Benzene ring properties and structure || What is Benzene Used for?

After the discovery of any organic compound, its properties are studied and its structure is established on the basis of its properties. The structure of benzene and benzene ring is thus established.

Molecular Formula

Based on the quantitative determination of elements and the use of molecular weight determination, it is known that the molecule formula of benzene is C₆H₆.

Open Chain Structure of Benzene

The molecule of the corresponding alkane of benzene is C₆H₁₄ according to the formula C_nH_2n. Benzene ring has eight hydrogen atoms less than it. Therefore, if the carbon atom forms an open chain in the structure of benzene, it should have four double bond or corresponding double bond and three bond. On this basis, the following open chain structures of benzene are possible.

(A)             HC=C-CH₂-CH2-C=CH

(B)             H2C=CH-C≡C-CH=CH2

(C)             H3C-C≡C-C≡C-CH3

Open chain structures of benzene are not possible due to the following reasons.

I – The above structures demonstrate that, like ethylene and other aliphatic unsaturated hydrocarbons, benzene will also color Br₂/CCl₄ and change the color of the Bayer reagent. Benzene does not do this. Hence the above structure of benzene is defective.

II – Benzene exhibits halogensation, nitrification, sulfonization, and other substitution reactions easily. In these reactions one or more hydrogen atoms present in the benzene molecule are displaced by other atoms or groups.

III – Aliphatic unsaturated hydrocarbons do not exhibit this type of reaction. Therefore, these reactions of benzene cannot be explained on the basis of the above structures.

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The above structures show that one molecule of benzene will add four molecules of hydrogen. In fact, one molecule of benzene is the sum of three molecules of hydrogen. Therefore, the above structures of benzene ring are defective.

From the above discussion, it is clear that the detailed chain structure of benzene is not possible, there are three carbon-carbon two-bound present and the nature of the double-bond present in it is different from the nature of double-bond present in the unsaturated hydrocarbons.

Benzene Ring Structures

Wide chain structures of benzene ring are not possible. Hence its structure is cyclic. There are three twins in it. This gives only one mono-substitution product, meaning that the positions of all six hydrogen atoms in it are the same. Based on these facts, its three cyclical structures are possible.

Benzene ring gives three isomeric double-substitution products. Only 2 – 2 isomeric double-substitution products are possible from structures I and II. Three isotropic two-substitution products are possible from structure III.

On this basis, structures I and II of benzene are not possible and structure III is possible structure of benzene. This structure was proposed by kekule in 1865 and is called Kekule structure.

Kekule’s Dynamic Formula

The two-substituted products of Structure III are as follows:

1, 2-product or ortho-product

1, 3-product or meta-product

1, 4-product or pera-product

A closer study of structure III suggests that two ortho products are possible.

These two products are different from each other. The reason for this is that one of these products does not have a binding between the carbon atoms that carry the substituent group, while the other product has a binding between the carbon atoms that sister the substituent group.

Thus a total of four double-substitution products are possible from the structure of the cake, while a total of three double-substitution products are obtained from benzene.

kekule introduced the kinetic formula of benzene in 1872 to correct this defect of benzene’s structure III. According to the kinetic formula of kekule, benzene ring is an equilibrium mixture of the following two structures, in which these two structures rapidly change into each other.

Since the two Ortho-replacement products rapidly change into each other, they cannot be separated and are considered to be the same product.

Kekule’s Benzene Ring Structure:

The following evidence is prominent in favour of the formula of Kekule

• This explains the sum of three molecules of hydrogen on one molecule of benzene, three molecules of chlorine and three molecules of ozone.
• This explains the formation of one mono-substitution and three double-substitution products from benzene.
• This explains the synthesis of benzene from acetylene.
• This explains the formation of 3 molecules of glycals by ozonisation of benzene.
• Spectral analysis of benzene and its parachor value confirm its formula