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Hückel's Rule

In 1931, Erich Hückel postulated that monocyclic (single ring) planar compounds that contained carbon atoms with unhybridized atomic p orbitals would possess a closed bond shell of delocalized π electrons if the number of π electrons in the molecule fit a value of 4 n + 2 where n equaled any whole number. Because a closed bond shell of π electrons defines an aromatic system, you can use Hückel's Rule to predict the aromaticity of a compound. For example, the benzene molecule, which has 3 π bonds or 6 π electrons, is aromatic.

 
  • Number of π electrons = 4 n + 2
  • 6 = 4 n + 2
  • n = 1

However, 1,3,5,7‐cyclooctatetraene, which has 4 π bonds or 8 π electrons, is not only nonaromatic but is actually considered antiaromatic because it is even less stable than the open‐chain hexatriene.

  • Number of π electrons = 4 n + 2
  • 8 = 4 n + 2
  • n = 1.5

Nomenclature

In IUPAC nomenclature, benzene is designated as a parent name. Other compounds that contain the benzene molecule may be considered as substituted benzenes. In the case of monosubstitution (the replacement of a single hydrogen), the prefix of the substituent is added to the name benzene.





In other cases, the substituent, along with the benzene ring, forms a new parent system.





When a benzene molecule is disubstituted (two hydrogens are replaced), two nomenclature methods exist. Either a number system or name system indicates the relative position of one substituent to the other. In the number system, one substituent is given the number one position and the second substituent is assigned the lower possible second number. The number position is given to the atom or group that has the higher priority as determined by the Cahn‐Ingold‐Prelog nomenclature system rules.




Notice that in the previous examples, the atom of the higher atomic weight is given the higher priority (Br = 79.1 versus Cl = 15.5, and I = 126.0 versus N = 14.0). These assignments are based on the priority rules of Cahn‐Ingold‐Prelog nomenclature.

In the name system, one carbon atom containing a substituent is considered to be the initial (locator) position. The carbon atom bonded to the other substituent is then located by the number of carbon atoms separating it from the locator position, as shown in Figure 1.



Figure 1

The ortho position is one removed from the initial substituent's position. The meta position is two removed, and the para is three removed.





Unlike the number system, you can assign an equally correct name with the names of the substituents reversed.

Benzene compounds that contain three or more substituents are always named by the number system. In this system, numbers are assigned to substituents so that the substituents have the lowest possible combination of numbers.




Reactions of benzene

Although the resonance structures of benzene show it as a cyclohexatriene, because of its fully delocalized π system and the closed shell nature of this π system, benzene does not undergo addition reactions like ordinary unsaturated compounds. The destruction of the π electron system during addition reactions would make the products less stable than the starting benzene molecule. However, benzene does undergo substitution reactions in which the fully delocalized closed π electron system remains intact. For example, benzene may be reacted with a halogen in the presence of a Lewis acid (a compound capable of accepting an electron pair) to form a molecule of halobenzene.

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