Return to Contents Chapter 11 : Arenes and Aromaticity Ch 11 contents
Benzene and substituted benzenes

As a functional group, benzene, substituted benzenes and polybenzenes are called "arenes".

Nomenclature:
Functional group suffix = -benzene (review)
Functional group prefix = phenyl-
Substituted benzenes are usually named as such. The relative positions can be denoted as 1,2- = ortho-, 1,3- = meta- and 1,4- = para- substitution.
When polysubstituted, the numbers alone are used, e.g. 1,2,3-trimethylbenzene.
Many simple monosubstituted benzenes have common names that are still in use - you will need to be familiar with these.

Physical Properties:

Structure:
 
Kekule representations Robinson representation
Which representation is best ?

In benzene all the CC bonds are known to be of equal length (above) so there are no C=C and C-C. This is best represented by the resonance hybrid in the Robinson form.
However, since the key to organic chemistry is being able to understand mechansims and drawing curly arrows to account for the positions of the electrons, the Kekule structures give a more precise description of the electron positions that can avoid confusion. Therefore, it is a good idea to use a Kekule representation.

Stability:

Reactivity:
 
electrostatic potential for benzene The image shows the electrostatic potential for benzene. 
The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density. Note the nucleophilic character of the aromatic pi system. 

The reactivity issues can be separated into two types of reactions: 

  • reactions of electrophiles directly on the aromatic ring, and
  • reactions of the substituents (since the neighbouring aromatic group influences its reactivity).

For reactions directly on the aromatic ring:

generic electrophilic aromatic subsitution reaction
Electrophilic Aromatic Substitution (see chapter 12)

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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary