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.
