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AROMATIC COMPOUNDS
By khutjo

AROMATIC COMPOUNDS
ORIGIN OF NAME AROMATIC
• Very early in organic chemistry (late 18th, early 19th century)
chemists discovered a set of compounds which were unusually
stable
...
Hence they

were called 'aromatic' compounds
• Today the term aromatic is used regardless of the odor of the

compound
...

H
H

H

The three pi bonds in benzene are

conjugated
...


AROMATIC COMPOUNDS
ORIGIN OF NAME AROMATIC
• Many valuable compounds are aromatic in part
...


• Benzene is by then the typical aromatic compound
...


• Benzene is planar and has the shape of a regular hexagon
...


• Carbon-carbon bond lengths are ~1
...


AROMATIC COMPOUNDS

STABILITY OF BENZENE

• The stability of benzene is unusual compare to others
...

• Two pi bonds separated by a sigma bond is said a conjugated
system
...

• When pi electrons are delocalized, we form the resonance
form of the compound
...


AROMATIC COMPOUNDS

STABILITY OF BENZENE

-359 KJ/mol
151KJ
resonance energy

Energy
-240 KJ/mol
-120 Kj/mol

Most stable

-208 KJ/mol
-232 KJ/mol

AROMATIC COMPOUNDS
STABILITY OF BENZENE
H2
2H2

Catalyst

120 KJ/mol

Catalyst

240 KJ/mol

2H2

Catalyst

232 KJ/mol

3H2

Catalyst

208 KJ/mol

• Hydrogenation of alkenes, can be used to determine the
stability due to the presence of conjugated system in the
alkene
...

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

Resonance represention

H
resonance hybrid

H
Combined representation

AROMATIC COMPOUNDS
 AROMITICITY
• Hückel’s Rule:

- A molecule is aromatic only if it has a

planar,

monocyclic system of conjugation and contains a
total of 4n+ 2 π electrons, where n is an integer (n =
0, 1, 2, 3,…)
...

Aromatic

nonaromatics

AROMATIC COMPOUNDS
AROMITICITY
• Hückel’s Rule:

-Molecules with 4n pi electrons (4, 8, 12, 16,…) can not
be aromatic, said to be antiaromatic because

delocalization of their p electrons would lead to their
destabilization
...

• Some of them can still be using the systematic names
...

ɸ

Ph
Phenyl group

• Aromatic hydrocarbons are called sometimes called Arenes
...


Ar

The simplest aryl group is phenyl group
...


CH2
Benzyl group

• Disubstituted benzenes are named using the prefixes ortho-,
X
X
para-, and metaX
Y
1,2 or ortho

Y

1,3 or meta

Y

1,4 or para

AROMATIC COMPOUNDS
NOMENCLATURE OF AROMITIC
COMPOUNDS

khutjo
NOMENCLATURE OF AROMITIC
COMPOUNDS
• Benzenes with more than two substituents are named as we did for
hydrocarbons
...


AROMATIC COMPOUNDS
NOMENCLATURE OF AROMITIC
COMPOUNDS
COOH
HO

CH

OH
Cl

CH2
NO2

AROMATIC COMPOUNDS
 REACTIONS OF AROMATIC COMPOUNDS
• Aromatic compounds might react from the pi bonds c=c as for
alkenes
...


AROMATIC COMPOUNDS
 REACTIONS OF AROMATIC COMPOUNDS
 Halogenation
-Bromination: the reaction is done in presence of a strong
Lewis acid such as FeBr3

Br2

FeBr3
Catalyst

Br

FeBr3 + HBr

Mechanism:

Step 1: Formation of a strong electrophile
Br

Br

FeBr3

Br Br FeBr3

AROMATIC COMPOUNDS
 REACTIONS OF AROMATIC COMPOUNDS
 Halogenation
-Bromination

Step 2: Electrophilic attack
Br

Br

Br

Br Br FeBr3
Sigma complex Resonance forms

Step 3: Lost of electron
Br
H

Br
FeBr4-

HBr

FeBr3

FeBr4

AROMATIC COMPOUNDS
 REACTIONS OF AROMATIC COMPOUNDS
 Halogenation
-Bromination

Step 2: Electrophilic attack
Br

Br

Br

Br Br FeBr3
Sigma complex Resonance forms

Step 3: Lost of electron
Br
H

Br
FeBr4-

HBr

FeBr3

FeBr4

AROMATIC COMPOUNDS
 REACTIONS OF AROMATIC COMPOUNDS
 Halogenation
-Bromination
NB
...

because if the addition reaction occurs, the resonance stabilization
of the aromatic ring would be lost and the overall reaction would be
energetically unfavorable
...
H

H

H

H

Br2
H

X

H

Br

H

Br

H

Addition does not occurs

AROMATIC COMPOUNDS
 REACTIONS OF AROMATIC COMPOUNDS
 Halogenation
-Chlorination
• Chlorination of benzene works much like bromination, in addition of
FeBr3, AlCl3 can be used also as Lewis acid catalyst
...
There are three products that might form on reaction of toluene
(methylbenzene) with Br2
...


