Benzene and substituted benzenes
||Chapter 11 : Arenes and Aromaticity
As a functional group, benzene, substituted benzenes and polybenzenes are called "arenes".
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-
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.
- In the absence of polar substituents,
arenes are typical of hydrocarbons : low melting and boiling points, low solubility
in polar solvents.
Which representation is best ?
- 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 π-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).
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.
- 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 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).
directly on the aromatic ring:
- The cyclic array of pi-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
Electrophilic Aromatic Substitution (see chapter