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Benzene: Structure

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Benzene: Structure - Lesson Summary

Benzene (C₆H₆) was first isolated by Michael Faraday in 1825. Due to its characteristic properties and unusual stability, the determination of the actual structure of benzene took many years.

Following were taken into consideration to determine the structure of Benzene. The ratio of carbon to hydrogen in Benzene is the same. Thus it is highly unsaturated compound. When Benzene is treated with Ozone, it forms tri-ozonide. The formation of tri-ozonide indicates that it has three double bonds.

It produces only one mono-substituted derivative, which indicates that all its six carbon and hydrogen atoms are equivalent. On the basis of these facts, in 1865 August Kekule proposed the structure of Benzene in which the six carbon atoms were arranged to form a hexagonal ring with each carbon atom carrying one hydrogen atom.

The structure had alternate single and double bonds.  The structure of Benzene suggested by Kekule is now known as the Kekule’s structure. Kekule’s structure could not explain all the properties of benzene.

Limitations to Kekule’s Structure:
This structure cannot explain the observed bond length of carbon-carbon bonds which is 139 picometers. According to this structure two bond lengths are expected due to the presence of alternate single and double bonds they are 154 picometers due to carbon-carbon single bonds and 133 picometers due to carbon-carbon double bonds.

This structure cannot explain the formation of only one 1, 2 - disubstituted benzene, according to this structure it forms two isomeric 1, 2 disubstituted benzenes.

Ex: In isomer I, the two chlorine atoms are attached to carbon -carbon single bond, whereas in isomer II, the two chlorine atoms are attached to carbon-carbon double bond. Hence there is a possibility of two isomeric 1, 2- dichlorobenzene. But experimentally only one 1, 2 -dichloro benzene product is obtained.

In order to explain, Kekule proposed two interchangeable isomeric structures for benzene.  These structures are obtained by interchanging the positions of the double bonds, which happens due to the delocalization of the pi bonds between the six carbon atoms in the benzene ring. However, Kekule’s structures alone do not explain the unusual stability and the characteristic substitution reactions of Benzene, which was explained by using the of ‘Resonance’ in Kekule’s structures.

If a molecule represented by two or more structures and these structures differ only in the arrangement of electrons, then the molecule is said to be in Resonance and the set of several possible structures are called resonance structures or canonical structures or canonical forms.

The actual structure of benzene is a hybrid of these two resonance structures. It is called a resonance hybrid. This resonance hybrid is more stable than any of the contributing resonating structures.

The difference between the energy of any one of the equivalent contributing structure and the energy of the resonance hybrid is known as resonance energy. The resonance energy of benzene is found to be 36 kilo Cal/mole. Benzene prefers to undergo substitution over addition, due to resonance. 
In 1931, Eric Huckel proposed the modern theory of aromaticity or aromatic character of compounds. Cyclic rings or heterocyclic rings or cyclic ions are said to be aromatic, if they show the essential properties such as complete delocalization of the pi electrons, planarity and presence of (4n+2) pi electrons in the ring.

The molecule should contain a cyclic cloud of delocalized pi electrons. For delocalization of pi electrons, the ring must be planar to allow cyclic overlap of the p-orbital.  Therefore for a molecule to be aromatic the ring must be planar.

In order to be aromatic a cyclic compound must contain a total of (4n+2) pi electrons, where n is an integer, which can be equal to 0, 1, 2, 3 and so on. For a compound to show aromaticity, it must be planar or cyclic and it should have delocalized (4n+2) pi electrons. This is known as the Huckel’s rule.

Thus, according to Huckel’s rule, the aromatic compounds should have a delocalized electron cloud of pi electrons such as 2, 6, 10, 14 electrons.

Examples for aromatic compounds are benzene with 6 pi electrons, naphthalene with 10 pi electrons, and anthracene with 14 pi electrons, when n=1 ,2,3 respectively


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