What do you mean by Benzene? Preparation and Properties

Benzene was first discovered by Michael Faraday in 1825. Faraday obtained benzene from a distillation of while fish oil. Hofmann obtained it in 1845 from the fractional distillation of bitumen. A few decades from now, bitumen(coil tar) was the major source of benzene. Now the major source of benzene is petroleum.

Benzene is the basic compound of the aromatic group. Most aromatic compounds are derivatives of benzene. All aromatic compounds have a cyclic structure like benzene or Toluene.

Benzene preparation

Laboratory method: sodium benzoate

In the laboratory, benzene is made by distillation of sodium benzoate and soda lime.

C6H5COONa + NaOH → C6H6 + Na2CO3

A round bottom distillation flask takes about 30 grams of sodium benzoate and about 45 grams of soda lime. Gently heat the condenser by placing it in the flask.

Benzene is collected as distilled with water. The benzene being insoluble in water forms a separate surface with water, which is separated by a practical funnel. Thus obtained benzene is dried on anhydrous calcium chloride to obtain pure benzene by re-distillation.

Acetylene

Berthelot synthesized benzene from acetylene in 1870 when acetylene is transported into the red heated tube and benzene is obtained.

3C2H2 → C6H6

Chlorobenzene

Benzene is obtained by reduction of Chlorobenzene by Ni — Al alloy and NaOH.

C6H5Cl + 2H → C6H6 + HCl

Phenol

Benzene is obtained by heating Phenol with zinc powder.

C6H5OH + Zn → C6H6 + ZnO

Benzenesulfonic Acid

Benzene is obtained by boiling Benzenesulfonic acid with dilute HCl or dilute H2SO4, or when heated with steam.

C6H5SO3H + H2O → C6H6 + H2SO4

Aniline

Benzenediazonium chloride is obtained when the aniline is reacted with a mixture of NaNO2 and HCl at 0–5°C. Benzene is obtained by reduction of Benzenediazonium chloride with hypophosphorous acid(H3PO2), formic acid or ethanol, Stanous chloride.

C6H5NH2 + HNO2 + HCl → C6H5-N=N-Cl + 2H2O

C6H5-N=N-Cl + 2H → C6H6 + N2 + HCl

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Benzene Physical Properties

Benzene is a colourless, volatile, flammable and distinctly smelling liquid. Its boiling point is 80.4°C and freezing point is 5.4°C. It is insoluble in water and due to being lighter than water, forms a separate surface over the water.

It is soluble in alcohol, ether and chloroform. It is a good solvent for resins, fats, sulphur and iodine.

Benzene Chemical Properties

The chemical reactions of benzene can be divided into three parts.

  • Addition reactions
  • Oxidation reactions
  • Substitution reactions

Addition reactions

Addition of Hydrogen

Cyclohexane is obtained as a result of high pressure in the presence of nickel or platinum and the addition of benzene and hydrogen at about 250°C.

Addition of Halogens

Benzene Hexachloride is obtained as a result of the reaction of chlorine with benzene in the presence of sunlight (hv).

C6H6 + 3Cl2 → C6H6Cl6

Banzene Hexachloride is also known as BHC, gammaxene, Lindane or 666. It is a strong germicide. The addition of benzene to bromine is similar.

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Oxidation Reactions

Ozonolysis

The addition of benzene to the ozone results in the addition of benzene triozonide, which results in three atoms of Glyoxal by water decomposition.

The addition of benzene to the ozone results in the addition of benzene triozonide, which results in three atoms of Glyoxal by water decomposition.

Catalytic Oxidation

By heating at 450°C in the presence of vanadium pentoxide(V2O5), it is oxidized by oxygen in the air to form a maleic anhydride.

Substitution Reactions

Halogenation

Benzene reacts with chlorine or bromine at room temperature in the absence of sunlight and in the presence of halogen carriers such as Fe or FeCl3 to form replacement products.

C6H6 + Cl2 → C6H5Cl + HCl

C6H6 + Br2 → C6H5Br + HBr

Nitration

Benzene reacts with concentrated nitric acid in the presence of concentrated sulfuric acid to form nitrobenzene.

C6H6 + HNO3(conc.) → C6H5NO2 + H2O

This reaction occurs at a slower rate than normal temperature and faster at increasing temperature. At higher temperatures and greater use of nitric acid, di and tri substitution products ie m-dinitro benzene and 1,3,5 tri-nitro-benzene are obtained.

Sulphonation

benzene sulphuric acid is obtained by heating benzene with concentrated sulphuric acid. This reaction takes place with ordinary sulphuric in normal temperature.

C6H6 + H2SO4 → C6H5SO3H + H2O

Friedel-Crafts alkylation

The reaction of benzene to an alkyl halide in the presence of a lewis acid such as AlCl3 results in alkylation of benzene.

C6H6 + CH3Cl → C6H5CH3 + HCl

Friedel-Crafts Acylation

The reaction of benzene to an acyl halide in the presence of a lewis acid such as AlCl3 results in acylation of benzene.

C6H6 + CH3COCl → C6H5COCH3 + HCl

Gattermann Koch Aldehyde Synthesis

Benzaldehyde is obtained when a mixture of carbon mono oxide and hydrogen chloride gas flows into benzene in the presence of AlCl3. This reaction is called Gattermann Koch Aldehyde Synthesis.

C6H6 + CO → C6H5CHO

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Gattermann Aldehyde Synthesis

Benzaldehyde is obtained when a mixture of carbon mono oxide and hydrogen chloride gas flows into benzene in the presence of AlCl3. This reaction is called Gattermann Koch Aldehyde Synthesis.

C6H6 + HCN → C6H5CH=NH + HCl

C6H5CH=NH + H2O → C6H5CHO + NH3

Chloromethylation

benzene chloride is obtained when a mixture of pharmaldehide and HCl gas flows into benzene in the presence of anhydrous ZnCl2. In this reaction, a hydrogen atom present in the benzene ring is displaced by a chloromethyl (-CH2Cl)group.

C6H6 + CH2O + HCl → C6H5CH2Cl + H2O

Mercuration

In alcoholic solution, benzene is heated with mercuric acetate to obtain Phenylmercuric acetate.

C6H6 + (CH3COO)2Hg → C6H5-Hg-O-CO-CH3 + CH3COOH

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Benzene uses

It is used to make many other aromatic compounds. Many aromatic compounds are used as dyes, medicines, explosives, and other useful substances.

It is also used in dry cleaning of clothes.

A mixture of benzene and petrol is used as fuel in motor cars.

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