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W07 Aromatic

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WEEK 07 –
AROMATIC COMPOUNDS
(BENZENE AND TOLUENE)
1
Aromatic compounds
• Organic compound that contains a benzene
ring in its molecule is known as an aromatic
compounds.
• Sometimes called arenes.
• Molecular formula: C6H6
• Represented as a regular hexagon containing
an inscribed circle.
2
Structure of Benzene
• Can be represented in two abbreviated ways.
• The corner of each hexagon represents a carbon and a
hydrogen atom.
3
Kekulé Structure of Benzene
Molecular formula is C6H6
All the hydrogen atoms are equivalent
Each carbon atom must have four covalent bonds.
4
The Structure of Benzene
Any structure for benzene must account for the following facts:
1. It contains a six-membered ring and three additional degrees of
unsaturation.
2. It is planar.
3. All C—C bond lengths are equal.
The Kekulé structures satisfy the first two criteria but not the third,
because having three alternating π bonds means that benzene
should have three short double bonds alternating with three longer
single bonds.
5
The Structure of Benzene
• In benzene, the actual bond length (1.39 Å) is intermediate between the carbon—
carbon single bond (1.53 Å) and the carbon—carbon double bond (1.34 Å).
6
Delocalization and Resonance Structures Rules
In resonance structures, the electrons are able to
move to help stabilize the molecule.
This movement of the electrons is called
delocalization.
Resonance structures should have the same number
of electrons, do not add or subtract any electrons.
(check the number of electrons by simply counting
them).
7
Delocalization and Resonance Structures Rules
All resonance structures must follow the rules of
writing Lewis Structures.
The hybridization of the structure must stay the
same.
The skeleton of the structure can not be changed
(only the electrons move).
Resonance structures must also have the same
amount of lone pairs.
8
Resonance Structure
• Resonance theory: the structure of benzene is a resonance
hybrid structure of two Kekulé cononical forms.
• The hybrid structure is often represented by a hexagon
containing an inscribed circle.
represents a resonance hybrid between the two
9
• Hexagonal ring – 6 carbon-carbon bonds are
equal.
• Circle – delocalised electrons of the benzene
ring
10
CRITERIA OF AROMATIC COMPOUNDS
• Structure must be cyclic, containing some number of conjugated pi
(π) bonds.
• Each atom in the ring must have an unhybridized p orbital. (The
ring atoms are usually sp2 hybridized or occasionally sp
hybridized).
• The unhybridized p orbitals must overlap to form a continuous ring
of parallel orbitals. The structure must be planar (or nearly planar)
for effective overlap to occur.
• Delocalization of the pi electrons over the ring must lower the
electronic energy.
* Antiaromatic compound: fulfills the first three criteria, but
delocalization of the pi electrons over the ring increase the
electronic energy.
11
12
Huckel’s rule
• Used to determine aromaticity for planar, cyclic organic
compounds with a continous ring of overlapping porbitals.
• If the number of pi (π) electrons in the monocyclic
system is (4N+2), the system is aromatic. N is 0, 1, 2,
3…..
• Systems that have 2, 6 and 10 pi electrons for N = 0, 1, 2
is a aromatic.
• Systems that have 4, 8, and 12 pi electrons for N = 1, 2,
3 are antiaromatic.
13
Naming Aromatic
Compounds
14
• A substituted benzene is derived by replacing one
or more of benzene’s hydrogen atoms with an
atom or group of atoms.
• A monosubstituted benzene has the formula
C6H5G where G is the group that replaces a
hydrogen atom.
• All hydrogens in benzene are equivalent.
• It does not matter which hydrogen is replaced by
G.
15
Monosubstituted
Benzenes
16
Nomenclature of Benzene Derivatives
• To name a benzene ring with one substituent, name the substituent and add the
word benzene.
• Many monosubstituted benzenes have common names which you must also
learn.
17
• Some monosubstituted benzenes are
named by adding the name of the
substituent group as a prefix to the word
benzene.
