Kekulé had a dream in which he envisioned snakes grasping their own tails and forming rings, inspiring his hypothesis that carbon atoms in benzene are arranged in a ring structure with alternating single and double bonds. This ring structure explains benzene's stability and resistance to addition reactions compared to open-chain alkenes. Aromatic compounds are named for many originally having pleasant aromas, though the term now refers to chemical stability conferred by conjugated planar ring structures like benzene that allow for resonance. These compounds are important industrially as precursors to dyes, drugs, polymers, and other chemicals.

1. 1

2. 2

3. AROMATIC COMPOUNDS
“LET US LEARN TO DREAM
GENTELMAN, AND THEN
PERHAPS, SHALL LEARN THE
TRUTH”.
(KEKULE)
3

4. AROMATIC
COMPOUNDS
 KEKULE fall asleep while sitting in front of fire,
dreamed about chains of atoms in form of twisting
snakes. one of snake caught hold of its own tail ,
forming a whirling ring.
 KEKULE awoke , freshly inspired , spent remainder
night working on his now-famous hypothesis.
4

5. AROMATIC COMPOUNDS
• The term aromatic compounds was 1st used by
Kekule to classify benzene & its derivatives.
• Benzene and its derivatives, many of which possess
fragrance/ aroma.
• According to new fact, aromatic is associated with
“chemical stability” rather than aroma.
• So aromatic is used for series containing benzene
ring.
• May be called as “ Arenes and derivatives.”
• Include H-carbons, ketones, aldehydes, amines,
ethers etc.
5

6. Benzene and its
derivatives
• Constitute the most important class of Arenes
(Aromatic hydrocarbons).
• Aryl group. By dropping H from ring.
• Aryl alkyl group. By dropping H from side chain.
6

7. AROMATIC HALOGEN COMPOUNDS &
AROMATIC SULPHONIC ACID
• Halogen atom bonded direct to benzene ring (aryl
halide) or bonded to side chain (aryl alkyl
halide)are aromatic halogen compounds.
• One –OH group of sulfuric acid is replaced by aryl
group, compounds are called aromatic sulphonic
acids.
7

8. AROMATIC NITRO
COMPOUNDS & PHENOLS
• Derivative of aromatic hydrocarbon, H atom is
replaced by –NO2 group are called
aromatic nitro compounds.
• Compounds containing –OH group directly attach
to ring, are called phenols.
8

9. AROMATIC AMINES &
AROMATIC CARBOXYLIC ACIDS
• H atom is replaced by an amino group.
• Aniline give reactions same as
aliphatic amines.
• Compounds containing –COOH group directly
attached to ring, are called aromatic carboxylic
acids.
9

10. AROMATIC ALDEHYDES
& KETONES
• Aromatic aldehyde: -CHO group attached to
benzene ring directly.
• Aromatic ketones: -Carbonyl group attached to
two aryl groups or one aryl and one alkyl group.
10

11. BENZENE
• Molecular formula: C6H6
• No straight chain structure is possible.
• All C-atoms are sp2 hybridized.
11

12. IDENTIFICATION TEST
FOR BENZENE
• Gas chromatography : used to identify benzene,
xylene, toluene etc. (compounds that can be
vaporized without decomposition).
• Reaction with KMNO4: benzene will not show
reaction as it doesn’t oxidize while methyl benzene
will discharge colour by oxidizing to acid.
• Introducing a glowing splint will burn with smoky
flame.
12

13. IDENTIFICATION TEST
FOR BENZENE
• Add conc.H2SO4 and conc.HNO3 to benzene at
55oC,
yellow oil with bitter almond smell produced.
13

14. PREPARATION OF
BENZENE
• Benzene and other aromatic
compounds are readily obtained in
large quantities from coal and
petroleum . Benzene and some other
hydrocarbons can also be obtained
from petroleum by special cracking
methods .some of the methods
generally used for the preparation of
benzene are as following :
14

15. GENERAL METHODS
1 : Dehydration of Cyclohexane .
2 : From Acetylene .
3 : From Alkanes .
4 : Preparation in the Laboratory .
5 : Wurtz-Fittig Reaction .
15

16. 1- Dehydrogenation of
Cyclohexane :
• When cyclohexane or its
derivative is dehydrogenated we
get benzene . The reaction is
carried out by the use of a
catalyst , pt or pd ,at elevated
temperature e.g, 2500c .
Cyclohexane Benzene
16

17. 2- From Acetylene :
• Benzene is formed by passing
acetylene under pressure over an
organo-nickle catalyst at 700c .
17

18. 3- From Alkanes :
• Benzene and toluene are also
prepared by passing the
vapours of n-hexane or n-
heptane over a mixture of
catalysts Cr2O3 +Al2O3+SiO2 at
5000C .
18

19. 19

20. 4- Preparation in the
Laboratory :
• Benzene can be prepared by
any of the following methods :
• * By heating sodium salt of
benzoic acid with soda lime :
20

