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In organic chemistry, Hückel's rule predicts that a planar ring molecule will have aromatic properties if it has 4n + 2 π electrons, where n is a non-negative integer. The quantum mechanical basis for its formulation was first worked out by physical chemist Erich Hückel in 1931.[1][2] The succinct expression as the 4n + 2 rule has been attributed to W. v. E. Doering (1951),[3][4] although several authors were using this form at around the same time.[5]

Benzene, the most widely recognized aromatic compound with six delocalized π electrons (4n + 2, for n = 1).

In agreement with the Möbius–Hückel concept, a cyclic ring molecule follows Hückel's rule when the number of its π-electrons equals 4n + 2, although clearcut examples are really only established for values of n = 0 up to about n = 6.[6] Hückel's rule was originally based on calculations using the Hückel method, although it can also be justified by considering a particle in a ring system, by the LCAO method[5] and by the Pariser–Parr–Pople method.

Aromatic compounds are more stable than theoretically predicted using hydrogenation data of simple alkenes; the additional stability is due to the delocalized cloud of electrons, called resonance energy. Criteria for simple aromatics are:

  1. the molecule must have 4n + 2 (a so-called "Hückel number") π electrons[7] (2, 6, 10, ...) in a conjugated system of p orbitals (usually on sp2-hybridized atoms, but sometimes sp-hybridized);
  2. the molecule must be (close to) planar (p orbitals must be roughly parallel and able to interact, implicit in the requirement for conjugation);
  3. the molecule must be cyclic (as opposed to linear);
  4. the molecule must have a continuous ring of p atomic orbitals (there cannot be any sp3 atoms in the ring, nor do exocyclic p orbitals count).

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