Arenes and Aromaticity

B.sc 1st year Book
Organic Chemistry
(Page 1)
Introduction: At the beginning of the study of organic chemistry, the sweat-smelling compounds were called ‘aromatic’ (Greek aroma means pleasant smell.). Further studies on these compounds revealed that they contained benzene rings involving 6 carbon atoms in a ring. Later a large number of aromatic compounds were discovered which did not have pleasant smells, some of them had even foul smells. Therefore, the word ‘aroma’ lost its significance in relation to aromatic compounds. Aromatic compounds are now regarded as a class of compounds that contain at least one benzene ring. They are also known as ‘benzenoids‘. Aromatic compounds which do not contain benzene rings are called non-benzenoids.

Arenes

Arenes are mixed aromatic aliphatic compounds. An alkyl(R= -CH3, -CH2CH3,-CH(CH3)2), -C(CH3)3 ), etc., Alkenyl(-C=CHR’, where R’ = H or alkyl group) or alkynyl (-C=CR’, R’=H or Alkyl group) group attached to a benzene ring constitutes an arena

  1.  Alkylbenzenes

2. Alkenylbenzenes

Arenes and Aromatic-(styrene to 3-phenylpropene-1 formation)

3. Alkynylbenzenes

Arenes and Aromatic-alkylbenzenes

Benzene

Molecular formula : C6H6

Methods of Preparation :

  1. Laboratory methods :

    (a) By Distillation of phenol with Zn-dust(Reduction):
    Arenes and Aromatic-(phenol with Zn-dust rection)(b) By Decarboxylation of benzoic acid or phthalic acid by soda lime(NaOH+CaO) :

    Arenes and Aromatic-(Decarboxylation of benzoic acid or phthalic acid by sodalime

    (c) By Reduction of benzene diazonium chloride with alkaline sodium stannate :
    benzene diazonium chloride benzene
    Reduction of benzene diazonium chloride with alkaline sodium stannate(d) By Hydrolysis of benzene sulphonic acid with super-heated steam or boiling with HCl under pressure :

    Hydrolysis of benzene sulphonic acid with super-heated steam

Industrial Methods :

(a) From Coal Tar:

The light oil fraction (boiling range 80−170∘C ) of coal tar contains mainly benzene, toluene, and xylenes; (C6H4(CH3)2). This on treatment with conc.H2SO4, remove basic impurities of pyridine and thiophene. The acidic impurities like phenol from the organic layer are removed successively with water and aqueous, KOH. When acidic and basic impurities are completely removed, the organic layer is subjected to fractional distillation to obtain the following three main fractions –
(i) 90%  Benzol (boiling range 80−110∘C) consists of 70% benzene, 24% toluene, and the rest of the xylenes.
(ii) 10% Benzol (boiling range 110−140∘C) consists mainly of xylenes and small amounts of benzene and toluene.
(iii) Solvent naphtha or Benzyne (boiling range 140−170∘C) consists mainly of xylenes.

(b) From Acetylene:

acetylene to benzene formation

(c) From Petroleum:
hexane to benzene formation

Chemical Properties of Benzene:

Benzene undergoes mainly three types of reactions: substitution, addition, and oxidation reactions, which are discussed below –

(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.

chlorination and bromination formations

(ii) Nitration: On warming benzene with a mixture of conc. nitric acid and conc. sulphuric acid, nitrobenzene is formed.

Benzene to nitrobenzene formations
(iii) Sulphonation: When benzene is treated with hot concentrated sulphuric acid gives benzene sulphonic acid. In this reaction, a sulphonic acid group replaces one of the nuclear hydrogens.
benzene react with sulphuric acid to create benzene sulphonic acid

(iv) Friedel-Craft alkylation: When benzene is treated with an alkyl halide in the presence of anhy. AICl3, gives alkylbenzene. Example:

benzene react with methyl chloride to from toluene plus hcl

(v) Friedel-Crafts acylation: Acylhalides react with benzene in the presence of anhy. AlCl3 to yield acyl benzene( aryl ketone).

Friedel-Crafts acylation

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.

Catalytic Reduction

On reduction with Na+ ethyl alcohol in the presence of liq.NH3, benzene gives 1,4-vyclohexadiene. This action is called the ‘Birch Reaction‘.

birch reaction
At higher temperatures, even in the absence of ethyl alcohol, Birch reduction takes place to give cyclohexene.
benzene to cyclohexene formations

(ii) Addition of Halogens: Benzene on treatment with halogens in the presence of sunlight gives BHC.

benzene to benzene hexachloride(BHC) or HCH
(iii) Addition of Ozone: Benzene on treatment with ozone forms triozonide, which decomposes in the presence to give 3 molecules of glyoxal.
benzene to ozonide formations

(3) Oxidation reactions:

When a mixture of benzene vapours and the air is passed over a heated vanadium pentoxide catalyst, maleic anhydride is obtained.
Benzene to maleic anhydride formations

When burnt in the air or in the presence of oxygen, CO2 and water are obtained.

2C6H6+15O2⟶12CO2+6H2O 

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.

Kekule structure of benzene
Drawbacks of Kekule structure:
Kekule structure was unable to explain the following observations
(i) Benzene is 1,3,5-cyclohexatriene(consists three carbon-carbon double bonds.), it does nt decolourise Baeyer’s reagent. It readily undergoes electrophilic substitution reactions in which the benzene ring is retained.

(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-

Kekule structure two ortho
Kekule himself discarded this objection by suggesting that the carbon atoms in benzene we continually in a state of vibration, each C-C, pair had a single bond half of the time and double bon the other half. i.e. benzene was a mixture of two forms ( 3 and 4 ) in rapid equilibrium. Thus, there is no difference between the pairs of adjacent carbon atoms and thus only one ortho-disubstituted product is possible.
benzene-3-benzene-4
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