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Huckel Theory of Aromaticity, Lecture notes of Organic Chemistry

Huckel 4n+2 rule, Benzenoid and Non-benzenoid aromatic compounds.

Typology: Lecture notes

2020/2021

Uploaded on 05/24/2021

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Dr Anju K. Gupta
Associate Professor
Department of Chemistry
A.N. College, Patna
1
Huckel Theory of Aromaticity
Organic compounds that contain one or more aromatic rings such as
benzene or naphthalene are called aromatic compounds. For a compound to be
classified as aromatic, it must fulfil the following criteria:
(1) The molecule must be cyclic and planar. All the atoms participating in the ring
must lie in the same plane.
(2) The molecule must have an uninterrupted cyclic cloud of π electrons above and
below the plane of the molecule. For this, every atom in the ring must have a p-
orbital and each p-orbital must be able to overlap with the p-orbitals on either side
of it.
(3) To ensure maximum overlapping of p-orbitals, each carbon atom in the ring
must be sp2 hybridised. This is also applicable to extended unsaturated
heterocyclics and fused ring compounds.
(4) Only those rings are aromatic which involve cyclic delocalization of (4n+2)π
electrons, where n is any positive integer (n= 0,1,2,…..). It means that the aromatic
ring must contain either of 2, 6, 10,……..π electrons. This criteria is known as Huckel
4n+2 rule.
(5) Cyclic compounds with 4nπ electrons would be even less stable than their open-
chain analogues and such cyclic compounds are called anti-aromatic compounds.
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Dr Anju K. Gupta Associate Professor Department of Chemistry A.N. College, Patna 1

Huckel Theory of Aromaticity

Organic compounds that contain one or more aromatic rings such as benzene or naphthalene are called aromatic compounds. For a compound to be classified as aromatic, it must fulfil the following criteria: (1) The molecule must be cyclic and planar. All the atoms participating in the ring must lie in the same plane. (2) The molecule must have an uninterrupted cyclic cloud of π electrons above and below the plane of the molecule. For this, every atom in the ring must have a p- orbital and each p-orbital must be able to overlap with the p-orbitals on either side of it. (3) To ensure maximum overlapping of p-orbitals, each carbon atom in the ring must be sp^2 hybridised. This is also applicable to extended unsaturated heterocyclics and fused ring compounds. (4) Only those rings are aromatic which involve cyclic delocalization of (4n+2)π electrons, where n is any positive integer (n= 0,1,2,…..). It means that the aromatic ring must contain either of 2, 6, 10,……..π electrons. This criteria is known as Huckel 4n+2 rule. (5) Cyclic compounds with 4nπ electrons would be even less stable than their open- chain analogues and such cyclic compounds are called anti-aromatic compounds.

Dr Anju K. Gupta Associate Professor Department of Chemistry A.N. College, Patna 2

Benzenoid and Non-benzenoid aromatic compounds

Compounds which contain one or more benzene rings are called benzenoid aromatic compounds. These compounds are unique due to the special stability possessed by them which is called aromaticity or aromatic character. But, on the other hand, there are several compounds, which have little resemblance to benzene and yet show the typical aromatic character. Such aromatic compounds are known as non-benzenoid aromatic compounds.