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Benzene

 

Benzene is an organic compound having the molecular formula C6H6. It is a carbon ring made of 6 carbon atoms, each attached with one hydrogen atom in a ring formation. Benzene is regarded as a hydrocarbon as it is formed of carbon and hydrogen only. The carbon atoms are attached to each other with pi- bonds which makes benzene an aromatic hydrocarbon. It is colourless and highly flammable and found in a liquid state with a characteristic odour.

Structure of benzene

Benzene exists as a ring structure of hydrocarbons. It is a six-membered carbon ring. Each carbon is attached to another carbon on both sides and a hydrogen atom. The carbon atoms have alternate double bonds.

1

The two figures from above represent benzene. The double bond indicates a delocalised pi-bond between the carbon atoms. The phenomenon of resonance is noticed in the benzene molecule. The two structures are benzene's resonance structures, which are formed due to the delocalisation of pi- bonds in benzene. The delocalisation causes the benzene bond to have a slightly double bond nature making it more strong and shorter in length.

Resonance in benzene

As we know, the benzene ring has pi- bonds. Pi- bonds are the double bonds formed between the carbon atoms of the benzene ring. The pi- bonds are delocalised in the benzene ring. This means that the pi- bonds are free to move. This enables the pi- bonds to move from one carbon atom to another. The movement of pi- bond among the carbon atoms is known as delocalisation of pi- bonds. This free movement of pi- bonds leads to the formation of different structures of the benzene ring, which differ only in the placement of pi- bonds. However, these different structures have the same chemical formula and are called resonance structures of benzene.

1

The resonance structures can be explained on the basis of valance bond theory. All the carbon atoms in the benzene ring are sp2 hybridised. The sp2 hybridised orbitals overlap with each other to form sigma bonds which are single bonds between carbon atoms. The leftover orbitals bond with hydrogen atoms to give carbon-hydrogen sigma bonds. The unhybridised p- orbitals form pi- bonds that are free to move to result in resonance.

*yha pic add krni hai*

Properties of benzene

Benzene possesses the following properties:

Physical properties-

1. It is an aromatic ring.

2. It is among the most prominently found hydrocarbon ring in organic compounds.

3. It is a colourless liquid at room temperature.

4. It has a characteristic odour.

5. It is highly inflammable.

6. It has a boiling point of 80.1°C.

7. It has a melting point of 5.5°C.

8. A benzene ring exhibits resonance.

9. It is lighter than water in density.

10. It is insoluble in water but soluble in organic solvents.

Chemical properties-
 

1. Nitration

Benzene reacts with sulphuric acid to form nitrobenzene. Benzene is treated with a mixture of nitric acid, water and sulphuric acid, giving nitrobenzene the product. Nitration of benzene takes place at a temperature not exceeding 50°C.

2. Sulphonation

Sulfonation is the process of replacing a hydrogen atom from a benzene ring with sulphonic acid SO3H. It produces sulphonic benzene acid as the product. Benzene is treated with sulphur trioxide and fuming sulphuric acid. It is a reversible reaction out of which benzene can be obtained by adding aqueous acid to benzene sulphonic acid.

3. Halogenation

Halogenation of benzene gives aryl halides as a product. It is an electrophilic substitution reaction where benzene reacts with bromine or chlorine in the presence of a catalyst that replaces a hydrogen atom with chlorine or bromine.

4. Friedel crafts alkylation

In Friedel crafts alkylation, an alkyl group attaches to the benzene ring by electrophilic substitution. The product formed is alkylbenzene.

5. Friedel crafts acylation

In Friedel crafts alkylation, an acyl group attaches to the benzene ring by electrophilic substitution. The product of Friedel crafts acylation of benzene gives acyl halide.

6. Uses of benzene

1. It is used in the preparation of phenols.

2. It is used to prepare aniline for dyes.

3. Detergents are manufactured with the help of benzene.

4. It is useful in making other chemicals such as nitrobenzene, alkylbenzene, cyclohexane etc.

5. It is also used as a solvent for many organic compounds.

6. The compound form the beginning reactant for toluene which is used to make trinitrotoluene(TNT) commonly known as dynamite.

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