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?
The organic chemical compound benzene (also known as Cyclohexatriene) has the molecular formula C6H6. The benzene molecule consists of six carbon atoms assembled in a planar ring, each containing a single hydrogen atom. Benzene is classified as a hydrocarbon because it only has carbon and hydrogen atoms.
One of the most common petrochemicals is benzene, which is a component of crude oil. Benzene is categorized as an aromatic hydrocarbon due to its cyclic continuous pi bonds between the carbon atoms. PhH is a common abbreviation. Benzene is a colorless, highly combustible liquid with a pleasant odor that contributes to the scent of gasoline (gasoline) stations. It is largely utilized as a precursor in the production of compounds with more complex structures, such as ethylbenzene and cumene, which are manufactured in billions of kilograms each year. In spite of being an important industrial chemical, benzene has limited application in consumer products due to its toxicity.
2. How is benzene prepared?
Benzene is prepared in the following ways:
Benzene Production from Alkynes: By cyclic polymerization, ethyne is utilised to produce benzene. At a temperature of 873K, ethyne is allowed to travel through a red-hot tube, where it undergoes cyclic polymerization and yields benzene.
Benzene Production from Aromatic Acids: To make Benzene, aromatic acids undergo a decarboxylation process. Benzene and sodium carbonate are formed when sodium benzoate, or the 'Na' salt of benzoic acid, interacts with soda lime in the presence of heat.
Benzene Production from Phenol: Benzene is created by reducing phenols. Phenol vapours are transported overheated zinc dust in this process. Zinc dust degrades phenols to create benzene and zinc oxide.
Benzene Production from Sulphonic Acid: Benzene is produced via hydrolysis of sulphonic acid. Benzene is produced by passing the chemical benzene sulphonic acid through a large amount of hot steam.
3. Describe the resonance in benzene.
Three twofold bonds and three single bonds are represented as 1,3,5-cyclohexatriene or 2,4,6-cyclohexatriene and Benzene's conventional design. The true structure of Benzene, on the other hand, resembles a blend of the two because all-electron thicknesses go through all P-orbitals in the same way. As a result, each side creates a bond that is a medium of solitary and twofold security, which sways inside the ring. The sp2 hybridization is present in all carbon particles within this ring. Because there are two sp2 hybridized orbitals, one of the links with the sp2 hybridized orbital of the nearby Carbon molecule, forming a C-C bond.
The sp2 hybridized orbital that follows connects with the s orbital of hydrogen to form a C-H bond. As a result, six C-C sigma bonds and six C-H sigma bonds are formed. There are still some unhybridized p orbitals around. By horizontal cross-over, they will shape bonds with the following carbon particles.
4. What are the effects on health due to benzene?
Benzene is a known cause of bone marrow failure and is categorised as a carcinogen, which increases the risk of cancer and other disorders. Benzene has been linked to aplastic anaemia, acute leukaemia, bone marrow abnormalities, and cardiovascular illness in a large number of epidemiologic, clinical, and laboratory studies. Acute myeloid leukaemia (AML), aplastic anaemia, myelodysplastic syndrome (MDS), acute lymphoblastic leukaemia (ALL), and chronic myeloid leukaemia are some of the hematologic cancers linked to benzene (CML).
"It is generally recognised that the only perfectly safe quantity for benzene is zero," the American Petroleum Institute (API) noted in 1948. There is no such thing as a safe quantity of exposure; even minute amounts can be harmful. Benzene is classified as a human carcinogen by the US Department of Health and Human Services (DHHS). Leukaemia is a potentially lethal malignancy of the blood-forming organs caused by long-term exposure to high quantities of benzene in the air. Acute myeloid leukaemia (AML) and acute non-lymphocytic leukaemia (ALL) are both known to be caused by benzene. The International Agency for Research on Cancer classified benzene as "known to be harmful to humans."
Human exposure to benzene is a global health hazard because it is found in gasoline and other hydrocarbon fuels used all over the world. Benzene can cause DNA strand breakage and chromosomal damage in the liver, kidney, lung, heart, and brain. In animals, including humans, benzene causes cancer. Multiple species of laboratory animals exposed to benzene via various methods have been proven to develop cancer in both sexes.
5. What are the uses of benzene?
The uses of benzene are the following:
Aniline and phenol, both of which are used in colors, are made from benzene.
The chemical benzene is also used to make nylon fibers.
The chemical benzene is also used to make cyclo-head, nitrobenzene, cumene, and ethylbenzene.
Benzene is rarely used in the household due to its toxic and carcinogenic qualities.
It is also used to make cleaning supplies, tobacco, glue, and adhesives, among other things.
It is a chemical reagent used in the manufacture of pesticides, soaps, cleansers, engineered filaments, plastics, pharmaceuticals, and pigments, among other things.
Volcanoes and forest fires are other natural sources of benzene.
In the case that it spills from capacity tanks, it swiftly evaporates in water and soil. It is, however, harmful and can lead to the contamination of surrounding water wells and water supplies.
