As a functional group, benzene and substituted benzenes 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.

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 curved 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 neighboring 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.

    As we saw in chapter 10, the positions adjacent to C=C, the allylic position, often show enhanced reactivity compared to simple alkanes due to the proximity of the adjacent p system.  Similarly, the positions adjacent to a benzene ring, known as the benzylic position also show enhanced reactivity compared to simple alkanes.
Students often confuse the term benzyl with phenyl, for example compare bromobenezene and benzyl bromide:
 
 

Benzylic carbocations
 

The p system of a benzene ring can stabilize an adjacent carbocation by donating electron density through resonance. Remember that delocalising charge is a stabilizing effect.

    Note that in the resonance forms of the benzylic cation, the positive charge is located on the ortho and para positions of the benzene ring, but not the meta positions.  This is reflected in the resonance hybrid.
    Due to the stability of these benzylic cations, they are readily formed as intermediates during chemical reactions, for example S_N1 reactions of benzylic halides.

Benzylic radicals

Benzyl radicals can also be stabilized by resonance in the same manner as shown above for carbocations.
 

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