Benzene

What is benzene?


Benzene is an aromatic organic hydrocarbon with the formula C6H6. It is the parent compound of many aromatic compounds. 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 with one hydrogen atom and two other carbon atoms. 

Benzene

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

                                      Benzene: Electrons clouds above and below the aromatic ring

Properties of benzene –


Physical properties of benzene- 

1. Benzene is colorless liquid with distinct odor. 

2. Its 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 following chemical reactions - 

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

 Reaction - 

2. Sulphonation – Benzene is heated with fuming sulphuric acid and produces benzene sulfonic acid. 

Reaction – 

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

Reaction – 

      Benzene                                                             Chlorobenzene 


4. Friedel craft’s alkylation reaction – On treating benzene with alkyl halide, in presence of lewis acid such as anhydrous aluminum chloride, it forms alkyl benzene. This reaction is known as Friedel craft alkylation reaction. 

Reaction – 

5. 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 –

Resonance form of a compound is nothing but another way of representing 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 arrangement of electrons. So, resonance structure of benzene differs only in arrangement of electrons, number of carbon atoms and hydrogen atoms in benzene remain same. Double bonds present in benzene rotates in resonating structures of benzene and provide stability to benzene. Resonance structure shows possible structures of a compound. 

Resonance structures of benzene ring or benzene ring resonance structure – 

   

Resonating structures of benzene         Hybrid structure of benzene 


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. 


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

Reaction – 

Sodium benzoate sodium hydroxide                   Benzene Sodium carbonate


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

Reaction – 

Phenol                         Zinc             Benzene Zinc oxide 


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 fibres. 

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