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

Coal tar

It is obtained from the efficient distillation of Coal tar in commercial quantities.

When distillation of Coal tar at 170°C a fractional liquid obtained. It is called light oil. It is lighter than water.

In addition to benzene, toluene and xylene, basic substances like pyridine and acidic substances like phenol and cresol are present. To remove them, light oil is first reacted with sodium hydroxide, which removes acidic impurities. After this, its diluents are reacted with H₂SO₄, which results in the separation of basic substances such as pyridine.

Now the light oil is washed with water. Due to which acidic elements are removed. In this way, efficient distillation of purified oil is done, which gives the following effect.

90% benzoyl (80 – 110°C) – 90% benzoyl means that distillation of 100 ml at 100°C gives 90 ml distillate. It contains 70% benzene and the rest toluene and xylene.

50% benzoyl (110 – 140°C): 50% benzoyl means that distillation of 100 ml at 100°C gives 50 ml distillate, it contains 45% benzene and the remaining toluene, xylene and other hydrocarbons.

Solvent naphtha or benzoic 140 – 170°C – It is used in dissolving dyes, rubber and resins, on re-effecting distillation of the above constituents, benzene at 80°C gives toluene at 110°C and xylene at 135°C.

Petroleum

In commercial quantities it is obtained from aromatization of n-heptane obtained from petroleum.

It’s C₆ – C₈ component is heated at high pressure and high temperature in the presence of appropriate catalysts to form benzene, toluene and xylene from petroleum.

In this action, benzene toluene and xylene are obtained as a result of aromatisation of aliphatic and alicyclic hydrocarbons. This action is called catalytic reforming or plat forming. Its ingredients are separated by Fractional distillation from a mixture of benzene, toluene and xylene.

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.

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.

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

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

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.