Hückel's rule

With the Hückel rule, named after Erich Hückel, can be determined whether it is an aromatic molecule with a chemical compound. They can be derived from the broader Huckel approximation.

A through-conjugated cyclic molecule with [4n 2] π - electrons ( an odd number of electron pairs, for example: n = 1, [4.1 2 = 6], three electron pairs ) are particularly stable and is referred to as aromatic.

Of [ 4n] π - electrons ( an even number of pairs of electrons, for example: n = 2, [4.2 = 8 ], four -pair ) it is unstable, and is referred to as Antiaromat. Here, n are positive integers including zero can be used (n = 0, 1, 2, 3 ..).

Examples

Hydrocarbons

The most important example benzene having 6 π - electrons ( n = 1) is satisfied, the control and also provides the ideal hexagon as bond angle for sp2 -hybridized carbons. Polycyclic ( condensed ) aromatic compounds are formed when a further 4 carbon atoms adds one or more times at an edge for them, however, applies the Hückelregel not more (see restrictions ).

Heteroaromatics

Aromatic compounds, which do not consist of only carbon and hydrogen are known as heteroaromatic. You can be rings 5 ​​or 6 - he - he. In 5-series rings ( eg, pyrrole, furan, thiophene, selenophene ) represents the heteroatom an entire free electron pair available. With 6- rings ( e.g., pyridine ) is the free electron pair of the hetero atom to the outside.

Note that is bonded because of the described bonding of the nitrogen in the pyrrole to a hydrogen, but not pyridine. Uninvited, cyclic conjugated 6- rings with oxygen or sulfur are not possible.

Aromatic ions

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

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

The Hückel rule applies only to monocyclic molecules. Only there can the Hückel approximation make a general statement as to let the energy schemes of these compounds is particularly easy to calculate ( Musulin Frost diagram). It does not state systems with multiple rings, eg, polycyclic aromatic hydrocarbons ( PAHs).

The following PAHs behave according to the Hückelregel: naphthalene (10 e-), anthracene (14 e-), phenanthrene (14 e-).

The following PAHs behave contrary to the Hückelregel pyrene (16 e-), benzopyrene (20 e-), fluoranthene (16 e-).

Furthermore, the rule applies only to molecules that can have two sides. The molecule has only one side, such as a Möbius strip, the aromaticity disappears. There are even molecules that can have (one or two-sidedness ) both properties, depending on what material they are dissolved.