Chapter 11 : Arenes and Aromaticity

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:

• In the absence of polar substituents, arenes are typical of hydrocarbons : low melting and boiling points, low solubility in polar solvents.

• All 12 atoms in benzene, C₆H₆, lie in the same plane.
• Benzene has a planar, cyclic, conjugated structure.
• If one draws benzene as alternating C=C and C-C then the two different Kekule structures are obtained.
• These are two equally valid resonance contributors.
• Alternatively, these two forms can be combined in the resonance hybrid and the p system represented by a circle as in the Robinson structure.
• Note that all of the CC bonds are 1.4 A (between typical C=C and C-C distances).

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:

• The aromatic character of  benzene infers an inherent stability to the system. As a result, benzene is not particularly reactive compared to alkenes.
• This stability can be expressed by the resonance energy, which for benzene is about 36 kcal /mol.

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 p 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:

• The cyclic array of p-bonds is a region of high electron density so arenes are typically nucleophiles (like alkenes and alkynes).
• Unlike alkenes and alkynes (which undergo addition reactions), arenes typically undergo substitution reactions in which a group (usually -H) is replaced and the aromatic system is retained.
• The stability of the aromatic system favours substitution over addition which would destroy the aromatic system.

Electrophilic Aromatic Substitution (see chapter 12)