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Arenes are also known as aromatic hydrocarbons. Arenes are an important class of hydrocarbons. They are highly un reactive even though they are highly unsaturated.

Benzene (C6H6) is the simplest member in this series. It is a mono-cyclic aromatic hydrocarbon (Fig. 16.16). Arenes with more than one cyclic system are also known, e.g. naphthalene (C10H8). The general formula of arenes is CnH2n-6y, where y is the number of cyclic rings in the arene. For benzene, y = 1 and n = 6 and hence the formula of benzene is C6H6. Similarly, the formula of naphthalene (n = 10 and y = 2) is C10H8 (Fig. 16.16).

Fig. Structures of benzene and naphthalene

Structure of Benzene
The molecular formula of benzene is C6H6. The six carbon atoms in benzene are linked together to form a hexagonal ring system. One hydrogen atom is attached to each of the carbon atoms in the benzene molecule. The molecule of benzene is a planar molecule, i.e. all the six carbon atoms and six hydrogen atoms are in one plane. In other words, a molecule of benzene is flat. In order to account for the tetra-valency (valency of 4) of a carbon atom, Kekule in 1945 proposed that the three double bonds are present in conjugation with each other, i.e. in the benzene ring there is an alternate system of double and single bonds. Such double bonds are called conjugated double bonds.
However, the proposed structure of benzene with alternate double and single bonds could not explain the following:
  1. Stability
    Benzene is highly stable. It is quite unreactive in spite of the presence of three double bonds. It behaves as a saturated hydrocarbon as substitution reactions are more common than addition reactions.
  2. Only one disubstituted product
    When two hydrogens in benzene are substituted, then according to the Kekule structure two isomers should exist . Only one such isomer is known. For example, the two possible ortho-dichlorobenzenes are:

Fig. Two possible isomers of o-dichlorobenzene

  1. Identical carbon-carbon bond length
    Kekule's formula for benzene contains carbon-carbon single bonds and carbon-carbon double bonds. Therefore, a benzene molecule is expected to have bond lengths as 154 pm (for the carbon-carbon single bond) and 134 pm (for carbon-carbon double bond). However, it has been found by X-ray studies that all the carbon-carbon bonds in benzene have the same bond length (139 pm) and the bond angle is 120° .

Benzene as Resonance Hybrid

In order to overcome these objections Kekule suggested that the double bond in the benzene rings keeps changing its position, i.e. benzene is a reasonance hybrid of the two forms. The resonating electrons are also represented by showing a circle in the benzene ring . The resonance reduces the energy of the system and is responsible for the greater stability of benzene.

Fig. Resonance in benzene

The structure of benzene and bond angle of 120° in between the carbon atoms are better explained on the basis of the concept of sp2 hybridization and delocalization of p-electrons.

Delocalization in Benzene

The carbon atoms of benzene are sp2 hybridized and, therefore, the bond angle is 120°. The 2p-orbital is perpendicular to the plane of three sp2 hybrid bonds. The 2p-orbital extends on both sides of the ring of carbon atoms. The six electrons in the 2p-ortbital of each carbon atom are so close to each other that sideways overlapping is possible.This forms π -bonds which are extended (spread or delocalized) over all the six carbon atoms, forming an electron cloud above and below the plane of the ring of the carbon atoms.

Fig. Bond angle in benzene

Fig. In benzene p-orbital forms over and below the plane of ring in benzene

Delocalization of p-electrons is responsible for the extreme stability or unreactive nature.

Isomerism in Arenes

Benzene is a symmetrical molecule and all the hydrogen atoms are identical. Therefore, monosubstituted benzene exists only as one isomer.

However, three position isomers (Fig) are known in the case of disubstituted benzene, e.g. xylene (dimenthylbenzene). More position isomers are possible in the case of tri- or tetra-substituted benzene. Moreover, xylene (C8H10) is also isomeric with ethylbenzene. 

Fig.  1, 2-Dimethylbenzene or ortho-xylene or o-xylene

Fig. 1, 3-Dimethylbenzene or meta-xylene or m-xylene

Fig.1, 4-Dimethylbenzene or para-xylene or p-xylene

Fig. Ethyl benzene

Aromaticity in Benzene and Related System


Aromatic compounds are those which resemble benzene. But there are many substances which do not bear any resemblance to benzene superficially. Thus, we are to know the properties which impart aromatic character to the molecule. The common properties of the aromatic compounds are:
  1. These are highly unsaturated compounds but are resistant to addition reactions. They show substitution reactions instead.
  2. They are usually stable. They possess low values of heats of hydrogenation and heats of combustion than expected.
  3. All aromatic compounds undergo electrophilic substitution reactions like those of benzene.
  4. These are cyclic compounds containing five, six or seven membered rings and are found to have flat molecules, i.e. Planer Structure when only delocalisation is possible.
  5. An aromatic compound has a delocalised π cloud which is uniformly spread on the whole ring system.
  6. Huckel's Rule: It states that in a conjugated, planar, cyclic system of the number of delocalised π -electrons is(4n+2) where ' n' is an integer , i.e, 1,2,3 etc.
Examples of aromatic compounds obeying Huckel's Rule


(4n+2)π electrons

Structures & Names of Aromatic Compounds







(14π electrons  n = 3)


Cycloctatetrene ,

n = 1 (4n+2) = 6π - electron but it has 8π electron. It does not obey Huckel's rule so it is not aromatic.

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