• The name is written as one word.
nitro group
nitrobenzene
ethyl group
ethylbenzene
18
• Certain monosubstituted benzenes have special
names.
• These are parent names for further substituted
compounds.
hydroxy
group
methyl group
toluene
phenol
19
carboxyl group
amino group
benzoic acid
aniline
20
Disubstituted Benzenes
21
•
Three isomers are possible when two substituents
replace hydrogen in a benzene molecule.
• The prefixes ortho-, meta- and para- (o-, m- and p-)
are used to name these disubstituted benzenes.
22
Nomenclature of Benzene Derivatives
• There are three different ways that two groups can be attached to a benzene ring,
so a prefix—ortho, meta, or para—can be used to designate the relative position
of the two substituents.
ortho-dibromobenzene
or
o-dibromobenzene
or 1,2-dibromobenzene
meta-dibromobenzene
or
m-dibromobenzene
or 1,3-dibromobenzene
para-dibromobenzene
or
p-dibromobenzene
23
or 1,4-dibromobenzene
ortho disubstituted benzene
substituents on adjacent carbons
ortho-dichlorobenzene
(1,2-dichlorobenzene)
mp –17.2oC, bp 180.4oC
24
meta disubstituted benzene
substituents on adjacent carbons
meta-dichlorobenzene
(1,3-dichlorobenzene)
mp –24.82oC, bp 172oC
25
para disubstituted benzene
substituents are on opposite sides
of the benzene ring
para-dichlorobenzene
(1,4-dichlorobenzene)
mp 53.1, bp 174.4oC
26
When one substituent corresponds to a monosubstituted
benzene with a special name, the monosubstituted
compound becomes the parent name for the
disubstituted compound.
phenol
3-nitrophenol
27
When one substituent corresponds to a
monosubstituted benzene with a special name, the
monosubstituted compound becomes the parent
name for the disubstituted compound.
toluene
3-nitrotoluene
28
Tri- and Polysubstituted
Benzenes
29
• When a benzene ring has three or more
substituents, the carbon atoms in the ring are
numbered.
• Numbering starts at one of the substituent groups.
• The numbering direction can be clockwise or
counterclockwise.
• Numbering must be in the direction that gives the
substituent groups the lowest numbers.
30
6-chloro
clockwise
numbering
1-chloro
6
4-chloro
5
1
4
2
3
1,4,6-trichlorobenzene
31
counterclockwise
numbering
chlorine
substituents
have lower
numbers
4-chloro
2-chloro
1-chloro
2
3
1
4
6
5
1,2,4-trichlorobenzene
32
• When a compound is named as a derivative of
the special parent compound, the substituent of
the parent compound is considered to be C-1 of
the ring.
33
6
1
6
2
5
3
5
1
4
2
3
4
toluene
2,4,6trinitrotoluene
(TNT)
34
• When the hydrocarbon chain attached to the
benzene ring is small, the compound is named as
benzene derivative.
• Example:
CH2CH3
ethylbenzene
35
Naming compounds that cannot be easily
named as benzene derivatives

Benzene named as a substituent on a molecule with another
functional group as its root by the prefix phenyl.
diphenylmethane
4-phenyl-2-pentene
36
The phenyl group, C6H5CH2
phenyl
CH=CH2
common
name
phenylethene
benzyl
NH2
CH2Cl
phenylamine
benzyl chloride
37
• If the hydrocarbon chain contains more than three
carbon atoms, phenyl is used as part of the name.
• Examples:
CH3
CH2(CH2)5CH3
1-phenylheptane
C CH2 CH3
Br
2-bromo-2-phenylbutane
38
PHYSICAL PROPERTIES OF BENZENE AND ITS
DERIVATIVES
• Benzene derivatives tend to be more symmetrical than
similar aliphatic compounds, and pack better into
crystals and have higher melting points.
• Density:
- Slightly dense than non-aromatic analogues, but still
less dense than water.
- halogenated benzenes are denser than water.
• Insoluble in water
• Boiling points depends on the dipole moments of
compounds.
