Hückel rule

The Hückel rule , named after Erich Hückel (1896–1980), can be used to predict whether a chemical compound is an aromatic molecule. It can be derived from the more comprehensive Hückel approximation .

A planar, cyclically conjugated molecule is particularly stable with [4n + 2] π electrons (an odd number of electron pairs; example: n = 1, [4 · 1 + 2 = 6], ie three electron pairs) and is called an aromatic .

With [4n] π electrons (an even number of electron pairs; example: n = 2, [4 · 2 = 8], that is, four electron pairs) it is unstable and is called an anti-aromatic. For n here are positive integers and 0 can be used (n = 0, 1, 2, 3 ...).

Examples

Hydrocarbons

benzene

The most important example is benzene , which fulfills the rule with 6 π electrons (n = 1) and, as a hexagon, also forms the ideal bond angle for sp ² -hybridized carbons. Polycyclic (condensed) aromatics are created if you add 4 more carbon atoms to one edge one or more times, but the Hückel rule no longer applies to them (see restrictions).

Heteroaromatics

Pyrrole

Pyridine

Aromatic compounds that consist not only of carbon and hydrogen are called heteroaromatics . They can be 5 or 6 rings. In the case of rings of five (e.g. pyrrole , furan , thiophene , selenophene ) the heteroatom provides a whole free electron pair. With 6 rings (e.g. pyridine ) the lone pair of electrons of the heteroatom points outwards.

It should be noted that due to the bonding conditions described, the nitrogen in pyrrole is bonded to a hydrogen, but not in pyridine. Uncharged, cyclically conjugated 6-ring rings with oxygen or sulfur are not possible at all.

Aromatic ions

Cyclopentadienyl anion

The cyclopropenium cation and the cyclopentadienyl anion fulfill the Hückel rule with 2 (n = 0) and 6 (n = 1) π electrons, respectively. They are therefore particularly stable for ions. Cyclopentadienyl is an important singly negatively charged ligand in organometallic chemistry .

Trichlorocyclopropenyl cation

Pyrylium cation

An example of heteroaromatic ions is the pyrylium cation (n = 1), which belongs to the group of oxonium ions . Compounds with a substituted pyrylium structure also occur in nature, for example in anthocyanins .

restrictions

The Hückel rule only applies to monocyclic molecules. Only there the Hückel approximation can make a general statement, since the energy schemes of these connections ( Frost-Musulin circle ) can be calculated particularly easily. It does not make any statement about systems with several rings, e.g. B. polycyclic aromatic hydrocarbons (PAH).

The following PAHs behave according to the Hückel rule: naphthalene (10 π electrons), anthracene (14 π electrons), phenanthrene (14 π electrons).

The following PAHs behave contrary to the Hückel rule: pyrene (16 e ^- ), benzopyrene (20 e ^- ), fluoranthene (16 e ^- ).

Furthermore, the rule only applies to molecules that have two sides. If the molecule has only one side, such as a Möbius strip , the aromaticity disappears. There are even molecules that can have both properties (unilateral or bilateral), depending on the substance in which they are dissolved.

Individual evidence

↑ Entry on Hückel (4 n + 2) rule . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.H02867 Version: 2.1.5.
↑ Stepień M, Latos-Grazyński L, Sprutta N, Chwalisz P, Szterenberg L: Expanded porphyrin with a split personality: a Hückel-Möbius aromaticity switch . In: Angew. Chem. Int. Ed. Engl . 46, No. 41, 2007, pp. 7869-73. doi : 10.1002 / anie.200700555 . PMID 17607678 .

[IUPAC-1] Entry on Hückel (4 n + 2) rule . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.H02867 Version: 2.1.5.

[2] Stepień M, Latos-Grazyński L, Sprutta N, Chwalisz P, Szterenberg L: Expanded porphyrin with a split personality: a Hückel-Möbius aromaticity switch . In: Angew. Chem. Int. Ed. Engl . 46, No. 41, 2007, pp. 7869-73. doi : 10.1002 / anie.200700555 . PMID 17607678 .