Arenes and Aromaticity
B.sc 1st year Book Organic Chemistry (Page 1)
Molecular formula : C6H6
Methods of Preparation :
Laboratory methods :
(a) By Distillation of phenol with Zn-dust(Reduction):
(b) By Decarboxylation of benzoic acid or phthalic acid by soda lime(NaOH+CaO) :
(c) By Reduction of benzene diazonium chloride with alkaline sodium stannate :
benzene diazonium chloride benzene
(d) By Hydrolysis of benzene sulphonic acid with super-heated steam or boiling with HCl under pressure :
Industrial Methods :
(a) From Coal Tar:
The light oil fraction (boiling range ) of coal tar contains mainly benzene, toluene, and xylenes; (. This on treatment with conc., remove basic impurities of pyridine and thiophene. The acidic impurities like phenol from the organic layer are removed successively with water and aqueous,. When acidic and basic impurities are completely removed, the organic layer is subjected to fractional distillation to obtain the following three main fractions –
(i) Benzol (boiling range) consists of benzene, toluene, and the rest of the xylenes.
(ii) Benzol (boiling range ) consists mainly of xylenes and small amounts of benzene and toluene.
(iii) Solvent naphtha or Benzyne (boiling range ) consists mainly of xylenes.
(b) From Acetylene:
(c) From Petroleum:
Chemical Properties of Benzene:
(1) Electrophilic Substitution reactions :
(i) Halogenation: Chlorine or bromine reacts with benzene in the presence of FeCl3 or FeBr3 as a catalyst to give chlorobenzene or bromobenzene respectively. e.g.
(ii) Nitration: On warming benzene with a mixture of conc. nitric acid and conc. sulphuric acid, nitrobenzene is formed.
(iv) Friedel-Craft alkylation: When benzene is treated with an alkyl halide in the presence of anhy. AICl3, gives alkylbenzene. Example:
(v) Friedel-Crafts acylation: Acylhalides react with benzene in the presence of anhy. AlCl3 to yield acyl benzene( aryl ketone).
2. Addition reactions:
(i) Addition of Hydrogen (Catalytic Reduction): When a mixture of benzene vapours and hydrogen gas is added over a nickel catalyst at 200∘C, cyclohexane is formed.
On reduction with Na+ ethyl alcohol in the presence of liq.NH3, benzene gives 1,4-vyclohexadiene. This action is called the ‘Birch Reaction‘.
(ii) Addition of Halogens: Benzene on treatment with halogens in the presence of sunlight gives BHC.
(3) Oxidation reactions:
When burnt in the air or in the presence of oxygen, CO2 and water are obtained.
Structure of Benzene :
Kekule Structure of Benzene: August Kekule(1865) was the first to suggest a ring structure of benzene: According to him, benzene consists of a cyclic planar hexagonal structure of six carbons with alternate double and single bonds. To each carbon, one hydrogen atom is linked as shown in Figure 7.01.
Drawbacks of Kekule structure:
(ii) Heat of combustion: On the basis of the Kekule structure, the heat of combustion of benzene is expected t be 3449.0 kJ mol−1, but the experimental value is 3298.5 kJ mol−1. The lack of 150.0 kJ mol−1 energy: benzene than Kekule structure indicates benzene may exist in another stable form.
(iii) Heat of hydrogenation: Similarly, on the basis of the Kekule structure, the heat of hydrogenation of benzene an expected to be 358.0 kJ mol−1, but the experimental value is 208.5 kJ mol-1. The lack of 149.5 kJ mol−1ene? of benzene than Kekule structure again indicates benzene may exist in another stable form.
(iv) Benzene undergoes electrophilic substitution reaction to form only one ortho-disubstituted product whereas Kekule structure predicts two ortho-disubstituted products as shown below-