What is Benzene?
Benzene is basically just an Organic compound. Some of its characteristics are: -
Flammable
Volatile
Toxic
Odour: Gasoline-like
Carcinogenic (potential cancer-causing agent)|
It is an aromatic hydrocarbon; whose chemical formula is C6H6.
Where is It found?
Benzene is present in crude oil, which is unrefined petroleum. Also as a natural byproduct of oil refining.
Structure
As you know, carbon usually can form just 4 single bonds. And as benzene has six carbon molecules and six hydrogen molecules, no structure could account for all the bonds, the structure of Benzene has remained a mystery. It was the chemist Kekulé who finally found the answer to this mystery when he saw a dream about a snake eating itself, it gave him the idea of a ringed structure. Which led him to develop a six Carbon membered ring, each attached to one hydrogen atom. The benzene ring forms three delocalized π -orbitals shared with all six of the carbon atoms, with respect to the molecular orbital theory. Whereas, valence bond theory suggests two stable resonance structures for the ring.
Discovery of Benzene
Benzene was first discovered in illuminating gas, by Michael Faraday who was an English Scientist. The origin of the word Benzene was from gum benzoin which was known as an aromatic resin.
Preparation
Benzene can be prepared in many ways: -
1. Decarboxylation of Sodium Benzoate
This is the laboratory method to obtain Benzene from Sodium benzoate. In this process, Sodium benzoate and Soda-lime (Sodium Hydroxide, along with Calcium Oxide) is heated which causes decarboxylation i.e., removal of carbon dioxide, to produce Benzene and Sodium Carbonate as the by-product.
2. Heating Phenol with Zinc
To make Benzene from Phenol, Phenol reacts with Zinc dust at a higher temperature, the phenol is converted to a phenoxide ion and a proton, which accepts an electron from Zn forming an H radical. Which results in the formation of ZnO and the phenoxide ion that was formed, converts itself into Benzene.
3. Polymerization of Ethyne
To produce Benzene from Ethyne (acetylene), it has to undergo cyclic polymerization. For this, Ethyne is made to pass through a red hot tube at a temperature of 873K, which in turn, converts itself into Benzene.
4. Reduction of Benzenediazonium Chloride
Making Benzene from Benzene-Diazonium Chloride requires the reduction of Benzenediazonium chloride with hypophosphorous acid at room temperature, resulting in the formation of Benzene and the reagent will get oxidised to phosphorus.
5. Hydrolysis of Sulfonic Acid
Hydrolysis of Sulfonic acid, accompanied by superheated steam produces Benzene from sulphonic acid.
Properties of Benzene
Benzene is immiscible in water and cannot form a homogeneous mixture with it. Whereas, it is soluble in organic solvents.
Benzene is a liquid, colourless aromatic compound which has an aromatic odour.
Benzene is highly inflammable and upon combustion, will produce a sooty flame.
Benzene shows resonance and can exist in different forms depending upon the position of the double bond, making it extremely stable.
Benzene is found to be lighter than water as the density of Benzene is 0.87g cm3
Benzene has a moderate boiling point of 80.5oC and a high melting point of 5.5oC.
Resonance of Benzene
The usual representation of the structure of Benzene consists of 3 double bonds and three single bonds drawn as 1,3,5-cyclohexatriene or 2,4,6-cyclohexatriene However, the real structure of Benzene is like a hybrid of the two as all the electron density flows through all P-orbitals equally. Therefore, every side, in reality, forms a bond that is an intermediate of a single and a double bond, which keeps oscillating, inside the ring. All the carbon atoms that are present inside this ring have sp2 hybridization. As there are two sp2 hybridised orbitals, one of these, attaches itself to the sp2 hybridised orbital of the Carbon atom lying next to it, forming a C-C bond. The next sp2 hybridised orbital, attaches itself to the s orbital of Hydrogen, forming a C-H bond. Therefore, forming six C-C sigma bonds and six C-H sigma bonds. Now, there are unhybridized p orbitals remaining, they will form π bonds with the next carbon atom by lateral overlap.
Aromaticity of Benzene
What makes Benzene an Aromatic Compound?
Aromatic compounds are divided into two: -
Given that, they follow Huckel's rule. According to this rule, for a given ring to be aromatic, it must have the following properties.
1. The compound must be planar.
2. There should be complete delocalization of the π electrons in the ring.
3. Should have the presence of (4n + 2) π electrons in the ring where n is an integer (n = 0, 1, 2, . . .)
Uses of Benzene
Benzene is an industrial chemical that is widely used in the production of pesticides, resins, detergents, synthetic fibres, plastics, drugs, dyes. Benzene can be naturally produced from volcanoes and forest fires. It evaporates rapidly from soil and water, if it leaks from storage tanks it can lead to the contamination of water wells and water sources situated close by.
