Hückel (4n + 2) rule

https://doi.org/10.1351/goldbook.H02867
Monocyclic planar (or almost planar) systems of trigonally (or sometimes digonally) hybridized atoms that contain (4n + 2) π-electrons (where n is a non-negative integer) will exhibit @A00441@ character. The rule is generally limited to n = 0–5. This rule is derived from the Hückel MO calculation on planar monocyclic conjugated @H02889@ (CH)m where m is an integer equal to or greater than 3 according to which (4n + 2) π-electrons are contained in a closed-shell system. Examples of systems that obey the Hückel rule include:
H02867.png
Systems containing 4n π-electrons (such as cyclobutadiene and the cyclopentadienyl @C00907@) are 'antiaromatic'.
See also:
conjugation
,
Möbius aromaticity
Source:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1122 [Terms] [Paper]