39
REACTION OF BENZENE
ELECTROPHILIC SUBSTITUTION REACTIONS OF
BENZENE
 stability of π-electron system is lost when benzene
undergoes addition reactions.
 benzene and its derivatives undergo substitution
reaction rather than addition reactions.
 product of substitution reactions: aromatic
compounds and not saturated compounds.
40
Mechanism of electrophilic substitution
of benzene
Step 1: Electrophilic addition of the benzene ring
H
+
E
E
slow
arenium ion (a carbocation)
Step 2: Deprotonation of the arenium ion
H
E
E
Nu
-
fast
H Nu
nucleophile
41
ELECTROPHILIC SUBSTITUTION REACTIONS
a) Halogenation
X
H
X2
H2SO4
or FeX3
HX
halobenzene
b) Nitration
NO2
H
HNO3
H2SO4
2H2O
nitrobenzene
c) Sulphonation
SO3H
H
SO3
H2SO4
benzenesulphonic acid
42
ELECTROPHILIC SUBSTITUTION REACTIONS
d) Friedel-Crafts alkylation
H
CH3
AlCl3
HCl
CH3Cl
toluene
e) Friedel-Crafts acylation
H
O
AlCl3
O
C CH3
HCl
CH3CCl
acetophenone
43
Reagents, electrophiles and catalysts in
electrophilic substitution reactions
Reactions
Reagents
Catalysts
Electrophiles
Halogenation
Cl2 or Br2
AlCl3, AlBr3,
FeCl3 or FeBr3
Cl , Br
Nitration
HNO3
H2SO4
NO2
Alkylation
RCl
AlCl3
R
RCH=CH2
H2SO4
RCH-CH3
RCOCl
AlCl3
Acylation
RCO
Sulphonation
SO3
H2SO4
SO3H
44
HALOGENATION OF BENZENE
a)Chlorination
Cl
Cl2
AlCl3
HCl
chlorobenzene
b)Bromination
Br
Br2
FeBr3
HBr
bromobenzene
c) Iodination
I
1/2I2
NO2
HNO3
iodobenzene
H2O
45
MECHANISM: BROMINATION OF BENZENE
Step 1: Formation of a stronger electrophile
Br Br
Br Br FeBr3
FeBr3
Br2.FeBr3 intermediate
(a stronger electrophile than Br2)
Step 2: Electrophilic attack and formation of the sigma complex
H
H
H
H
H
Br
H
Br
Br
H
H
H
H
H
Br
H
H
H
H
H
Br
H
FeBr3
H
H
H
H
H
H
H
sigma complex
FeBr4Step 3: Loss of a proton gives the products
H
H
H
Br
H
H
H
FeBr4
-
H
H
Br
HBr
H
FeBr3
H
H
46
MECHANISM: NITRATION OF BENZENE
Step 1: Formation of the nitronium ion, NO2+
HO SO3 H
H2O + NO2+ + HSO4-
HO NO2
Step 2: Formation of an arenium ion as a result of electrophilic addition
H NO2
NO2+
slow
nironium ion
arenium ion
Step 3: Loss of a proton gives the products
H NO2
HSO4-
NO2
fast
H2SO4
47
MECHANISM: FRIEDEL-CRAFTS ALKYLATION
Step 1: Formation of electrophile
CH3
H
C Cl
CH3
AlCl3
CH3
H
C
CH3
-
AlCl4
carbocation (electrophile)
Step 2: Formation of an arenium ion
H
C CH3
CH3
H CH(CH3)2
arenium ion
Step 3: Loss of a proton
H CH(CH3)2
AlCl4-
CH(CH3)2
HCl + AlCl3 48
MECHANISM: FRIEDEL-CRAFTS ACYLATION
Step 1: Formation of electrophile
O
O
CH3 C Cl
AlCl4-
CH3 C
AlCl3
Step 2: Formation of an arenium ion
O
H C
O
CH3
CH3 C
Step 3: Loss of a proton
O
H C
AlCl4-
O
CH3
C
CH3
HCl + AlCl3
49
Ortho-Para and Meta Directing
Substituents
• When substituted benzenes undergo further
substituents, the substituent group present in the
benzene derivative will influence electrophilic
substitution in 2 ways which are:
i) Reactivity
ii)Orientation
50
EFFECTS OF SUBSTITUENTS ON THE
REACTIVITY OF ELECTROPHILIC
AROMATIC SUBSTITUTION
• Substituent group present in the benzene ring can
influence the rate of reaction of further substitutions.
