AROMATIC HYDROCARBONS
Introduction
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Compounds having similar chemical properties to benzene
are called aromatics.
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The name aromatic is used for characteristic pleasant odor.
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Aromatic hydrocarbons are also called arenes.
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The simplest aromatic hydrocarbon is benzene.
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The source of aromatic hydrocarbons are coke and
When two substituents are attached to the benzene ring, three
isomeric structures are possible.
petroleum.
1.
Benzene and Its Structure
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Benzene is the oldest known organic compound, firstly
discovered by Michael Faraday in 1825.
Its structure was first proposed by German chemist August Kekule in
1865
It does not undergo an addition reaction with bromine despite having
a double bond.
Benzene is a water insoluble compound boils at 80oC, has a molecular
formula C6H6.
Each of the six carbon atoms undergoes sp2 hybridization.
It has a planar shape.
2. Nomenclature of Aromatic Compounds
Benzene is parent name. For simple structures common name is often
used. For complex structures either numbering of letter prefixes are
used.
When three substituents are attached to the benzene ring, numbers
are used to show the position.
Example 1
Name the following aromatic compounds.
a.
b.
c.
d.
m-ethyl phenol (3-ethylphenol)
1-ethyl-3-hydroxylbenzene (phenylcyclohexane)
p-bromotoluene (3,5-dichlorostyrene)
1, 3-dichloro-5-vinylbenzene
Example 2
Draw the structures of the following aromatic compounds.
m-nitrotoluene ,m-dinitrobenzene, 1,3-dichloro-5-nitrobenzene,
nitrobenzene, triphenylmethane
3. Isomerism in Benzene Derivatives
Monosubstituted benzene derivatives do not exibit isomerism.
However, when two substituents are present there are three possible
isomers.
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•
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Main natural source of benzene is coal tar. It contains
benzene, toluene, xylene, phenol, naphthalene and
anthracene.
Also be produced from petroleum hydrocarbons by
aromatization.
Alkanes can be dehydrogenated to produce benzene and its
derivatives by heating them over catalysts.
6.1 Trimerization of Acetylene
Isomer structures
6.2 Decarboxylation of Sodium Benzoate
4. Physical Properties of Benzene
Colorless, poisonous liquid with a specific odor.
Dissolves in organic solvents such as ether, alcohol, acetone and
acetic acid.
Good solvent for non-polar substances
Used to dissolve fats, resins, rubber, iodine and sulfur.
Carbon content is high, therefore, when burned, it gives a sooty flame
6.3 Dehydrogenation of Hexane and Cyclohexane
5. Chemical Properties and Reactions of Benzene
Benzene is a stable compound it undergoes substitution reaction and
addition reaction under special condition.
5.1 Substitution Reactions
Halogenation
Nitration
7. Other Aromatic Hydrocarbons
Naphthalene, anthracene, and phnanthrene are aromatic
hydrocarbons with fused benzene rings.
Alkylation or FridelCraft Reaction
Sulfonation
Hydrogenation
Halogenation
6. Occurrence and Preparation of Benzene
Naphthalene
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Two benzene rings are fused together.
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Colorless, crystalline compound which sublimes easily.
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Insoluble in water but soluble in organic solvents.
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Has a sharp, aromatic odor.
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Used to protect clothes from moths and to dispel the bad
odors in bathrooms.
Anthracene
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Formed by fusing three benzene rings together in a linear
geometry.
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Colorless, crystalline compound used in the production of
paints.
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It is also used in wood preservatives, insecticides, and
coating materials.