The document discusses aromatic hydrocarbons and provides details about benzene. It defines aromatic compounds as those containing benzene or benzenoid rings. Benzene's structure was originally proposed by Kekulé as alternating single and double bonds, but it is now understood to be a resonance hybrid of two contributing structures with delocalized pi electrons. This explains benzene's stability and reactivity toward substitution rather than addition reactions. The document provides examples of aromatic compounds and substitution patterns on benzene derivatives. It also discusses the orientation and directing effects of substituents on benzene during electrophilic substitution.

1. AROMATIC
HYDROCARBON
1

2. INTRODUCTION
• The term 'aromatic' was derived from Greek word 'aroma' meaning
sweet fragrance.
• The term 'aromatic' was first used by August Kekule.
• According to Kekule, benzene and its derivatives are aromatic
compounds.
• But later, it was found that they also contain one or more benzene
rings.
• Hence 'Aromatic compounds' refers to the whole class of compounds
containing one or more benzenoid rings ( benzene rings ).
• Aromatic hydrocarbons are also called ARENES. In this sense, arenes
and their derivatives are all 'Aromatic compounds'.
• But there are aromatic compounds containing no benzene ring.
2

3. CHARACTERISTICS OF AROMATIC
COMPOUNDS
• Aromatic compounds have cyclic & flat structure.
• They are highly unsaturated but do not give unsaturation test.
• Addition reactions are much less favorable than electrophilic
substitution reactions.
• They are stabilized due to resonance or due to delocalization
of π-electrons, so they have low values of heats of
hydrogenation than expected.
• They obey Huckel's rule: A conjugated ring system containing
(4n+2)πe- will show aromatic character.(where
n=0,1,2,3,4,5,etc)
3

4. FEW EXAMPLES
n=1, the conjugated ring system
containing 3π bonds= 6π electrons.
4n+2= ….π electrons
Or If n=1, 4×1+2=6π electrons, It obeys
Huckel's rule.
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Π bond=2 electrons
1.BENZENE
σ
σ
σ
Π
Π
Π

5. vgvg 5
2. NAPHTHALENE
Π bond=2 electrons
n=2, the conjugated ring system containing
5π bonds= 10π electrons.
4n+2= ….π electrons
OrIf n=2, 4×2+2=10π electrons, It
obeys Huckel's rule.
Π
Π
Π
Π
Π
σ
σ
σ σ
σ
σ

6. 3.Anthracene
vgvg 6
n=3, the conjugated ring system containing
7π bonds= 14π electrons.
4n+2= ….π electrons
Or If n=3, 4×3+2=14π electrons,
It obeys Huckel's rule.

7. 4.Phenanthrene
vgvg 7
n=3, the conjugated ring system containing 7π bonds=
14π electrons.
4n+2= ….π electrons
Or If n=3, 4×3+2=14π electrons, It obeys Huckel's rule.

8. FACTS ABOUT BENZENE
❑ Benzene does not discharge pink colour
of alkaline KMnO4 solution (also known as
Baeyer's reagent) and red colour of Br2/CCl4.
❑ It gives substitution reactions easily than addition reactions.
❑ It forms one mono-substituted, three di-substituted (one ortho,
one meta, and one para) & three tri-substituted products.
❑ It adds three molecules of chlorine, three molecules of hydrogen
and three molecules of ozone separately under suitable conditions.
❑Molecular formula of benzene is C6H6.
8

9. STRUCTURE OF BENZENE
1. From quantitative analysis and molecular weight
determination: Molecular formula of benzene is
C6H6.Thus benzene is highly unsaturated.
Proves for unsaturation:
hf
a) Benzene + 3Cl2 Benzene hexachloride
b) Benzene + 3O3 Benzene tri-ozonide
9

10. 10
Ni/2000c
c)Benzene +3H2 Cyclohexane
POSSIBLE STRUCTURE OF BENZENE
* Open Chain and
* Closed chain

11. POSSIBLE OPEN CHAIN STRUCTURES OF BENZENE
But any of the above donot fulfill all the facts of the benzene, so
no one of the above is the structure of benzene that is open chain
structure is discarded.
11

