## Wikipedia - Aromatyczność (pl) Wikipedia - Hückel-Regel (de) Wikipedia - Regola di Hückel (it) Wikipedia - Związki aromatyczne (pl) Wikipedia - قاعدة هوكل (ar) Wikipedia - ヒュッケル則 (ja) 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 @[email protected] character. The rule is generally limited to n = 0–5. This rule is derived from the Hückel MO calculation on planar monocyclic conjugated @[email protected] (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 @[email protected]) are 'antiaromatic'.