21. • *By distilling phenol with
zinc dust .
21

22. • *By the hydrolysis of
benzene sulphonic acid
with superheated steam
or by boiling with dilute
HCl .
22

23. 5- Wurtz-Fittig Reaction :
• The Wurtz reaction for the synthesis of
alkanes was extended by Fittig in 1864
,to the synthesis of alkyl aromatic
hydrocarbons.
23

24. PHYSICAL
PROPERTIES OF
BENZENE
24

25. • Benzene is a colorless liquid.
• Benzene is soluble in organic solvents but immiscible in
water.
• It is an aromatic compound so it has a typical
aromatic odor. (Aroma in Greek means pleasant
smelling).
• Benzene is highly inflammable and burns with sooty
flame as compared to alkanes & alkenes which
usually burn with a bluish flame.
• Its vapors are highly toxic which on inhalation
produce loss of consciousness.
• Benzene poisoning in the longer run can prove fatal,
destroying red & white blood corpuscles.
25

26. • Benzene is lighter than water ,specific gravity is
0.87g/cm3.
• It has high melting point (5.50c)
• It has moderate boiling point(80.1)
• For homologous series m.p increases with increasing
molecular mass due to vander waal’s forces.
• All c-c bonds are equal in length of 140 pm.
• Bond angel is 120 degree.
• Each carbon is sp2 hybridized.
• Benzene also shows resonance that is it can exist in
different forms based on the positioning of double
bond and this property of benzene makes it stable.
That is why Benzene does not undergo addition
reactions readily but it undergoes substitution
reactions.
26

27. CHEMICAL
PROPERTIES
OF BENZENE
27

28. REACTIONS OF BENZENE
• Electrophilic Substitution Reactions
• Addition reactions
• Oxidation Reaction
28

29. Electrophilic Substitution reactions
Benzene undergoes electrophilic substitution reaction. The
benzene ring with its delocalized pi electrons is an
electron rich system . It is attacked by electrophiles,
giving substitution products.
These reactions can be represented as :
Where E+ is any electrophile . Such reactions in
which hydrogen atom of aromatic ring is
replaced by an electrophile are called
electrophilic aromatic substitution reactions.
29

30. Halogenation
Benzene react with bromine in the presence of
AlBr3 or FeBr3 at room temperature to form bromo-
benzene . Iron powder can be used in place of
ferric chloride.
Benzene Bromobenzene
30

31. Mechanism of Halogenation
31

32. Nitration
Benzene react with concentrated nitric acid in
The presence of concentrated sulphuric acid at
600C to form nitrobenzene.
Benzene Nitrobenzene
32

33. Mechanism of Nitration
• The Nitro group can be reduced to an Amino group
if needed
33

34. Sulphonation
• Benzene react with conc. sulphuric acid at 1200C
or fuming sulphuric acid at room temperature to
give benzene sulfonic acid .
34

35. Mechanism of Sulphonation
35

36. Friedel-Crafts Alkylation
• Benzene reacts with alkyl halides in the presence of
aluminum chloride ( AlCl3) to form alkyl benzenes.
Benzene Toluene
36

37. Drawbacks Of Friedel-crafts Alkylation
It is useful in the synthesis of certain alkyl benzenes ,
the reaction has two serious drawbacks:
 It is difficult to stop the reaction when one alkyl
group has entered the ring . Di- and tri- alkyl
benzenes are also formed.
 The alkyl group often tends to rearrange .
For Example:
37

38. Friedel-Crafts Acylation
• Benzene reacts with acid chlorides in the presence
of aluminum chloride to give aromatic ketones .
38

39. ADDITION REACTIONS
• Addition of hydrogen:
Benzene reacts with hydrogen in the presence of
nickel (or platinum) catalyst at 1500C under pressure
to form cyclohexane .
39

40. Addition of halogens
• Benzene reacts with chlorine (or Br) in the presence
of ultraviolet light to form benzene hexachloride .
• Benzene hexachloride is a powerful insecticide.
Benzene Benzene hexachloride
40

41. OXIDATION REACTIONS
• Ozonolysis:
Benzene reacts with ozone to give a triozonide which
on treatment with Zn/H2O yields glyoxal.
41

42. Vapour phase oxidation
• Benzene undergoes oxidation with air/oxygen in the
presence of vanadium (V2O5) at 4500C to form maleic
anhydride.
42

43. EFFECT OF SUBSTITUENTS
ON FURTHER SUBSTITUTION
43

44. CONCEPT…
 Some students get confused over which group
does the directing, the incoming electrophile, E+,or
the initial substituent,-X.
 Try thinking about it in terms of an aircraft (the E+)
coming into land at an airport (the Ar-X)…
 It is the control tower at the airport on the ground (-
X) that does the directing of which runway and
which ramp the aircraft should go to.
44

45. SUBSTITUTION IN MONO
SUBSTITUTED BENZENES
 All hydrogen atoms of the benzene ring are
equivalent. Therefore, only one mono-substitution
product (C6H5- S) is possible.
45

