8._Aromaticity,_Substitution_Reactions Nadir.pdf

BS-Organic Chemistry-I 15-Jan-20
Dr Ahmad Raza (PhD, QAU Islamabad) 1
Aromaticity
• The term “Aromatic” was initially associated with benzene
• Now linked with cyclic compounds having chemical properties similar to
benzene
• The characteristic behavior exhibited by aromatic compounds is called
“Aromaticity”
• Property of sp2 hybridized planar ring
• p-orbital on each atom allow cyclic delocalization of π-electrons
• Aromatic compounds exhibit high degree of unsaturation and undergo
substitution reactions rather than addition
• Have extra stability (resonance energy)
Dr Ahmad Raza 1
Aromaticity
• The aromatic system possess closed system of π-electrons
 Huckle’s rule
• Given by Huckle in 1931 for monocyclic compounds
• Cyclic system having 4n+2 delocalized π-electrons exhibit aromatic character
Where n = 0, 1, 2, 3, 4, 5……etc.
• In general planar cyclic structures with 2, 6, 10, 14, 18, 22 etc. will be aromatic
n = 0 we get (4 × 0) + 2 = 2 n = 1 we get (4 × 1) + 2 = 6
n = 2 we get (4 × 2) + 2 = 10 n = 3 we get (4 × 3) + 2 = 14
Note: The system can also be an ion
Dr Ahmad Raza 2
Aromaticity
 The necessary conditions for aromaticity
• For a compound to be aromatic, it must fulfill following requirements:
1. Must have cyclic and planar structure
2. Each atom of ring must be sp2 hybridized
3. Must obey Huckle’s rule
 Examples:
H
H
Non-aromatic
H
Aromatic
• Although cyclopropane is cyclic,
planar and have 2 pi electrons, yet is
non-aromatic
• It does not have delocalization of pi
electrons
Cyclopropene Cyclopropenyl cation
Dr Ahmad Raza 3
Aromaticity
 Examples:…………
Pyrrole
N
H
O
Furan
S
Thiophene
N
Pyridine
Benzene
H H
1,3-cyclopentadiene
Non-aromatic
H
1,3-cyclopentadienyl cation
Non-aromatic
H
1,3-cyclopentadienyl anion
aromatic
Dr Ahmad Raza 4

Aromaticity
Cycloheptatriene
Dr Ahmad Raza 5
Aromaticity
Naphthalene
aromatic
Cyclodecapentaene
Non-aromatic Not planar
cyclooctatetraene
Non-aromatic
Dr Ahmad Raza 6
Aromaticity
 Annulenes
• monocyclic compounds containing alternating ring double bonds,
such as benzene, but of different sizes.
• According to systematic nomenclature, cyclobutadiene is
[4]annulene, benzene is a [6]annulene, and cyclooctatetraene is an
[8]annulene
• Digit within brackets indicate no. of pi electrons
• [10]-Annulene was expected to be aromatic but is non-aromatic due to
ring strain e.g. Cyclodecapentaene
• Higher annulnes i.e. 14, 18 etc are aromatic
Dr Ahmad Raza 7
Electrophilic substitution
• The exposed pi electrons of benzene are susceptible to electrophilic (E+)
attack, hence undergoes electrophilic aromatic substitution
• The reactions in which a proton (H+) of aromatic ring is substituted by an
elctrophile (E+) is called as electrophilic aromatic substitution
 Mechanism
• In the first step, an electrophile (E+) attacks the π-electrons of benzene ring
• A carbocation (arenium ion or σ-complex) is generated which is delocalized
over the ring
• This carbocation is non-aromatic (has 4 π-electrons)
1. Step one
Dr Ahmad Raza 8

 Mechanism…….
• E+ may be positive ion or dipole
• Arenium ion is resonance stabilized and hybrid of three structures
• First step is slow and is therefore rate determining step
Dr Ahmad Raza 9
 Mechanism…….
2. Step two
• In the second step, arenium ion looses a proton to base present in mixture
• The proton is lost from the carbon which bears E+
• The released electron pair of C-H bond becomes part of cyclic π-system
and restore the six π-electrons to regain aromatic character
• Since stability is gained by the molecule hence this step is fast
• The driving force of substitution is rearomatization
Dr Ahmad Raza 10
 Energy diagram
Dr Ahmad Raza 11
Electrophilic substitution reactions
1. Nitration of benzene
• Commonly carried out by heating benzene with mixture of conc. HNO3 and
conc. H2SO4
• The nitronium ion (NO2
+) is generated as electrophile (observed
spectroscopically)
Dr Ahmad Raza 12

Nitration of benzene………
 Mechanism
Dr Ahmad Raza 13
2. Sulphonation of benzene
• By heating benzene with conc. H2SO4 at 80 °C
Or
Fuming sulphuric acid (H2SO4 +SO3) at 25 °C
• Although SO3 is neutral but still act as E+ since S is electron deficient
Dr Ahmad Raza 14
Sulphonation of benzene……….
 Mechanism
1 = Generation of E+
2 = Attack of E+
3 = Restoration of aromaticity
by abstraction of proton form
sp3 carbon by HSO4
-1
4 = Proton transfer from
H2SO4 to benzene sulphonate
ion Dr Ahmad Raza 15
3. Halogenation of benzene
• The substitution of halogen atom (Cl, Br or I) in place of hydrogen is called
halogenation
• Benzene reacts with halogens in the presence of Lewis acid (FeCl3 or FeBr3)
• Fe reacts wit halogen to from ferric halide
2Fe + 3X2 2FeX3
25°C Heat
Dr Ahmad Raza 16