• Electron-donating groups make the ring more reactive
(called activating groups) thus influence the reaction
become faster.
• Electron-withdrawing groups make the ring less reactive
(called deactivating groups) thus influence the reaction
become slower.
51
EFFECTS OF SUBSTITUENTS ON THE
ORIENTATION OF ELECTROPHILIC
AROMATIC SUBSTITUTION
• A substituents group already in the ring influences the
position of further electrophilic substitution whether at
ortho, meta or para position.
• Ortho-para directors: the groups that tend to direct
electrophilic substitution to the C2 and C4 positions.
• Meta directors: the groups that tend to direct
electrophilic substitution to the C3 position.
52
Effetcs of substituent groups on the benzene ring
Activating groups
(electron donating)
-NH2
-OH
-OR
-NHCOCH3
-R
Deactivating groups
(electron-withdrawing)
-F
-Cl
-Br
-I
O
C
O
C
R
OH
ortho-para directors
ortho-para
directors
C
N
NO2
O
C
SO3H
OR
NR3
meta directors
53
Example:
CH2CH3
Br2
CH2CH3
Br
CH2CH3
CH2CH3
Br
FeBr3
Br
-CH2CH3 = ortho and para directors
ortho position
para position
major products
meta position
minor product
54
Example:
NO2
NO2
NO2
NO2
Br
Br2
FeBr3
-NO2 = meta director
Br
meta position
Br
ortho position
para position
major product
minor products
55
REACTIONS OF BENZENE
DERIVATIVES
• Alkylbenzene such as toluene (methylbenzene)
resembles benzene in many of its chemical
properties.
• It is preferable to use toluene because it is less
toxic.
• The methyl group activates the benzene nucleus.
• Toluene reacts faster than benzene in all
electrophilic substitutions.
56
Reactions
of toluene
Reactions of the
methyl group
Substitution
-halogenation
Oxidation
Reactions of the
benzene ring
Electrophilic
substitutions
- Halogenation
- Nitration
- Friedel-Crafts reactions
- Sulfonation
Addition reaction
-hydrogenation
57
58
SIDE-CHAIN REACTIONS
59
OXIDATION REACTION OF ALKYLBENZENE
O
+
CH2 R hot, conc., KMnO4/H
C OH
reflux
examples:
CH3
O
+
hot, conc., KMnO4/H
C OH
reflux
+
CH2 CH3 hot, conc., KMnO4/H
reflux
CH3
CH3
hot, conc., KMnO4/H+
reflux
O
C OH
COOH
COOH
60
HALOGENATION OF TOLUENE
Side chain substitution
CH3
CH2 Cl
Cl2
uv light
HCl
(chloromethyl)benzene
CHCl2
CH2 Cl
Cl2
uv light
HCl
(dichloromethyl)benzene
CCl3
CHCl2
Cl2
uv light
HCl
(trichloromethyl)benzene
* Bromination of toluene takes place under similar conditions to yield
corresponding bromine derivatives.
61
SYNTHESIZING A SUBSTITUTED AROMATIC
COMPOUNDS
Synthesis m-chloronitrobenzene starting from benzene
NO2
?
Cl
• Two substituents: -NO2 (meta-directing) and –Cl (orthoand para-directing)
• Cannot nitrate chlorobenzene because the wrong isomer
(o- and p-chloronitrobenzenes) would formed.