Benzene also has household uses too, but the extent of its use is limited due to its toxic and carcinogenic nature. In homes, Benzene is used in glue, adhesive, cleaning products, tobacco smoke, etc.
It is also used to prepare phenol and aniline which is used in dyes.
It is used to manufacture nylon fibres.
Degreasing metals.
One of the most important uses of benzene is to manufacture different chemicals such as ethylbenzene, cyclohexane, cumene, nitrobenzene, etc.
FAQs on Preparation of Benzene
1. What is benzene and what is its chemical formula?
Benzene is a fundamental organic chemical compound classified as an aromatic hydrocarbon. Its chemical formula is C₆H₆, which indicates that its molecule consists of six carbon atoms arranged in a planar ring, with one hydrogen atom attached to each carbon atom.
2. What are the main laboratory methods for preparing benzene?
For laboratory-scale synthesis, benzene can be prepared through several common methods. These include:
- Cyclic Polymerisation of Ethyne: Passing ethyne gas through a red-hot iron tube at 873 K.
- Decarboxylation of Aromatic Acids: Heating sodium benzoate with soda-lime.
- Reduction of Phenol: Passing phenol vapours over heated zinc dust.
- Hydrolysis of Sulphonic Acids: Passing benzene sulphonic acid through superheated steam.
3. How is benzene synthesised from ethyne (acetylene)?
Benzene is prepared from ethyne through a process called cyclic polymerisation. In this reaction, three molecules of ethyne (C₂H₂) are passed through a red-hot iron tube at a high temperature of 873 K. The ethyne molecules trimerise, or join together, to form the stable six-carbon ring structure of benzene.
4. What is the process of preparing benzene from phenol using zinc dust?
The preparation of benzene from phenol is a reduction reaction. When vapours of phenol (C₆H₅OH) are passed over heated zinc dust, the zinc removes the hydroxyl (-OH) group from the phenol molecule. This process results in the formation of benzene (C₆H₆) and zinc oxide (ZnO).
5. What are some important industrial uses of benzene?
Industrially, benzene is a vital starting material, or precursor, for manufacturing other chemicals. Its primary uses include producing:
- Ethylbenzene, which is used to make styrene for plastics and synthetic rubber.
- Cumene, which is a key ingredient for making phenol and acetone.
- Cyclohexane, used in the production of nylon fibres.
- Aniline, which is used in the manufacturing of dyes and pharmaceuticals.
6. Why is the resonance structure of benzene so important for its stability?
The resonance in benzene is crucial because the six pi-electrons are not confined to specific double bonds. Instead, they are delocalised and spread out evenly across the entire ring. This creates a resonance hybrid structure that is more stable and has lower energy than any single contributing structure. This enhanced stability is why benzene does not undergo addition reactions like typical alkenes.
7. What makes benzene an aromatic compound, and how does it differ from a non-aromatic cyclic compound like cyclohexene?
Benzene is an aromatic compound because it is cyclic, planar, has a continuous ring of p-orbitals, and follows Hückel's rule with 6 delocalised pi-electrons. In contrast, cyclohexene is a non-aromatic cyclic alkene with a single, localised double bond. Because of this, cyclohexene readily undergoes addition reactions, while the stable aromatic system of benzene favours substitution reactions.
8. Why is the decarboxylation of sodium benzoate a useful method for preparing benzene?
This method is effective because it involves heating sodium benzoate with soda-lime (a mixture of NaOH and CaO). The soda-lime acts as a decarboxylating agent, removing the carboxyl group (-COONa) from the molecule as a stable product, sodium carbonate. This removal drives the reaction to completion, reliably forming the stable benzene ring.
9. How does Hückel's rule (4n+2)π help explain the stability of benzene?
Hückel's rule is a condition for aromaticity. It states that a planar, cyclic molecule with a continuous ring of p-orbitals is exceptionally stable if it has (4n+2)π electrons, where 'n' is a non-negative integer (0, 1, 2, ...). For benzene, there are 6 pi-electrons, which fits the rule for n=1 (4(1)+2 = 6). This specific number of electrons creates a stable, closed shell of delocalised electrons, explaining its aromatic character and high stability.
10. What is the main difference between laboratory and industrial methods for preparing benzene?
The main difference lies in the source and scale. Laboratory methods, like the reduction of phenol, are suitable for preparing small quantities for study. Industrial production, however, focuses on cost-effective, large-scale methods. It primarily relies on refining crude oil through processes like catalytic reforming of naphtha or steam cracking, which yield benzene and other useful hydrocarbons in massive quantities.