AROMATIC COMPOUNDS
 REACTIONS OF AROMATIC COMPOUNDS
 Nitration of Benzene
• Aromatic compounds can be nitrated by reaction with a mixture of
concentrated nitric acid and sulfuric acid
...


• The product of the reaction is nitrobenzene
...


NO2

Zn, Fe, or Sn
HCl aq
...

• Mechanism
Step 1: Formation of electrophile

O
S
O

O
O

O

S
O

O
S

S

O

O

O

O

O

AROMATIC COMPOUNDS
 REACTIONS OF AROMATIC COMPOUNDS
 Sulfonation of benzene
O
S
O

H
O

• Step 2:
H
H
H

H

H O
H

O
S

O
O S O
O

H
O

H

H

HH

H

O
S

H

O

O

SO3H

H

H

H

HH

H

HSO4-

H

SO3H H
H

• Step 3: Lost of a proton from sigma complex
...
The reaction
is catalysed by a Lewis acid such as AlCl3, AlBr3, FeCl3

H

R

R

Lewis Acid
X AlCl3, AlBr3, FeCl3

HX

• With X = Cl, Br, and I

• There is a formation of an intermediate carbocation
...


AROMATIC COMPOUNDS
 REACTIONS OF AROMATIC COMPOUNDS
 Friedel-Crafts Alkylation
Mechanism:
Step 1: Formation of carbocation and stabilization of it
...
The shift of an hydrogen can form
a tertiary carbocation which is more stable than the primary
...

• For this case, the rearrangement might involve the shift of hydrogen
atom or a methyl group in order to stabilize the carbocation
...

CH3
CH3
Cl
H
C CH3
C CH3
Cl Al Cl
CH3
CH3
Cl

HCl + AlCl3

AROMATIC COMPOUNDS
 REACTIONS OF AROMATIC COMPOUNDS
 Friedel-Crafts Alkylation
 Limitation of Friedel-Crafts Alkylation
- Only alkyl halides R-X can be used but not Ar-x
...
Not all
alkyl benzene can be synthetized from this reaction
...


• At second step of the reaction, the acyl group is introduced into the

ring to form the ketone (acyl benzene)
...

CH3

O
C Cl

Cl
Al Cl
Cl

CH3

Cl
O
C Cl Al Cl
Cl

Step 2: Electrophilic attack forms sigma complex:
O
O
O
C
C CH
CH3
3
C

CH3

Sgma Complex

CH3

O
C

Acylium ion

CH3 C O

O
C

CH3

AlCl4

AROMATIC COMPOUNDS
 REACTIONS OF AROMATIC COMPOUNDS
 Friedel-Crafts Acylation reaction
Mechanism:
Step 3: The lost of proton regenerates the aromatic system
...
The reaction is called
Clemmensen Reduction

O
C

H
CH3

Acetylbenzene

Zn or (Hg)
aq
...


HNO3 / H2SO4

H

H
C
NH2

Sn
aq
...
Ph C CH2CH(CH3)2 b
...
1-phenyl-2,2-dimethylpropane
d
...
3-methyl-1-phenylbutane

AROMATIC COMPOUNDS
 REACTIONS OF AROMATIC COMPOUNDS
 Substituent effects in electrophilic Aromatic substitution
• When the electrophilic substitution is taking place to a benzene, only
one substituent can be formed at the time
...

- some substituents can activate the ring by making it more active
than benzene itself
...


 The substituent might affects the orientation of a reaction
...


AROMATIC COMPOUNDS
 REACTIONS OF AROMATIC COMPOUNDS
 Substituent effects in electrophilic Aromatic substitution
EX
...


• The common characteristic about all deactivating groups is that they
withdraw electrons from the ring, thereby making the ring more

electron – poor, destabilizing the carbocation intermediate, and raising
the activation energy for it formation
...
For example,
the nitration of phenol can occurs in 3 orientations (ortho, para, and
meta)
...
Only the meta
and para intermediates are more stable because of the number of

resonance forms (they have 4 resonance forms each) but the ortho
orientation have only 3 resonance forms
...


Ex
...
In both, ortho and para

intermediates, the third resonance form is particularly unfavorable
because it is placed directly on the carbon that bears the aldehyde

group
...
Hence the meta
position is more favored
...


Ex
...

• Only, if the benzene ring have an alkyl group as a substituent, the
alkyl will be converted to carboxylic acid to form a benzoic acid
...


AROMATIC COMPOUNDS
 REACTIONS OF AROMATIC COMPOUNDS
Exercises
1
...
m – chloronitrobenzene
b
...
Propose the syntheses of the following by starting from benzene
...
Carbocations generated by reaction of an alkene with a strong acid
catalyst can react with aromatic rings in a Friedel-Crafts reaction
...
Identify the reagents represented by the letters a through d in the
following scheme
...
Ribavirin, an antiviral agent used against hepatitis C and viral
pneumonia, contains a 1,2,4 – triazole ring
...




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