Unit-2-AROMATICITY & AROMATIC SUBSTITUTION
Aromaticity – definition – Huckel’s rule – consequence of aromaticity – stability, carbon-carbon
bond lengths of benzene, resonance energy and participation of substitution vs addition –
examples. Non-benzenoid aromatic compounds Aromatic electrophilic subs titution – general
pattern of the mechanism , role of σ and π complexes, Mechanism of nitration, halogenation,
sulphonation and Friedel-Crafts reaction. Activating and deactivating substituents, orientation
in mono substituted benzenes, ortho/para ratioOrientation- Korner’s absolute method, dipole
mom ent method – direct influence of substituents – rules of orientation - Aromatic Nucleophilic
substitutions- unimolecular, bimolecular and benzyne mechanisms.
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Aromatic Compound
Definition - Huckel's Rule
Consequence 1.Stability
2.C=C Bond Length
3.Resonance Energy
4. Addition Vs Substituion
Aromatic Eelctrophylic Substituion (AES)
1. General Mechanism
2. Role of Sigma and Pi Complexes
3. Mechanism
Nitration
Sulphonation
Halogination
Friedel-Craft Rean.-Alkylation,Acylation
4.ReactivityActivating & Deactivating Group
5.Orientation of Di-Substitution-o/m/p-Or ientation
-Determination
Korners Absolute
Dipole moment Methods
Influence of Sustituents, Rules of Orientation
Aromatic Nucleophilic Substitutions (ArNS)
Mechanism
Unimolecular
Bimolecular
Benzyne Mechanism
Aromaticity Benzanoid
Non-Benzanoid
1. Aromaticity: This is the extra stability of cyclic
conjugated alkenes like benzene due to the
delocalization of electrons present in the π-π orbitals.
These compounds are called aromatic compounds .
(Eg) Benzene
2. Huckel’s Rule: Compounds obey 4n+2
rule of π - electrons are called aromatic
compounds. Here 'n' is a whole number. (Eg)
Benzene has 6 π - electrons. 4n+2 =
6 and n=1. Hence benzene is aromatic.
3. Consequence of Aromaticity: Some of the consequence of aromaticity are; stability,
carbon-carbon bond lengths of benzene, resonance energy and
participation of substitution vs addition.
i. Stability of Aromatic Compounds: Aromatic
Compounds are highly stable than the corresponding
aliphatic com pounds. (eg) Estimated heat of formation of
1,3,5-cyclohexatriene is -85.8kcal/mole. But the
heat of formation of benzene is -
49.8kcal/mole. This higher stability of -36kcal/mole is
called aromaticity or resonance energy
ii. Resonance Energy: It is the difference between the
energy of a conjugated molecule and a Kekuléé localized
structure. (eg) Es timated heat of form ation of 1,3,5-
cyclohexatriene is -85.8kcal/mole. But the heat of
formation of benzene is -49.8kcal/mole. This higher
stability of -36kcal/mole is called aromaticity or resonance
energy.
iii. Carbon-Carbon Bond Lengths of Benzene: Due to aromaticity the C-C
bond length of benzene (1.39) is in between C-C (1.54) s ingle and C=C
(1.34)double bonds.
iv. Participation of Substitution vs
Addition: Normally unsaturated
compounds undergo addition reactions
easily. Benzene has three double bonds
but, not undergo addition reactions .
Undergo substitution reactions
4. Non-benzenoid Aromatic Compounds: These are
aromatic compounds that do not contain benzene
ring. (Eg) Pyridine, Naphthalene
N
Pyridine Naphthalene
5. i.Aromatic Electrophilic Substitution(AES): Thes e are important reactions . In which an
atom/group in the aromatic compound is replaced by electrophile (positive ion/group). (Eg)
nitrations , sulphonation, and Friedel-Crafts reactions of halogenation, alkylation and
acylation.
ii. Genral Pattern of AES: In this
reaction the electrophile replaced a
hydrogen atom in the benzene ring.
