CH‐103

Aromaticity

Dr. Umesh A. Kshirsagar
Assistant Professor
Chemistry, IIT‐Indore

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 Aromaticity:

Why to study this?
Why to study this?
‐Reactivity of aromatic compounds is connected to their structure, their characteristic 
aromaticity.
‐ Aromatic compounds provide a sensitive probe for studying relationship between 
structure and reactivity
d i i

Benzene: typical example of aromatic compound. 
yp p p
Benzene was first discovered in 1825 but its structure was not generally agreed upon 
until 1946.

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Unlike, aliphatic carbon‐carbon bonds (where C‐C and C=C bond length are different 
148 and 134 pm respectively), all of the Carbon‐Carbon bonds in benzene are of the 
same length (139 pm) and intermediate in length between single and double bonds.

• All bonds are 139 pm (intermediate between C‐C and C=C)
g
• C–C–C bond angles are 120°
• Structure is planar, hexagonal

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‐Generally, alkenes (C=C) undergoes addition reaction but Benzene does not undergo addition
reaction, instead undergoes substitution reactions.

Undergo addition
d ddi i

undergo substitution
g

undergo substitution

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 Heats of hydrogenation are far lower than they should be

cyclohexene

1,3 cyclohexadiene

actual
benzene
expected
p
called 
resonance energy
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 Heats of Hydrogenation as Indicators of Stability
Heats of Hydrogenation as Indicators of Stability

• The addition of H2 to C=C normally gives off about 28.6 kcal/mol 
yg /

• Two conjugated double bonds in cyclohexadiene add two H2 to give 

off 55.2 kcal/mol 

• three double bonds are expected to give off 85.8 kcal/mol 
Benzene has three unsaturation sites but gives off only 48.8 kcal/mol on 
reacting with three H
i ih h H2 molecules
l l

• Therefore it has about 36 kcal more “stability” than an isolated set of 
three double bonds called as resonance energy
three double bonds, called as resonance energy.

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 what we have said thus far about benzene
h h d h f b b

ƒBenzene is cyclic and conjugated.

ƒBenzene is unusually stable, having a heat of hydrogenation 36 kcal/mol less 
th
than we might expect for a conjugated cyclic‐triene.
i ht tf j t d li t i

ƒBenzene is planar and has the shape of a regular hexagon. 

ƒAll bond angles are 120°, all carbon atoms are sp2‐hybridized, and all carbon–
carbon bond lengths are same i.e. 139 pm.

ƒBenzene undergoes substitution reactions that retain the cyclic conjugation 
rather than addition reactions that would destroy the conjugation.

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 What makes molecule Aromatic?
Cyclic conjugation is necessary, but not sufficient criteria for aromaticity.
y j g y, y
See the following examples.
First two are having cyclic conjugation but not aromatic,.
Next two are cyclic but not fully conjugated (conjugation in interrupted) 

In Short:‐
Benzene is Aromatic
1) The Benzene is cyclic (a ring of atoms) 

2) The Benzene is planar/flat (all atoms in the molecule lie in the same plane) 

) y j g (p
3) The Benzene is fully conjugated (p orbitals at every atom in the ring) 
y g)

4) The Benzene has 4n+2 π electrons (n = 0 or any positive integer) 
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 Hückel's Rule of Aromaticity
In 1931,
1931 German chemist and physicist Sir Erich Hückel proposed a theory to help
determine if a planar ring molecule would have aromatic properties. His rule
states that “if a cyclic, planar, fully conjugated molecule having 4n+2 π
electrons, it is considered aromatic”. This rule would come to be known as
Hückel's Rule.

Criteria for Aromaticity
There turn out to be 4 conditions a molecule must meet in order for it to be aromatic 
(It’s all or nothing, If any of these conditions are violated, no aromaticity is possible).

1) it must be cyclic.
it must be cyclic

2) molecule must be fully conjugated (every atom in the ring must be conjugated).

3) the molecule must be planar (flat). 

4) the molecule must have [4n+2] pi electrons. Then it is AROMATIC.

Note: If molecule fulfilled first 3 conditions but instead of 4n+2 pi electron, if it has 
4n pi electron, it is called as ANTI‐AROMATIC (highly unstable).
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 What is the theoretical base of Hückel's Rule?

According to Hückel's Molecular Orbital Theory (MOT), a compound is particularly stable if

all of its bonding molecular orbitals are filled with paired electrons.

‐ This is true of aromatic compounds, meaning they are quite stable.

‐ With aromatic compounds, 2 electrons fill the lowest energy molecular orbital, and 4
electrons fill each subsequent energy level (the number of subsequent energy levels is
denoted by n), leaving all bonding orbitals filled and no anti‐bonding orbitals occupied. This
gives a totall off 4n+2π electrons.
l

‐ As for example: Benzene has 6π electrons. Its first 2π electrons fill the lowest energy
orbital,, and it has 4π electrons remaining.
g These 4 fill in the orbitals of the succeedingg
energy level. Notice how all of its bonding orbitals are filled, but none of the anti‐bonding
orbitals have any electrons.

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According to Hückel's Molecular Orbital Theory (MOT), a compound is particularly
stable if all of its bonding molecular orbitals are filled with paired electrons.

See the following MO diagram of Benzene (6 pi electron making 6 molecular orbital)

Anti‐bonding Molecular Orbital
(ABMO)

Non‐Bonding Molecular Orbital

Bonding Molecular Orbital
(BMO)

All bonding electrons are filled and paired hence benzene is stable and Aromatic. 

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See the following MO diagram of cyclobutadiene (4 pi electron making 4 molecular orbital)

Anti‐bonding
Anti bonding Molecular Orbital
Molecular Orbital
(ABMO)

Non‐Bonding Molecular Orbital

Bonding Molecular Orbital
(BMO)

Cyclobutadiene has unpaired electrons in its non‐bonding molecular orbital 
hence highly unstable and is Anti‐aromatic
hence highly unstable and is Anti aromatic 

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 Solved problems on aromaticity

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 Solved problems on aromaticity

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 Polycyclic Aromatic Compounds
• Aromatic compounds can have rings that share a set of carbon atoms (fused rings) 

naphthalene anthracene h th
phenanthrene
-cyclic -cyclic -cyclic
-fully-conjugated -fully-conjugated -fully-conjugated
-planar/flat -planar/flat -planar/flat
-4n+2 p pi electron ((10 p
pi)) -4n+2 pi electron (14 pi) -4n+2 pi electron (14 pi)
= AROMATIC = AROMATIC = AROMATIC

Heterocyclic Aromatic Compounds
• Aromatic compounds can have rings that share a set of carbon atoms (fused rings) 
• Compounds from fused benzene or aromatic heterocycle rings are also aromatic

N O S N
H
pyrole furan thiophene pyridine

-cyclic -cyclic -cyclic -cyclic

-fully-conjugated -fully-conjugated -fully-conjugated -fully-conjugated
-planar/flat
l /fl t planar/flat
-planar/flat -planar/flat planar/flat
-planar/flat
-4n+2 pi electron (6 pi -4n+2 pi electron (6 pi -4n+2 pi electron (6 pi -4n+2 pi electron (6 pi
including N lone pair) including O lone pair) including S lone pair) without N lone pair)
= AROMATIC = AROMATIC = AROMATIC = AROMATIC
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