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Benzene Reactions


Benzene (C6H6) the simplest aromatic hydrocarbon, occupies a prominent position in the study of the chemistry of aromatic compounds. Aromatic hydrocarbons containing more than one benzene rings are also referred to as bezenoid hydrocarbons. Benzene is a common trace gas originating primarily from the use of motor vechicles. Benzene was designated a hazardous air pollutant by the Environmental protection. 

Benzene is the parent compound of a group of substances which are pleasant, pungent smells and hence the name "aromatic". Some like benzene are highly poisonous, but others play an intimate role in the cell and in living processes. Benzene has the molecular formula C6H6, it contains a ring of six carbon atoms with a hydrogen atom attached to each one. 

Nitration of Benzene

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Benzene undergoes nitration when heated with a a mixture 1:1 (by volume) concentrated nitric acid and concentrated sulphuric acid (nitration mixture) at a temperature below 60oC. A mixture of concentrated nitric and sulphuric acids is used in the nitration of benzene. 

C6H6 + HNO3 $\rightarrow$ C6H5NO2 + H2O

The simpleset example of an aromatic nitro compound, nitro benzene is obtained by treating benzene at about 60oC with a mixture of concentrated nitric acid and sulphuric acid.

The mechanism of nitration of benzene is a two step process involving initially an electrophilic attack of the nitronium ion on the benzene molecule to form the intermediate mesomeric species followed by removal of the proton by a suitable base to give nitrobenzene. 

Sulfonation of Benzene

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Benzene will react with concentrated sulfuric acid at about 150oC to produce benzenesulfonic acid. The mechanism involves the initial formation of sulfur trioxide (SO3) from sulfuric acid. Though sulfur trioxide is a neutral molecule rather than a cation it is till a relatively powerful electrophile. The generation of sulfur trioxide from sulfuric acid and the sequence of reaction is given below.

Halogenation of Benzene

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Benzene and bromine react reather sluggishly if mixed under similar conditions. However, in the presence of a Lewis acid as catalyst benzene and other aromatic compounds have a strong tendency to undergo substitution with halogens leaving the aromatic ring intact. 

Benzene can be halogenated in the presence of Fe. In this reaction the iron(III) bromide acts as the catalyst which is formed from the iron an dthe bromine. The bromine-iron(III) bromide complex that is formed acts as the electrophile which attacks the benzene.

Uses of Benzene

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Benzene is used as a constitutient in motor fuels; as a solvent for fats, waxes, resins, oils, inks, paints, plastics and rubber in the extraction of oils from seeds and nuts and in photogravure printing. It is also used as a chemical intermediate. Benzene is also used in the manufacture of detergents, explosives, pharmaceuticals and dyestuffs.

The initial compound is used in the production of numerous organic aromatic compound including styrene, phenol and cyclohexane. Benzene is also used in tyre and shoe manufacturing. Exposure occurs mainly in petroleum industry. Benzene also found in cigarette smoke. Benzene is no longer used as a general solvent because of its high toxicity.