62
Cl
chlorobenzene
HNO3, H2SO4
NO2
NO2
Cl
Cl2, FeCl3
m-chloronitrobenzene
nitrobenzene
TWO STEPS:
benzene
NO2
NO2
HNO3
Cl2
H2SO4
FeCl3
nitrobenzene
Cl
m-chloronitrobenzene
63
SYNTHESIZING A SUBSTITUTED AROMATIC
COMPOUNDS
Synthesis p-bromobenzoic acid starting from benzene
COOH
?
Br
• Two substituents: -COOH (meta-directing) and –Br (ortho- and paradirecting)
• Cannot brominated benzioc acid because the wrong isomer
(m-bromobenzoic acid) would formed.
• Oxidation of alkylbenzene side chains yields benzoic acids.
• Intermediate precursor is p-bromotoluene
CH3
COOH
KMnO4
Br
Br
64
Immediate precursor of p-bromotoluene:
i) Bromination of toluene
or
ii) Methylation of bromobenzene
CH3
CH3
Br2
FeCl3
Br
or
Br
separate the isomer
CH3
CH3Cl
Br
CH3
AlCl3
Br
CH3
Br
separate the
isomer
65
Immediate precursor of toluene:
i) Benzene was methylated in a Friedel-Crafts reaction
CH3
CH3Cl
AlCl3
toluene
benzene
Immediate precursor of bromobenzene:
i) Bromination of benzene
Br2
FeBr3
benzene
Br
bromobenzene
66
TWO WORKABLE ROUTES FROM BENZENE TO
p-BROMOBENZOIC ACID
Br2
FeBr3
CH3Cl
Br
AlCl3
CH3
benzene
CH3Cl
AlCl3
CH3
Br2
Br
COOH
KMnO4
Br
FeBr3
67
USES OF BENZENE AND TOLUENE
• Benzene:
- as solvent for oils and fats
- starting material for making other chemicals. For
example, benzene is used in the cumene process to
produce phenol.
- making organic compounds such as phenylethene
(styrene) and nitrobenzene. These organic compounds
are then used to make plastics (polystyrene), dyes and
nylon.
68
USES OF BENZENE AND TOLUENE
• Toluene:
- A common solvent, able to dissolve paints, paint thinners,
silicone sealants, many chemical reactants, rubber, printing ink,
adhesives (glues), lacquers, leather tanners and disinfectants.
- As a solvent to create a solution of carbon nanotubes.
- Dealkylation to benzene (industrial uses).
- As an octane booster in gasoline fuels used in internal
combustion engines.
-As a coolant in nuclear reactor system loops.
69
Interesting Aromatic Compounds
• Benzene and toluene, the simplest aromatic hydrocarbons obtained
from petroleum refining, are useful starting materials for synthetic
polymers. They are also two of the components of the BTX mixture
added to gasoline to boost octane ratings.
• Compounds containing two or more benzene rings that share
carbon—carbon bonds are called polycyclic aromatic hydrocarbons
(PAHs). Naphthalene, the simplest PAH, is the active ingredient in
mothballs.
70
Figure 17.5
Selected drugs that
contain
a benzene ring
71
Stability of Benzene
• Consider the heats of hydrogenation of cyclohexene, 1,3cyclohexadiene and benzene, all of which give cyclohexane when
treated with excess hydrogen in the presence of a metal catalyst.
72
Stability of Benzene
• The low heat of hydrogenation of benzene means that benzene is
especially stable—even more so than conjugated polyenes. This
unusual stability is characteristic of aromatic compounds.
• Benzene’s unusual behavior is not limited to hydrogenation. Benzene
does not undergo addition reactions typical of other highly
unsaturated compounds, including conjugated dienes.
• Benzene does not react with Br2 to yield an addition product. Instead,
in the presence of a Lewis acid, bromine substitutes for a hydrogen
atom, yielding a product that retains the benzene ring.
73
Buckminsterfullerene—Is it Aromatic?
• Buckminsterfullerene (C60) is a third elemental form of carbon.
• Buckminsterfullerene is completely conjugated, but it is not aromatic
since it is not planar.
• It undergoes addition reactions with electrophiles in much the same
way as ordinary alkenes.
74
75
76
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