12. CYCLIC STRUCTURE OF BENZENE
PROPOSED BY AUGUST KEKULE (1865)
12
OR
Fig.Kekule structures

13. ABOVE KEKULE'S STRUCTURE STILL DOES
NOT EXPLAIN ALL FACTS
Above structure does not explain
• Why it does not discharge pink colour of KMnO4 solution and red
colour of bromine in CCl4.
• Why it gives substitution reactions more easily than addition
reactions.
• Why it forms only one ortho di- substituted isomer. On the basis
of above structure two ortho di-substituted isomers are possible.
For example:
13

14. Two possible ortho di-bromobenzene
14
Single bond
Double bond

15. ACTUAL STRUCTURE OF BENZENE
• In order to explain all the facts or to overcome
above difficulties, Kekule proposed his oscillation
theory.
• According to oscillation theory, the double bonds
and single bonds in benzene exchange their
positions very rapidly & the dynamic equilibrium
between the following two structures is
maintained.
15

16. ACTUAL STRUCTURE OF BENZENE ( contd. )
16
These two structures are inter-convertible to each other and are called
resonance structures of benzene. None of these two structures is the
actual structure of benzene. The actual structure of benzene is
resonance hybrid of these two canonical forms as shown below:

17. ACTUAL STRUCTURE OF BENZENE ( contd. )
17

18. ACTUAL STRUCTURE OF BENZENE ( contd. )
18

19. ACTUAL STRUCTURE OF BENZENE ( contd. )
From X-ray diffraction studies:
vgvg 19

20. STRUCTURE OF BENZENE ( contd. )
• The resonance hybrid structure of benzene can explain the
unusual property of double bonds in it which causes the extra
stability of benzene molecule.
• Resonance hybrid makes a molecule more stable than that
expected theoretically. This extra stabilization energy is called '
Resonance energy' which is 36 Kcal/mole for benzene.
• Resonance hybrid structure explains why benzene shows very
few addition reactions in spite of the presence of apparent
three aliphatic C=C bond in benzene.
vgvg 20

21. MOLECULAR ORBITAL PICTURE OF BENZENE
21

22. MOLECULAR ORBITAL PICTURE OF BENZENE (contd.)
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23. MOLECULAR ORBITAL PICTURE OF BENZENE (contd.)
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24. MOLECULAR ORBITAL PICTURE OF BENZENE (contd.)
24
Electron cloud ( π MO )

25. ISOMERISM IN BENZENE DERIVATIVES
• No isomerism in monosubstituted benzene.
• Disubstituted ( and trisubstituted ) benzene exhibits position
isomerism. They can have three isomers. Examples:
25

26. ISOMERISM IN BENZENE DERIVATIVES(contd.)
vgvg 26

27. ISOMERISM IN BENZENE DERIVATIVES(contd.)
27

28. ISOMERISM IN BENZENE DERIVATIVES(contd.)
28

29. ORIENTATION IN SUBSTITUTED BENZENE
• The term orientation indicates the assignments of the
positions of the substituents with respect to a substituent
already present in the ring during electrophilic substitution
reaction of a mono substituted benzene.
• The first substituent (G) may direct the next incoming group
(E+) to ortho, meta or para position, depending on the
nature of the substituent already present in the ring. This is
called directive or orientation effect. The substituent
already present in the ring is called director or directing
group.
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30. ORIENTATION IN SUBSTITUTED BENZENE(contd)
• ACTIVITY EFFECT. The substituent already present may activate or deactivate
the benzene ring towards further substitution.
Director
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31. vgvg 31
ORIENTATION IN SUBSTITUTED BENZENE(contd.)
Directing group
or director
ELECTROPHILE

32. ORIENTATION IN SUBSTITUTED BENZENE(contd)
TYPES OF DIRECTOR
Two types of Directors
1. Ortho-para directing group :
If the group present on the benzene ring is electron-
donating group then the incoming electrophile will
attack on the para and ortho position of the mono
substituted benzene ring. This is because the electrons
donated by electron-donating group are in conjugation
with the benzene ring which will generate negative
charge on ortho and para positions of the mono-
substituted benzene ring.
vgvg 32