The electrophile attacks the benzene
ring and forms an intermediate.
The intermediate give the product.
H
E
+
E
H
E
Benzene Electrophile
Intermediate Product
+
iii. Role of σ and π complexes AES: In the
AES, the electrophile attacks the π-cloud
(electrons ) of the benzene ring and formed a
π-complex between π-electron-
electrophile. Then the electrophile forms a
sigma complex with a carbon atom . The
base rem oves the proton and gives the
product.
EE
E
Sigma-Bond
pi-Complex
Reactant
Electrophile
6. Mechanism AES: The are three types of AES. They are nitration, sulphonation and
Friedel-Crafts reactions.
i. Nitration: Benzene reacts with con. HNO3
and con. H2SO4 at 50°C gives nitro
benzene.
HNO3/H2SO4
NO2
Mechanism: This has three steps.
Step 1: In this step the nitric acid accepts
a proton from sulphuric acid and gives
nitronium ion(NO2). This is a fast step
and not rate determining.
Step 2: In this step the electrophile
nitronium ion attacks the benzene ring and
formed an arenium ion.This is the slow
step and rate determining.
Step 3: In this step the arenium ion gives
the product nitrobenzene. This is a fast
step and not rate determining.
HNO3 + H2SO4
Fast
Step-1 NO2+ + H2O+HSO4-
Step-2
H
+
H
Benzene Electrophile
NO2+Slow
+
NO2
Step-3
H
Intermediate Nitrobenzene
+
Fast
NO2
NO2
ii. Sulphonation: Benzene on heating at
40°C with con.H2SO4 gives
benzenesulphonic acid.
H2SO4
SO3H
Mechanism: This has three steps.
Step 1: In this step one s ulphuric acid
accepts a proton from another sulphuric acid
and gives sulphonium ion(HSO3). This is a
fast step and not rate determining.
Step 2: In this step the electrophile
sulphonium ion attacks the benzene ring and
formed an arenium ion.This is the slow step
and rate determining.
Step 3: In this step the arenium ion gives the
product benzenes ulphonic acid. This is a
fast step and not rate determining
2H2SO4
Fast
Step-1 SO3H+ + H2O+HSO4-
Step-2
H
+
H
Benzene Electrophile
SO3H+Slow
+
SO3H
Step-3
H
Intermediate Benzenesulphonic acid
+
Fast
SO3H
SO3H
iii. Halogenation: Benzene reacts with
chloride in the presence of Lewis acid
(AlCl3/FeCl3) gives chlorobenzene.
Cl2/AlCl3
Cl
Mechanism: This has three steps.
Step 1: In this step Lewis acid takes up a halide
ion from chloride and gives the electrophile
chloronium ion(Cl+). .This is a fast step and not
rate determining.
Step 2: In this step the electrophile Cl+ ion
attacks the benzene ring and formed an
arenium ion.This is the slow step and rate
determining.
Step 3: In this step the arenium ion gives the
product chlorobenzene . This is a fast step and
not rate determining.
Cl2 + AlCl3
Step-1 Cl++ AlCl4-
Step-2
H
+
H
Benzene Electrophile
Intermediate
Cl+Cl
Slow
+
Step-3
Cl
H
Intermediate Chlorobenzene
Cl
+
Fast
.iv.Friedel–Crafts Reaction: i.Benzene reacts with
methylchloride (CH3Cl) in the presence of Lewis acid
(AlCl3/FeCl3) gives toluene.
ii. Benzene reacts with acetylchloride (CH3COCl) in the
presence of Lewis acid (AlCl3/FeCl3) gives
methylphenylketone.
CH3Cl/AlCl3
CH3
Benzene Toluene
iv. Friedel–Crafts Reaction: In this reaction Lewis
acids like AlCl3, FeCl3 attacks a neutral molecules
like CH3Cl, CH3COCl and removes the electron to
give electrophile like, CH3+ and CH3CO+.This
electrophile attacks the benzene ring and gives the
product toluene (hydrocarbon) or arylketone.
