Monocyclic planar (or almost planar) systems of trigonally (or sometimes digonally)
hybridized atoms that contain (4
n + 2) π-electrons (where
n is a non-negative integer) will exhibit
aromatic character. The rule is generally limited to
n = 0–5. This rule is derived from the Hückel MO calculation on planar monocyclic conjugated
hydrocarbons (CH)
m where
m is an integer equal to or greater than 3 according to which (4
n + 2) π-electrons are contained in a closed-shell system. Examples of systems that
obey the Hückel rule include:
Systems containing 4
n π-electrons (such as cyclobutadiene and the cyclopentadienyl
cation) are '
antiaromatic'.
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1122