46.  A second substituent, E, can occupy any of the
remaining five positions
 The positions 2 and 6 are equivalent, and would
give the ortho product.
 The positions 3 and 5 are equivalent, and would
give the meta product.
 The positions 4 is unique and would give the para
product. 46

47. 47

48. TWO TYPES OF INFLUENCE OF
SUBSTITUENTS
Directive or Orientation Effect.
Activity Effects.
48

49. DIRECTIVE OR ORIENTATION EFFECT.
The first substituent (S) may direct the
next incoming substituent (E) to ortho,
meta, or para position, depending on
the nature of the first substituent.
This is called the Directive or the
Orientation (Orient: to arrange) Effect.
49

50. ACTIVITY EFFECTS.
The substituent already present
may activate or deactivate the
benzene ring toward further
Substitution, these effects are
called the activity effects.
50

51.  Ortho = 2/5 of the total, or 40%
 meta = 2/5 of the total, or 40%
 para = 1/5 of the total, or 20%
51

52. ORTHO-PARA DIRECTING EFFECT.
 Certain substituents direct the second
substituent to the ortho and para positions
simultaneously. These are called Ortho-Para
Directors.
 For example, when phenol is nitrated, the only
products obtained are 0-nitrophenol and p-
nitrophenol.
52

53.  The substituent –OH is said to have directed the –
NO2 group to ortho and para positions on
the ring. Therefore,-OH is designated as
ortho –para Director.
 Some common ortho- para directing groups
are –Cl,-Br, -I ,-OH ,-NH2, -CH3 , -C2H5
53

54. META DIRECTING EFFECT
The substituents which direct the
second incoming substituent primarily
to the meta position are referred to as
meta-directors.
For example, nitration of nitro benzene
gives 94% of m –dinitrobenzene, and
only 5% of ortho and 1% of p-
dinitrobenzene 54

55. 55

56. EFFECT OF SUBSTITUENTS ON REACTIVITY
 A Substituent which activates the aromatic ring to
further substitution , is called an activating
substituent or ring activator.
56

57.  A substituent which deactivates the aromatic ring to
further substitution , is called a deactivating
substituent or ring deactivator.
57

58.  Ortho –para directors activate a ring toward
electrophilic substitution, whereas meta
directing groups deactivate a ring toward
electrophilic substitution.
 Although F , Cl , Br and I are ortho para
directors ,these substituents deactivate an
aromatic ring in electrophilic substituents
58

59. APPLICATIONS OF
AROMATIC COMPOUNDS
Different aromatic compounds
are used in different industries
and for different purposes.
59

60. USES
They are extracted from complex
mixture obtained by the refining of oil or
by distillation of coal tar. they are also
used to produce chemicals, polymers,
including nylon. Phenol and styrene.
60

61. OTHER AROMATIC
COMPOUNDS.
BENZENE: its most widely
produce derivative including
styrene . Which is used to
make polymer and plastics.
61

62. • Small amount of benzene are used
to make some type of rubber ,
lubricant , dyes detergent , drugs
and pesticides .
62

63. TOLUENE
• A common solvents able to dissolve
paints, resins & rubber.
• In the preparation of saccharin &
Tolbutamide (hypoglycemic agent).
• For blending petrol.
• As a starting material for benzyl
derivatives, Benzaldehyde, benzoic
acid.
63

64. Aromatic nitro compounds
• Useful in lab reagents for the identification of
organic compounds e.g.
• 2,4- DNPH used for the identification of carbonyl
compounds.
• 2,4- dinitrofluoro benzene is widely used in protein
chemistry, commonly known as Sangers reagent.
• In the preparation of TNT which is widely used as
explosive.
• In the preparation of Nitrazepam
(used in the management of
seizures & as sedative/hypnotic)
64

65. PHENOL
• Phenol is also used in preparation of
cosmetic including sunscreen and hair
dyes. In cosmetic surgery it serve as
exfoliator.
• In surgical procedure used to treat an
ungrown nail which is applied to nail
bed to prevent regrowth of nail.
65

66. NAPHTHALENE
• Act as raw material in the production
of dyes and insecticides for preventing
moths in clothes.
• Also used in production of some
pharmaceutical products.
66

67. ANTHRACENE
• Anthracene is converted mainly to
anthraquinone, a precursor to dyes &
has antitumor & laxative activity.
67

68. PHENANTHRENE
• An important skeletal nucleus in bile acids,
sex hormones & cardiac glycosides.
• Codeine is used as analgesic & cough
suppressant.
68

69. PYRENE
• Pyrene and its derivatives are used
commercially to make dyes.
• DI-BENZONE (A , L) PYRENE
• Most potent carcinogen.
• XYLENE: A mixture of xylene/
xylol is used as solvent &
diluent for lacquers.
69

70. Benzene sulphonic acid
• In the synthesis of saccharin & azo-
dyes.
• Sulfa drugs such as sulfanilamide.
70

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