CH3Cl/AlCl3
CH3
Benzene
a). Friedel–Crafts Alkylation: Benzene reacts with
alkylhalide(methyl chloride) in the presence of
Lewis acid gives hydrocarbon (toluene).
CH3Cl/AlCl3
CH3
Benzene
Mechanism: This has three steps .
Step 1: In this s tep Lewis acid takes up a halide
ion from methyl chloride and gives the electrophile
methyl group (CH3+). This is a fast step and not
rate determining.
Step 2: In this step the electrophile CH3+ attacks
the benzene ring and formed an arenium ion.This
is the slow step and rate determ ining.
Step 3: In this step the arenium ion gives the
product toluene . This is a fast step and not rate
determining
CH3Cl+AlCl3
Step-1 CH3++ AlCl4-
Step-2
H
+
H
Benzene Electrophile
Intermediate
CH3+CH3
Slow
+
Step-3
CH3
H
Intermediate \Toluene
CH3
+
Fast
b). Friedel–Crafts Acylation: Benzene reacts with acidhalide(acetyl chloride) in the presence
of Lewis acid gives ketone (methyl phenyl ketone).
Mechanism: This has three steps .
Step 1: In this step Lewis acid takes up a
halide ion from acid chloride and gives
the electrophile methylcarbonyl group
(CH3CO+). This is a fast step and not rate
determining.
Step 2: In this step the electrophile
CH3CO+ attacks the benzene ring and
formed an arenium ion. This is the slow
step and rate determining.
Step 3: In this s tep the arenium ion gives
the product methyl phenyl ketone . This
is a fast step and not rate determining..
CH3COCl+AlCl3
Step-1 CH3CO++ AlCl4-
Step-2
H
+
H
Benzene Electrophile
Intermediate
CH3CO+COCH3
Slow
+
Step-3
H
Intermediate Benzophenone
COCH3
+
Fast COCH3
7. Activating and Deactivating Groups:
i. Activating Substituents: If a substituent increases the rate of reaction relative to
hydrogen is called activating..
The order of activating groups is; -NH2, -NR2 > -OH, -OR> -NHCOR> -CH3
ii. Deactivating Substituents:If a substituent decreases the rate relative to hydrogen it is
called deactivating.
The order of deactivating groups is;: -NO2, -CF3> -COR, -CN, -CO2R, -SO3H > Halogens
among halogen is: F> Cl > Br > I
8. Orientation in Mono Substituted Benzenes: In Mono substituted benzene the position
and rate of the second substituent depends on the first substituent. This is called the
orientation effect.
i. Ortho/Para Orientation (Directors) of EAS: In
EAS, the first substituent on benzene direct the
electrophile (E) to the ortho– (1,2) and para– (1,4)
positions is called, ortho, para- director.
Substituent with +M character are ortho, para-
orienting. +M groups are; -CH3,-OH, -NH2,-X, -OR
CH3
Cl2
CH3Cl
Cl
Cl
o
p
o
o,p-orienting
ii. Meta Orientation (Directors) of EAS: In EAS, the
first substituent on benzene direct the electrophile
(E) to the meta– (1,3) positions is called, meta-
director. Substituent with -M character are meta-
orienting. -CHO, -COOH,-NO2 have -M character and
others have+M character.
Cl2
Cl
Cl m-orienting
NO2NO2
mm
iii. Ortho/Para Ratio in EAS: This is the
ratio of the amount of ortho and para
products
CH3CH3
+
CH3
NO2
NO2
p-61% o-39%
.
11. Determination of ortho/meta/para Substitution(isomer): This methods are used to find
out the position of substitution in the benzene ring. There are two methods.
i. Korner’s Absolute Method of Determine ortho/meta/para Substitution(isomer):
Korner's absolute method was used to determine the disubstituted benzene as ortho, meta,
or para isom er. It involves adding a third group (nitro group) and determining the number of
isomers formed. (Eg) o-xylene on nitration gives two isom ers.
