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:
Benzene is a colourless Compound, and the physical state of Benzene is liquid.
Benzene melts at 5.5 °C, and it boils at 80.1 °C.
Benzene is not miscible in water and is soluble in organic solvents.
It has an Aromatic odour.
The density of Benzene is 0.87 gm/cm3 and is lighter than water.
Benzene exhibits resonance.
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.
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.
Earlier, Benzene also helped in the degreasing activity of metals.
It is used for the manufacture of nylon fibers.
Benzene is used for effective formation of other Chemicals, like cumene, alkylBenzene, ethylBenzene, nitroBenzene, etc.
FAQs on Benzene - Physical and Chemical Properties
1. What is benzene and why is it called an aromatic compound?
Benzene (C₆H₆) is an organic chemical compound that is the simplest aromatic hydrocarbon. It consists of a six-carbon ring with one hydrogen atom attached to each carbon. It was originally called 'aromatic' due to its distinct, sweet odour. In modern chemistry, the term aromaticity refers to its special stability and chemical properties that arise from its unique, delocalised pi-electron system.
2. What are five key physical properties of benzene?
Here are five key physical properties of benzene:
- It is a colourless liquid at standard room temperature.
- It has a characteristic sweet, aromatic odour.
- Benzene is immiscible in water but is readily soluble in most organic solvents like alcohol, ether, and chloroform.
- It has a relatively low melting point of 5.5 °C and a boiling point of 80.1 °C.
- It is less dense than water with a density of 0.87 g/cm³ and is highly flammable, typically burning with a sooty flame due to its high carbon content.
3. What are the main electrophilic substitution reactions of benzene?
Benzene's primary chemical reactions are electrophilic substitutions, where a hydrogen atom is replaced by an electrophile, preserving the stable aromatic ring. The main examples as per the CBSE syllabus are:
- Nitration: Reaction with a mixture of concentrated nitric acid and sulphuric acid to form nitrobenzene.
- Halogenation: Reaction with a halogen (e.g., Cl₂ or Br₂) in the presence of a Lewis acid catalyst like FeCl₃ to form a halobenzene.
- Sulphonation: Reaction with fuming sulphuric acid (H₂SO₄ + SO₃) to produce benzenesulphonic acid.
- Friedel-Crafts Alkylation: Reaction with an alkyl halide (e.g., CH₃Cl) and a Lewis acid to attach an alkyl group to the ring.
- Friedel-Crafts Acylation: Reaction with an acyl halide (e.g., CH₃COCl) and a Lewis acid to form an acylbenzene.
4. What are some important industrial uses of benzene?
Despite its toxicity, benzene is a fundamental starting material in the chemical industry. Its most important uses include:
- The production of ethylbenzene, which is a precursor to styrene, used for making plastics and polymers like polystyrene.
- The synthesis of cumene, an essential intermediate for manufacturing phenol and acetone.
- The manufacturing of cyclohexane, which is a key component in the production of nylon fibres.
- The preparation of nitrobenzene, which is primarily used to produce aniline for the dye industry.
5. Why is benzene exceptionally stable compared to other unsaturated hydrocarbons like alkenes?
Benzene's exceptional stability comes from aromaticity, a result of its unique electronic structure. Unlike simple alkenes that have isolated double bonds, the six pi-electrons in the benzene ring are fully delocalised. This means they are shared evenly across all six carbon atoms, creating a stable, continuous electron cloud. This delocalisation, often represented by resonance, significantly lowers the molecule's internal energy, making it much more stable than a hypothetical ring with three distinct, non-interacting double bonds.
6. What is resonance in benzene, and what is the significance of its hybrid structure?
Resonance in benzene is a concept used to describe the delocalisation of its pi-electrons, as no single Lewis structure can accurately represent the molecule. It is depicted as a resonance hybrid of two contributing Kekulé structures. The significance of this hybrid is profound: the actual benzene molecule is more stable than either contributing structure suggests. Evidence for this is that all carbon-carbon bonds in benzene are of an identical, intermediate length (1.40 Å), which is between a standard C-C single bond (1.54 Å) and a C=C double bond (1.34 Å), confirming that the electrons are shared equally around the entire ring.
7. Why does benzene undergo substitution reactions rather than addition reactions, despite being unsaturated?
Although benzene is unsaturated, it strongly favours electrophilic substitution over addition reactions to preserve its stability. An addition reaction would require breaking the continuous ring of delocalised pi-electrons, thereby destroying the molecule's highly stable aromatic system. This loss of aromaticity would require a large input of energy. In contrast, a substitution reaction replaces a hydrogen atom but leaves the stable aromatic ring intact. Therefore, substitution is the much more energetically favourable pathway for benzene under typical reaction conditions.
8. What is aromaticity, and how does Huckel's rule apply to benzene?
Aromaticity is a chemical property of certain cyclic, planar molecules that gives them extraordinary stability. For a molecule to be considered aromatic, it must comply with Huckel's rule. This rule states that the molecule must have a continuous ring of p-orbitals and contain a total of (4n+2) pi-electrons, where 'n' is any non-negative integer (0, 1, 2, etc.). Benzene is a perfect example: it is cyclic, planar, and contains 6 pi-electrons. This fits the rule for n=1, since (4×1 + 2) = 6. This compliance confirms its aromatic character and is the fundamental reason for its chemical stability and unique reactivity.