Halogenation of benzene…………..
 Mechanism
• Aromatic halogenation reactions are carried out in the absence of light since
side chain substitution is catalyzed by light
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Halogenation of benzene…………..
• However in the absence of light toluene reacts with Cl2 or Br2 in the
presence of AlCl3 or FeCl3 to give mixture of o- and p- derivatives
Dr Ahmad Raza 18
• Developed by Charles Friedel and James Crafts in 1877
• Involves the introduction of alkyl or acyl group into benzene ring in the
presence of anhydrous AlCl3 or any other Lewis acid i.e. BF3, FeCl3 etc.
Friedal-Crafts reactions
4. Friedal-Crafts alkylation
• Carried out by treating benzene with alkylating reagent like alkyl halide (R-
X), alcohols (R-OH), or alkenes
• Used for introducing alkyl group in the benzene ring
• Reaction is carried out in inert solvent like nitrobenzene or carbon disluphide
Dr Ahmad Raza 19
Friedal-Crafts alkylation……
 Examples
CH3Cl
AlCl3
5 o
C
CH3
HCl
(CH3)2CHOH
BF3
-10 o
C
CH
H2O
H3C
CH3
CH3-CH=CH2
AlCl3
HCl
CH
H3C
CH3
Dr Ahmad Raza 20

 Mechanism
Dr Ahmad Raza 21
 Lmitations
a) Polyalkylation
• It is difficult to stop the reaction when one alkyl group has entered the ring
• Often poly-alkylated benzenes are produced
• For example: Benzene with excess of CH3Cl gives1,2,4,5- tetramethyl
benzene
C H 3 C l
A lC l3
C H 3
H 3 C
H 3 C
C H 3
Dr Ahmad Raza 22
 Lmitations
b) Products due to rearrangement of carbonium ion electrophile (E+)
• The generated carbocation of alkyl halide may undergo rearrangement
before attacking the benzene ring
• This rearrangement is in the order primary secondary tertiary
attributed to their relative stability
Dr Ahmad Raza 23
• This rearrangement usually takes place by transfer of H- or R-
• Therefore reaction of benzene with 1-chloropropane (propyl chloride) in
the presence of AlCl3 produces isopropylbenzene (cumene) as major
product rather than expected n-propylbenzene
 Lmitations
CH3-CH2-CH2-Cl
AlCl3
CH
CH3
CH3
H2
C
H2
C CH3
Major product
Minor product
Dr Ahmad Raza 24

5. Friedal-Crafts acylation
• Carried out by treating benzene with acylating reagent like acyl halide
(CH3COX), or acid anhydride, or ketenes
• Used for introducing acyl group (RCO-) in the benzene ring
• Reaction is carried out in inert solvent like nitrobenzene or carbon
disluphide
• Aromatic ketone is generated
Dr Ahmad Raza 25
Friedal-Crafts acylation……..
 Examples
AlCl3
C CH3
O
HCl
H3C C Cl
O
AlCl3
C CH3
O
CH3COOH
H3C C
O
H3C C
O
O
AlCl3
C CH3
O
H2C C O
Ketene
Acetophenone
Dr Ahmad Raza 26
 Mechanism
Dr Ahmad Raza 27
• Unlike alkyl group in Friedal craft alkylation, acyl does not undergo
rearrangement, not it allows the introduction of another acyl group in
the benzene ring
 Advantage of Friedal crafts acylation
Dr Ahmad Raza 28
• Friedal craft acylation can be applied to prepare straight chain alkyl
benzenes
• Acylation of benzene gives ketone which is then reduced to n-
alkylbenzenes by Clemmensen reduction or Wolf Kishner reduction

 Advantage of Friedal crafts acylation
Dr Ahmad Raza 29
5. Formylation
• Substitution of formyl group (aldehyde) -CHO- into aromatic ring
• Similar to acylation
• Can not be carried out with formyl chloride or formic anhydride
• Formyl chloride is stable for short time in CHCl3 at -40 °C
Dr Ahmad Raza 30
• Formic anhydride decomposes at room temperature
• May be accomplished by:
1. Gatterman-Koch reaction
2. Gatterman reaction
3. Formylation with formyl fluoride
1. Gatterman-Koch reaction
 Mechanism
• Similar to Friedel-Crafts acylation
• CO and HCl react to form formyl chloride which then reacts with benzene
to form benzaldehyde
• Formyl chloride can be generated by dropping chlorosulphonic acid on
formic acid
Dr Ahmad Raza 31
2. Gatterman reaction
• Gatterman used HCN in place of CO (poisonous) as source of formyl group
• Direct use of HCN can be avoided by using Zn(CN)2
Dr Ahmad Raza 32

3. With formyl fluoride
• Benzene is treated with formyl fluoride (HOCF) in the presence of BF3 ether
(solvent)
• Formyl fluoride is stable as compared to formyl chloride
Dr Ahmad Raza 33

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