Aromatic Hydrocarbon

Aromatic hydrocarbon

Introduction

The term aromatic has been derived from the Greek word ‘aroma’ which means pleasant smell or fragrant odor. Therefore the compounds having sweet or pleasant smells are called aromatic compounds.

They are cyclic compounds and benzene is the most common aromatic compound.

Most aromatic compounds have one or more benzene rings on their molecule. Hence benzene and its derivatives are called aromatic compounds.

A new term ‘arene’ has been introduced to indicate all aromatic hydrocarbons. Thus arenes and their derivatives are called aromatic hydrocarbons or compounds. The chief source of aromatic hydrocarbon is coal-tar and petroleum.

The general formula for aromatic hydrocarbon is CnH2n-6y
Where n = number of carbon atom
y = number of rings in the molecule.
For example, the molecular formula of benzene is C6H6

Here , n = 6 and y= 1

Few examples of aromatic hydrocarbon

General characteristics of aromatic compounds

1. Aromatic compounds burn with sooty flame because of the higher percentage composition of carbon.
2. They are cyclic compounds containing 5, 6, or 7 members of carbon having a flat or planar structure.
3. They are highly unsaturated compound and undergoes electrophilic substitution reaction.
4. The percentage of carbon is higher in aromatic compounds than in aliphatic compounds.
5. They follow Huckel’s rule i.e. (4n+2) π electron system.

Huckel’s rule of aromaticity

The modern theory of aromaticity was given by Erich Huckel in 1938 in order to predict which
arenes and their derivative would be expected to be aromatic or not. This rule states that “A cyclic conjugated ring system which consists of (4n+2) number of π electron will show aromatic character”. That means the compound which follows the (4n+2)π electron system are aromatic compounds. Where n is a whole number positive integer, i.e. 0, 1, 2, 3,
4, 5, 6…etc. Huckel’s rule of aromaticity
According to this rule, the following conditions (criteria) are necessary to be aromatic compounds.

1. The compound should be cyclic and planar.
2. The compound must contain a cyclic cloud of delocalized π electrons above and below the plane. (Carbon to carbon alternate single and double bond)
3. All carbon atoms of the ring compound must be sp²- hybridization.
4. The cyclic compound must contain Huckel’s number. i.e. 2, 6, 10, 14, or 18 number of π electrons. Some examples of aromatic compounds which follow Huckel’s rule are:

Structure of benzene

The most convincing structure of benzene is given by Kekule in 1885 which can explain many properties of benzene.

He suggested that the benzene ring consists of alternate carbon-to-carbon single and double bond along with one hydrogen atom in each carbon of cyclic chain (C₆H₆) with a flat structure.

However experimentally it was found that all carbon-to-carbon bonds in benzene are identical and have the same bond length (1.39A˚) due to the delocalization of π electrons all over the six carbon atoms. Thus the structure of benzene is;

[C-C→1.54A˚/C=C→1.34A˚]

General methods of preparation of Benzene

1. From sodium benzoate (Decarboxylation reaction):When sodium benzoate is heated with soda-lime (NaOH+CaO) then benzene is formed. The preparation of hydrocarbon by heating sodium salt of an organic acid with soda-lime is called a decarboxylation reaction.

2. From phenol: When the vapour of phenol is heated with zinc dust then benzene is formed.

3. From acetylene: When acetylene is passed to a red hot tube of Fe or Cu at 500-600˚C then benzene is formed

4. From chlorobenzene: When chlorobenzene is heated with Ni-Al alloy in presence of NaOH then benzene is formed.

5. From benzene sulphonic acid: When benzene sulphonic acid is boiled with dilute acid
under pressure then benzene is formed.

6. From benzene diazonium chloride: When benzene diazonium chloride is reduced with
copper in presence of ethanol at 300˚C then benzene is formed.

Physical properties of Benzene

1. It is a colorless liquid with an aromatic odor.
2. It is insoluble in water and soluble in almost all organic solvents.
3. Its boiling point is 80.4˚C and its melting point is 5.5˚C.
4. The vapor of benzene is highly inflammable and toxic.
5. It is a good solvent for many organic and inorganic compounds.
6. It produces a sooty flame when burnt with air.

Chemical properties of Benzene

[A] Electrophilic substitution reaction: When an electrophile is substituted by another
electrophile then the reaction is called an electrophilic substitution reaction. And when this reaction occurs in an aromatic ring then it is called an aromatic electrophilic substitution reaction.

1. Halogenation: When benzene reacts with chlorine in the presence of FeCl₃ or AlCl₃ as a catalyst under cold and dar the condition then chlorobenzene is formed.

Similarly, benzene reacts with bromine in presence of FeBr₃ then bromobenzene is formed.

Since the iodination of benzene is reversible. So strong oxidizing agent like HNO₃ or HIO₃ is used to increase the rate of forward reaction and a large amount of iodobenzene is obtained.

2HNO₃ + 2HI ⟶ I₂ + 2H₂O + 2NO₂
HIO₃ + 5HI ⟶ 3I₂ + 3H₂O

Benzene Idobenzene

2. Nitration: When benzene reacts with conc. Nitric acid in presence of Conc. Sulphuric acid is a catalyst at about 50-60˚C then nitrobenzene is formed.

Above 60˚C, meta-dinitrobenzene is formed.

Benzene 1,3-dinitrobenzene

Above 100˚C, trinitrobenzene is formed which is largely used as explosives.

3. Sulphonation: When benzene is heated with Conc. Sulphuric acid then benzene sulphonic acid is formed.

4. Friedel Craft’s alkylation: When benzene is heated with an alkyl halide in presence of anhydrous AlCl₃ then alkyl benzene is formed.

