[Chemistry Class Notes] on Benzene Pdf for Exam

Benzene is an aromatic organic hydrocarbon with the formula C6H6. It is the parent compound of many aromatic compounds. The presence of benzene can be recognized by its characteristic odor. 

Structure of Benzene- Eilhardt Mitscherlich, a German chemist heated benzoic acid with lime and produced benzene. Although benzene was 1st discovered by scientist Michael Faraday. Benzene has a cyclic structure with 6 carbon atoms and 6 hydrogen atoms. In benzene, each carbon atom is attached to one hydrogen atom and two other carbon atoms.

Benzene

Due to the delocalization of six electrons in benzene, actually there is no distinct single or double bond in benzene. That’s why bond length is

[1.39 dot{A}]

These 6 electrons make electrons clouds above and below the aromatic ring. 

Properties of Benzene 

Physical Properties of Benzene- 

  1. Benzene is a colorless liquid with a distinct odor. 

  2. It’s highly flammable and volatile. 

  3. It is soluble in organic solvents while insoluble in water. 

  4. Its boiling point is 80.5°C.

  5. It shows resonance.

Chemical Properties of Benzene – Benzene shows the following chemical reactions – 

  1. Nitration- Benzene reacts with nitric acid in presence of sulphuric acid at 55°C and forms nitrobenzene.

Reaction – 

  1. Sulphonation- Benzene is heated with fuming sulphuric acid and produces benzene sulfonic acid.

Reaction – 

  1. Halogenation- In presence of Lewis acid, benzene reacts with halogen to form aryl halide.

Reaction – 

 

  1. Friedel Craft’s Alkylation Reaction- On treating benzene with an alkyl halide, in presence of Lewis acid such as anhydrous aluminum chloride, it forms alkylbenzene. This reaction is known as the Friedel craft alkylation reaction.

Reaction – 

  1. Friedel Craft’s Acylation Reaction- On treating benzene with an acyl halide, in presence of Lewis acid, it forms acyl benzene. This is known as Friedel craft’s acylation reaction.

Reaction – 

 

Resonance in Benzene

The resonance form of a compound is nothing but another way of representing the Lewis dot structure of that compound. Benzene shows resonance as it has more than one way to place double bonds in the ring. Benzene shows 2 resonating structures. Resonance forms or resonating structures only differ in the arrangement of electrons. So, the resonance structure of benzene differs only in the arrangement of electrons, and the number of carbon atoms and hydrogen atoms in benzene remains the same. Double bonds present in benzene rotate in resonating structures of benzene and provide stability to benzene. Resonance structure shows possible structures of a compound.

Resonance structures of a benzene ring or benzene ring resonance structure –  

The hybrid structure of benzene is more stable than resonation structures of benzene as it has lower energy than resonating structures of benzene.

Preparation of Benzene 

  1. From alkynes – Ethyne is passed through a red-hot iron tube at 873K, it goes through polymerization and forms benzene. 

  1. Decarboxylation of aromatic acids- Sodium benzoate is heated with sodium hydroxide in presence of calcium oxide to produce benzene. 

Reaction – 

  1. Reduction of phenol – Phenol is heated with zinc to produce benzene. 

Reaction – 

Uses of Benzene 

  1. Benzene is used in the preparation of polystyrene. 

  2. It is used in the production of phenol.

  3. It is used in the preparation of aniline and detergents also. 

  4. It is used in the manufacturing of nylon fibers. 

  5. It is used as a solvent for organic compounds in the preparation of many useful compounds.

About Benzene

Benzene is considered to be one of the most important organic compounds which have a chemical formula C6H6. Benzene is considered the parent compound of a lot of aromatic compounds. Benzene is one of the simplest organic compounds and aromatic hydrocarbons. It is also one of the elementary petrochemicals and is one of those natural constituents of crude oil. It has an odor like gasoline and is a liquid that is colorless. Benzene is one of the highly toxic compounds and carcinogenic in nature. Primarily it is being used in the production of polystyrene.

Benzene is considered one of the naturally occurring substances which are produced by volcanoes and by forest fires and is present in many plants and animals, but benzene is also one of the major industrial chemicals which is made from coal and oil. Benzene is a clear and colorless liquid as it is a pure chemical. In many industries, benzene is being used to make other chemicals along with other types of plastics, pesticides, and detergents. Benzene is also considered a component of gasoline.

Characteristics of Benzene

  • Benzene was discovered by the great English physicist Michael Faraday in the year 1825 and Benzene was made available in the year 1842 in large quantities.

  • Now, a large amount of benzene is being extracted from petroleum.

  • Benzene is a liquid that is colorless.  

  • Benzene is a ring of six carbon atoms that are closed and which are linked by the bonds that alternate between the single and double bonds. Each atom of carbon is being bounded by a single hydrogen atom. 

  • The melting point of Benzene is 5.5°C and the boiling point of Benzene is 80.1°C. *Benzene and its derivatives are the important parts of an essential chemical community which is known as aromatic compounds. 

  • Benzene is a precursor in the production of oil, plastics, synthetic, rubber and dyes, and medicines. 

  • The hydrogenation of the benzene happens much more slowly in comparison with the hydrogenation of other organic compounds which contain the double bonds of carbon-carbon, and benzene becomes more difficult to oxidize than alkenes.

  • Most of the reactions of any benzene belong to a particular class which is called the electrophilic aromatic substitution and which eventually leaves the ring intact but it is being replaced by one of the hydrogens that are attached to it. These reactions are commonly used for preparing the derivatives of benzene and are versatile in nature. 

Properties of Benzene

The main properties of benzene are given below – 

  • Benzene is soluble in organic solvents but it is immiscible in water.

  • Benzene has an aromatic odor and is a liquid that is colorless.

  • Benzene has a density of 0.87g cm3.

  • Benzene is lighter than water.

  • Benzene has a moderate boiling point which is 80.5°C and a high melting point which is 5.5°C.

  • Benzene can show resonance.

  • Benzene is highly inflammable and can even burn with a flame of scooty.

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