33. ORIENTATION IN SUBSTITUTED BENZENE(contd)
• EXAMPLES OF ORTHO-PARA DIRECTORS
- OH, -NH2, -NHR, -NR2, -OCH3, -Cl, -Br, -I, -CH3 , -C2H5 etc.
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34. ORIENTATION IN SUBSTITUTED BENZENE(contd)
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35. ORIENTATION IN SUBSTITUTED BENZENE(contd)
2. Meta directing group:
If the group present on the benzene ring is electron-
withdrawing group then the incoming electro-phile will attack
on the meta position of the mono substituted benzene ring. This
is because the electrons withdrawing group will withdraw the
electron density of the ring by which positive charge will be on
ortho and para positions on the mono-substituted benzene
ring. Since, the ortho and para positions are electron deficient
only meta position will be rich in electron density. Hence,
electrophile will attack on meta position of the mono-
substituted benzene ring. As shown in the image attached.
vgvg 35

36. ORIENTATION IN SUBSTITUTED BENZENE(contd)
Examples of meta directing groups
-NO2-COOH,CHO, -COCH3, -CONH2, CN, -CCl3, -CBr3, -CF3, etc.
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36

37. ORIENTATION IN SUBSTITUTED BENZENE(contd)
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38. ORIENTATION IN SUBSTITUTED BENZENE(contd.)
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39. PREPARATION OF BENZENE
1. By decarboxylation of sod.benzoate.
vgvg 39
Soda lime

40. 2. By the reduction of phenol
Phenol Benzene
3. By the polymerization of ethyne
vgvg 40

41. 4.By the reduction of chlorobenzene
LiAlH4 or
Physical Properties of Benzene
⮚ Colourless liquid with pleasant odour.
⮚Boiling point 80.40C.
⮚Immiscible with water but miscible with organic solvents like alcohol,
petrol,etc
⮚Highly inflammable
⮚Lighter than water
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41

42. CHEMICAL PROPERTIES OF BENZENE
• Electrophilic substitution reactions
These reactions are highly favorable than addition reactions in benzene
due to its stability.
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42

43. ? Halogenation- takes place in DARK, in absence of light.
vgvg 43
Examples:
DARK
DARK

44. as an electrophile
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45. 2. NITRATION
+ -
HNO3 + H2SO4 NO2 + HSO4 + H2O
Nitronium ion
3. SULPHONATION
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46. vgvg 46
About
250C
2H2SO4 ↔ SO3 + H3O+ + HSO4
-

47. vgvg 47
4. FRIEDEL CRAFT’S REACTIONS : Two types
1. FRIEDEL CRAFT’S alkylation
2. FRIEDEL CRAFT’S acylation
1. FRIEDEL CRAFT’S alkylation
Warm
(Lewis acid)

48. vgvg 48
Examples

49. vgvg 49
Alkyl cation as
an electrphile

50. vgvg 50
2. FRIEDEL CRAFT’S acylation

51. 51
FORMATION OF ACYLIUM ION AS AN ELECTROFILE

52. 52
• ADDITION REACTIONS
• With hydrogen
Ni-cat
1500, press.
Cyclohexane
• With chlorine

53. 53
• With ozone
OR
CHO
CHO
3

54. 54
• COMBUSTION OF BENZENE
⮚ Complete combustion
C6H6 + 15/2 O2 6CO2 + 3H2O +Heat
Excess
⮚ Incomplete combustion
C6H6 + 5/2 O2 5C + CO2 + 3H2O + Heat
Limited Sooty flame Steam

55. 55
USES OF BENZENE
⮚ As a solvent for oils and fats.
⮚For dry cleaning or washing of wollen and silk clothes ( garments )
⮚As a fuel for automobiles in the name of Benzol. It is added in gasoline
to increase its OCTANE NUMBER.
⮚ For the manufacture of varieties of commercial derivatives like ethyl
benzene, phenol, chlorobenzene, nitrobenzene, BHC, etc.
⮚ In the manufacture of dyes, drugs, perfumes, explosives, etc.