easily oxidised in the vapour phase by passing its vapour combined with oxygen over V2O5 at 773 degrees Celsius. Maleic anhydride is formed when the ring splits.


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b. Birch Reduction:

Treatment with Na or Li in a combination of liquid ammonia and alcohol can convert Benzene to 1, 4-cyclohexadiene. It is the most practical way to make Cyclic dienes.


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4. Show the resonating structure of Benzene and explain?

The occurrence of two or more equivalent structures for a Compound being written with identical atom locations but different electron configurations is known as resonance, and the structures are known as resonating structures.


The resonance hybrid structure of a molecule is believed to be a hybrid structure of several resonating structures. Carbon Carbon bonds in Benzene have a bond length of 1.40A, which is halfway between the lengths of a single Carbon bond and a double Carbon bond. Benzene has a structure that is a mix of resonating structures.


Kekule's two structures (A) and (B) are major contributing structures.


As seen in (C) and (D), the hybrid structure is represented by placing a circle or a dotted circle in the hexagon (D). The circle depicts the six electrons that are delocalized around the Benzene ring's six Carbon atoms.


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5. Why is Benzene and its related Compounds very stable?

The heat emitted when double bonds in a six-Carbon ring are Hydrogenated (Hydrogen is added catalytically) to give cyclohexane as a common product was used to determine the stability of Benzene. Cyclohexane is shown as a low-energy reference point in the graphic below. The addition of Hydrogen to cyclohexene yields cyclohexane, which emits 28.6 kcal per mole of heat. On full Hydrogenation, a cyclohexadiene should release 57.2 kcal per mole, and 1,3,5-cyclohexatriene should release 85.8 kcal per mole, if this figure represents the energy cost of inserting one double bond into a six-Carbon ring. The relative thermodynamic stability of the Compounds would be reflected by these Hydrogenation temperatures. In actuality, 1,3-cyclohexadiene is somewhat more stable than predicted, with a 2 kcal difference, owing to double bond conjugation. Benzene, on the other hand, is 36 kcal/mole more stable than predicted. Aromaticity is the term for this type of stability improvement, and Aromatic substances are molecules having Aromaticity. The most common Aromatic Chemical is Benzene, although there are many more. The lack of reactivity of Benzene in comparison to other alkenes is due to Aromatic stabilisation.


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