Aromaticity and Huckel’s rule Aromatic, antiaromatic and non-aromatic systems

1. 1
B.Sc. First year
Students
B.Sc. Semester – I
Unit-IV
Aromaticity and Huckel’s rule
Aromatic, antiaromatic
and non-aromatic systems
by
Dr Pramod R Padole
Professor in Chemistry
Department of Chemistry
Shri Shivaji Science College, Amravati

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4. Unit IV- Aromatic Hydrocarbons
Orientation: Effect of substituent groups. Activating and
deactivating groups.
Theory of reactivity and orientation on the basis of inductive
and resonance effects (-CH3, -OH, -NO2 and –Cl groups).
D
Nomenclature and Isomerism of Aromatic Compounds:
Structure of Benzene: Kekule structure and Molecular orbital structure.
A
Aromaticity and Huckel’s rule Aromatic, antiaromatic and
non-aromatic systems
B
Mechanism of Electrophilic Aromatic Substitution: Nitration,
Friedal Craft Alkylation and Acylation.Nuclear and Side Chain
Halogination, Birch Reduction
C

5. LOGO
By Dr Pramod R Padole
Aromaticity &
Huckel’s Rule:

6. LOGO
Aromaticity & Huckel’s Rule:
Aromaticity & Huckel’s (4n+2) π e-s Rule:
Or
Characteristics of Aromatic Compound:
 E-Huckel (German scientist) put forward a
satisfactory explanation for aromaticity.
Q.1) Explain Aromaticity in the right of Hukel’s Rule. (W-06, 2 Mark)
Q.2) State Huckel’s rule for aromaticity. (S-10 & S-11, 2 Mark)
Q.3) Explain Huckel’s rule with examples. (W-11, 4 mark)
Q.4) What are the characteristics of aromatic compounds? (S-13 & S-15, 4 Mark)
Q.5) (4n+2) electron rule is called __________. (S-14 & W-16, ½ Mark)
Q.6) Explain the criteria of aromaticity. (W-14, 4 Mark)
Q.7) Give two examples of Aromatic compounds. (W-19, 2 Mark)

7. LOGO
Aromaticity & Huckel’s Rule:
Aromatic compounds possess the following
characteristics:
 Compounds should be cyclic and planar.
 The ring C-atoms have unhybridised p-orbital’s (because each
C-atom is sp2 hybridised).
 These unhybridised p-orbital’s are perpendicular to the plane of the
molecule and must be parallel to each other, so that continuous
overlapping is possible around the ring.
C6
C6
C6
G.S.
2s 2px 2py 2pz
E.S.
sp2
H.S.
3 sp2
H.O's
unhybridised orbital
( to form bond)
Valence Shell
( to form -bond)
Valence Shell

8. LOGO
Aromaticity & Huckel’s Rule:
 Compound must have a conjugated electron system.
 Cyclic molecular orbital shall have, (4n+2); π e-s.
Where, n is the integer number & n = 0,1,2,3,……, etc
Thus, the conjugated cyclic aromatic compounds containing 2, 6, 10,
14, ….., etc, π e-s possess aromatic character.
This is known as Huckel’s Rule.
 Or A cyclic, planar, completely conjugated compound with
(4n+2); π e-s, is known as aromatic compounds .
 Aromatic compounds give substitution reaction rather than
addition reaction.
 The aromatic compounds are stabilized by
 resonance energy.

9. LOGO
Aromaticity & Huckel’s Rule:
 Examples of Aromatic compounds:
Cyclopropenyl cation
(2 electrons)
Benzene
(6 electrons)
Naphthalene
(10 electrons)
Cyclopentadienyl anion
(6 electrons)
Cycloheptatrienyl cation
(6 electrons)

10. L o g o
Antiaromatic Compound or System:
Q.1) What are the characteristics of antiaromatic compunds?
(S-14, W-16 & W-19, 4 Mark)
Q.2) Which of he following compounds is antiaromatic? (S-16 & W-18, ½ Mark)
a) Benzene b) Cyclobutadiene c) Cyclodecapenene d) Cyclo-octatetraene
Q.3) Give two examples of non-aromatic compounds. (W-19, 2 Mark)

