The difference between aromatic,antiaromatic, nonaromatic and paeudo aromatic compounds.

1. Aromatic Compounds
Anti aromatic Compounds
Pseudo aromatic Compounds
Non aromatic Compounds
Prof. Mrs. Salunkhe A.S.
Associate Professor
Department of Chemistry
S.G.M. College, Karad
Difference between

2. Cyclic unsaturated compounds are of following types:
Aromatic Compounds
1
Anti aromatic Compounds
2
Pseudo aromatic Compounds
3
Non aromatic Compounds
4

3. Aromatic Compounds
➢ Compounds which obey Huckel’s rule of aromaticity are aromatic in nature.
➢ Huckel’s rule- Cyclic, planer, conjugated system having (4n +2)∏ electrons
is aromatic in nature. ( n= 0,1,2,3,4……………..)
➢ There should be delocalization of ∏ electrons.
➢ Cyclic system is more stable than corresponding acyclic system.
eg. Benzene is more stable than 1,3,5 hexatriene
CH2= CH – CH = CH – CH = CH2
1,3,5, hexatriene
Benzene
Cyclic
Planer – sp2 hybridization
Conjugated system
(4n +2)∏ electrons
When n=1,(4n +2)= 6
6 ∏ eles
Less stable
More stable

4. Aromatic compounds are of two types – 1) Benzenoid aromatic compound
2) Non benzenoid aromatic compound
1) Benzenoid aromatic compounds
Aromatic compounds which contains one or more benzene rings.
When n=2,(4n +2)= 10 When n=3,(4n +2)= 14
Anthracene
14 ∏ eles
When n=3,(4n +2)= 14
Phenanthrene
14 ∏ eles
Naphthalene
10 ∏ eles

5. 2) Non benzenoid aromatic compounds
Aromatic compounds which not contains benzene ring.
When n=0,(4n +2)= 2
Cyclopropenyl cation
2 ∏ eles
When n=1,(4n +2)= 6
Cyclopentadienyl anion
6 ∏ eles
When n=1,(4n +2)= 6
Cycloheptatrienyl cation
6 ∏ eles
Tropylium cation
When n=3,(4n +2)= 14
[14] Annulene
14 ∏ eles
When n=4,(4n +2)= 18
[18] Annulene
18 ∏ eles
When n=2,(4n +2)= 10
Azulene
10 ∏ eles

6. .. ..
..
.. ..
..
When n=1,(4n +2)= 6
Pyrrole
6 ∏ eles
When n=1,(4n +2)= 6
Furan
6 ∏ eles
When n=1,(4n +2)= 6
Pyridine
6 ∏ eles
When n=1,(4n +2)= 6
Thiophene
6 ∏ eles
Heterocyclic compounds
‘P’ atomic orbital
Sp2 hybrid orbital

7. Anti aromatic Compounds
➢ Cyclic, planer, conjugated system having (4n)∏ electrons is antiaromatic in nature.
( n= 0,1,2,3,4……………..)
➢ Cyclic system is less stable than corresponding acyclic system.
eg. Cyclobutadiene is less stable than 1,3 butadiene
Cyclobutadiene
When n=1,(4n)= 4
4 ∏ eles
Cyclic
Planer – sp2 hybridization
Conjugated system
(4n)∏ electrons
CH2= CH – CH = CH2
1,3 butadiene
4 ∏ eles
More stable
Less stable

8. Cyclopropenyl anion
4 ∏ eles
Cyclopentadienyl cation
4 ∏ eles
Cycloheptatrienyl anion
8 ∏ eles
Anti aromatic Compounds

9. Non aromatic Compounds
➢ Cyclic unsaturated system in which double bonds are not in conjugation and ring is not
planer is nonaromatic in nature.
➢ Cyclic system and acyclic system have similar stability.
Cyclopropene Cyclopentadiene 1,3 Cyclohexadiene 1,3,5 Cycloheptatriene
sp3 hybridised
2 ∏ eles 4 ∏ eles 4 ∏ eles 6 ∏ eles

10. Pseudo aromatic Compounds
Cyclic, conjugated system having (4n)∏ electrons but ring is not planer is known as
pseudo aromatic compound.
Cyclooctatetraene Pentalene Heptalene
12 ∏ eles
8 ∏ eles
8 ∏ eles

11. Summary
Aromatic compounds- Cyclic, planer, conjugated system having (4n +2)∏ electrons.
Anti aromatic compounds- Cyclic, planer, conjugated system having (4n)∏ electrons.
Non aromatic compounds- Cyclic unsaturated system in which double bonds are not
in conjugation and ring is not planer.
Pseudo aromatic compounds- Cyclic, conjugated system having (4n)∏ electrons
but ring is not planer.

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