Hückel 4n + 2 rule

Definition

The Hückel 4n + 2 rule is a principle used to determine if a planar ring molecule will have aromatic stability based on its number of pi electrons. For a molecule to be considered aromatic, it must contain a total of 4n + 2 π (pi) electrons, where n can be any non-negative integer (0, 1, 2, ...).

Related terms

Aromaticity:

A property of cyclic molecules with conjugated pi electron systems that meet specific criteria including planarity, fully conjugated pi electron system throughout the ring, and following the Hückel 4n + 2 rule, resulting in unusual stability.

Pi electrons: Electrons that reside in p orbitals and participate in forming pi bonds between atoms in organic molecules; these are the electrons counted when applying the Hückel rule.

Conjugation:

The overlap of one p orbital with another across an intervening sigma bond in adjacent atoms, creating a system of delocalized pi electrons over three or more atoms