5. Friedel Craft’s acylation: When benzene is heated with an acyl halide or acetic anhydride in presence of anhydrous AlCl₃ the acyl group is introduced to the benzene ring then acyl benzene is formed.

[B] Addition reaction:
Benzene contains three double bonds but it is less reactive toward addition reactions. Hence it undergoes additional reaction slowly with hydrogen, halogen, and ozone at the proper conditions of heat, light, pressure, catalyst, etc.

1. Addition of hydrogen (Catalytic hydrogenation): When benzene is heated with finely divided Ni or Pt catalyst at about 200˚C under pressure then cyclohexane is formed.

2. Addition of halogen: When benzene reacts with chlorine in presence of sunlight then benzene hexachloride is formed which is used as an insecticide.

3. Addition of ozone ( Ozonolysis): When benzene reacts with ozone followed by hydrolysis with zinc water then glyoxal is formed.

[C] Oxidation of benzene:


1. Combustion Benzene is completely oxidized into CO₂ when it is burnt with air to give a sooty flame.2C₆H₆ + 15O₂ → 12CO₂↑ + 6H₂O + Heat
   benzene
2. Catalytic oxidation: When benzene is oxidized with air in the presence of V₂O₅ catalyst at 500˚ then maleic anhydride is formed.

Uses of benzene

1. Benzene is starting material for the synthesis of dyes, drugs, explosives, plastics, perfumes, etc.
2. It is used as motor fuel along with petrol called benzol.
3. It is used as an industrial solvent for fats, oils, resins, rubber, etc.
4. It is used as a dry-cleaning agent for clothes
5. It is also used as an anti-knocking agent in gasoline.
6. It is used to prepare cyclohexane, cumene, aniline, nitrobenzene, polystyrene, adhesives, lubricants, detergents, pesticides, etc.

Orientation of benzene derivatives

The process of determining the position of substituents on the benzene ring is called the orientation of benzene. In the benzene ring, all six hydrogen atoms are chemically equivalent. Therefore it gives only one type of product in an electrophilic substitution reaction. But benzene derivatives give two or more different products. On the basis of orientation, the substituent group is divided into two classes.

1. Ortho and para-directing group(electron-releasing group)
2. Meta-directing group( electron-withdrawing group)

Ortho and para directing group

The substituent that directs the incoming electrophile at the ortho and para position is called the ortho and para directing group. All groups with unshared pairs of electrons (lone pair) on the atom attached to the benzene ring are ortho and para-directing in nature. They are also called ring activator groups. For example, – OH, -OR, -NH₂, -NHR, -NR₂, -R , -X etc. These groups are an electron-releasing group that releases electron toward the ring.

For example; phenol
In phenol, the -OH group present is the electron-releasing group. It releases electrons towards the ring and electron density is increased at the ortho and para position by delocalization of the electron. Hence incoming electron-lacking species (electrophile) attack at the ortho and para position. Therefore the –OH group in phenol is ortho and para
directing the group towards electrophilic substitution reaction. -OH group is called ortho para director or ring activator for electrophilic reaction.

or

For example; aniline.

In aniline, the -NH2 group present is an electron-releasing group. It releases electrons towards the ring and electron density is increased at the ortho and para position by delocalization of the electron. Hence incoming electron-lacking species (electrophile) attack at the ortho and para position. Therefore the –NH₂ group in aniline is the ortho and para-directing group towards electrophilic substitution reaction.

-NH2 group is called ortho para director or ring activator for electrophilic reaction.

or

Meta directing group

The substituent that directs the incoming electrophile at the meta position is called the meta-directing group. All groups with multiple bonds, attached to the benzene ring are meta-directing in nature. They are also called ring deactivator groups. For example, -NO₂, -CHO, -COOH, – C≡N, -COR, -COOR, etc. Meta directing group

Example: Nitrobenzene. In nitrobenzene, the –NO₂ group present is the electron-withdrawing group. It withdraws electrons from the ring and electron density is decreased at the ortho and para position by the delocalization of electrons. But at the meta position, the electron density is neither increased nor decreased. Hence an incoming electrophile can attack at meta this position to give meta substituted product. Therefore the –NO₂ group in nitrobenzene is a meta-directing group and ring deactivator toward electrophilic
substitution reaction.

or

Meta Group

Some important questions

1. State Huckel’s rule of aromaticity.
2. Write an example of Friedel-Craft’s acylation.
3. What is Friedel-Craft’s alkylation. Give example.
4. From phenol how would you obtain cyclohexane?
5. From sodium benzoate how would you obtain BHC?
6. What are aromatic compounds according to Huckel’s rule?
7. Write the characteristics of aromatic compounds.
8. What are the essential condition for a compound to be aromatic according to Huckel’s rule?
9. Write the resonance structure of the arene containing ortho and para-directing substituents.?
10. What happens when benzene is catalytically oxidized?

11. Why benzene is called an aromatic compound according to Huckel’s rule?
12. What happens when
    (a) sodium benzoate is heated with soda-lime?
    (b) benzene is heated with hydrogen in the presence of Ni?
    (c) phenol is heated with zinc dust?
    (d) benzene is heated with Conc. HNO₃ and H₂SO₄?
    (e) Does benzene react with chlorine in dark conditions?
    (f) acetylene is heated with a red hot tube at 500˚C?
    (g) benzene is heated with CH₃Cl and anhydrous AlCl₃?
13. How would you obtain benzene from
    (a) phenol (b) sodium benzoate (c) acetylene (d)
    chlorobenzene (e) benzene diazonium chloride