What is Huckel's rule



With the Hückel rule, named after Erich Hückel, can be used to determine whether a chemical compound is an aromatic molecule. It can be derived from the more comprehensive Hückel approximation.

A cyclically fully conjugated molecule is particularly stable with 4n + 2 π electrons (an odd number of electron pairs) and is called an aromatic. With 4n π electrons (an even number of electron pairs) it is unstable and is called an anti-aromatic. For n positive, whole numbers, including zero, can be used (n = 0, 1, 2, 3, ...).

Examples

Hydrocarbons

The most important example is benzene (benzene), which fulfills the rule with 6 π electrons (n ​​= 1) and, as a hexagon, also represents the ideal bond angle for sp2- forms hybridized carbons. Polycyclic (condensed) aromatics are formed 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

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) the heteroatom provides a whole lone pair of electrons. 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 relationships described, the nitrogen in pyrrole is bonded to a hydrogen, but not in pyridine. Uncharged, cyclically conjugated 6-membered rings with oxygen or sulfur are not possible at all.

Aromatic ions

   

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.

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 compounds (Musulin-Frost diagram) can be calculated particularly easily. It does not make any statements about systems with several rings, e.g. polycyclic aromatic hydrocarbons (PAH).

The following PAHs behave according to the Hückel rule: Naphthalene (10 e-), Anthracene (14 e-), Phenanthrene (14 e-).

The following PAHs behave contrary to the Hückel rule: Pyrene (16 e-), Benzopyrene (20 e-), Biphenyl (12 e-), Fluoranthene (16 e-).

See also

Category: Chemical Bond