11. L o g o

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Antiaromatic Compound
or System:
Antiaromatic molecules are cyclic systems
containing alternating single and double
bonds, where the electron energy of
antiaromatic compounds is higher than that
of its open chain analogue.
Therefore, antiaromatic compounds are
unstable and highly reactive.
By Dr Pramod R Padole

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Antiaromatic Compound
or System:
Antiaromatic compounds possess the following
characteristics:
 Compounds should be cyclic and planar.
 The ring C-atoms have unhybridised p-orbital’s (because
each C-atom is sp2 hybridised).
 Compound must have a conjugated electron system.
 Cyclic molecular orbital shall have, (4n); π e-s.
Where, n is the integer number & n = 0,1,2,3,……, etc
Thus, the conjugated cyclic anti-aromatic compounds
containing 0, 4, 8, 12, ….., etc, π e-s possess anti-
aromatic character.
By Dr Pramod R Padole

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Antiaromatic Compound
or System:
Antiaromatic compounds possess the following
characteristics:
Or
A cyclic, planar, completely conjugated compound
with (4n); π e-s, is known as anti-aromatic
compounds.
 Examples:
By Dr Pramod R Padole
Cyclopropenyl anion
(4 electrons) Cyclopentadienyl cation
(4 electrons)
Cycloheptatrienyl anion
(8 electrons)
Cyclobutadiene
(4 electrons)

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Aromatic & Antiaromatic
Compound
Q.1) Identify which of the following compounds are
aromatic and antiaromatic: (S-16, 4 Mark)
-----*****-----
Q.2) Predict the following compounds as aromatic or
antiaromatic with explanation: (S-17, 4 Mark)
By Dr Pramod R Padole
(i) (ii) (iii) (iv)
(i) (ii)
&

16. LOGO
Non aromatic
Compound or
System:
Q.1) Explain why, Cyclo-octatetraene, is non-aromatic compounds. (W-13, 2 Mark)

17. Non aromatic Compound or System:
Those compounds which are neither aromatic nor anti-
aromatic are known as non-aromatic compounds.
For example - 1:
 Cyclo-octatetraene is cyclic with conjugated system, does not follow
Huckel’s rule, (4n+2); π e-s while it fallows 4n electrons required for anti-
aromatic character, but its plane is distorted and transforms into stable
non planar Tub shape structure.
So, it is non aromatic.

18. Non aromatic Compound or System:
Those compounds which are neither aromatic nor anti-
aromatic are known as non-aromatic compounds.
For example - 2:
 Cyclo-decapentaene is cyclic with conjugated system and has 10
electrons so follows Hucke’s rule but the planar structure is strained
and transforms into stable non planar structure, therefore, it is non
aromatic. So, it is non aromatic.

19. Q.1) Classify the following as aromatic and anti-aromatic: (S-18, 4 Mark)
---------*****---------
Q.2) Identify which of the following compounds are aromatic and
antiaromatic: (W-18, 4 Mark)
(i)
(ii) (iii) (iv)
Ans: Aromatic Aromatic Anti-aromatic Anti-aromatic

20. Q.3) Identify the aromatic character for the following
compounds:-
Cyclohexane
e.g. (i) Cyclohexane molecule is cyclic, but non-planar.
1)
2) Applying Huckel.s Rule:-
4n+2 = 0 ; e
n = -2/4
n = -1/2 ( non interger no.)
So, Compound cyclohexane is non-aromatic.
e.g.ii)
Benzene molecule is cyclic and planer.
1)
2) Applying Huckel.s Rule:-
Benzene
4n+2 = 6; π e-s
4n = 6-2
n = 4/4
n = 1, integer no.
So, Benzene is aromatic compound.

22. LOGO

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Aromaticity and Huckel’s rule by Dr Pramod R. Padole
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