Term: Hückel (4n + 2) rule
https://doi.org/10.1351/goldbook.H02867

Definition:
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 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 (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 cation) are 'antiaromatic'.

Related Terms:
1) conjugation (http://doi.org/10.1351/goldbook.C01267).
2) möbius aromaticity (http://doi.org/10.1351/goldbook.M03957).
3) aromatic (http://doi.org/10.1351/goldbook.A00441).
4) hydrocarbons (http://doi.org/10.1351/goldbook.H02889).
5) cation (http://doi.org/10.1351/goldbook.C00907).

Image: Not defined (https://goldbook.iupac.org/img/inline/H02867.png) 

Source: PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1122 (https://doi.org/10.1351/pac199466051077)

Citation: 'Hückel (4<em>n</em> + 2) rule' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.H02867

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