STRUCTURE OF BENZENE

1.    Molecular formula:

The molecular formula of benzene as determined on the basis of chemical analysis and molecular mass determination is C₆H₆, and it corresponds to the general formula C_nH_n (n=6).

2.    Straight-chain structure not possible:

§     The molecule suggests a high degree of unsaturation since it has eight hydrogen atoms less than the alkane corresponding to the formula C₆H₁₄.

§     Keeping in view the tetra-co-valency of carbon, the following open-chain structures can be suggested.

              

              

§  However, the structures listed above are not in agreement with some of the properties of benzene.

For example:

i)             Benzene does not decolorize bromine water as is done by alkenes & alkynes, i.e., Benzene does not undergo addition reaction, instead undergo substitution reactions under suitable conditions.

ii)            Alkaline KMnO₄ solution (Bayer’s reagent) does not react with benzene in cold.

iii)           Benzene forms only one kind of mono-substituted product C₆H₅X (say X=Cl).

iv)           Benzene forms three kinds of di-substitution products, C₆H₄X₂ or C₆H₄XY, distinguished as ortho - (1, 2), meta - (1, 3) and para – (1, 4) substitution product.

3.    Ring (cyclic) structure for Benzene:

i)             Benzene gives only one mono-substituted product when subjected to the halogenation.

This indicates that all the six-carbon, as well as hydrogen atoms in the molecule, are identical.

ii)            Benzene forms three kinds of di-substitution products, C₆H₄X_2, or C₆H₄XY. {Distinguished as ortho - (1, 2), meta - (1, 3) & para - (1, 4) substitution products.}

§  In order to account for the above facts, a cyclic or ring structure for benzene was suggested.

§ A number of structures were suggested by different persons. However, the most accepted structure is by Kekule.

§  In 1865, Kekule (August Kekule) suggested a hexagonal structure for benzene composed of six carbon atoms joined by alternate single & double bonds and with one hydrogen atom attached to each one of them.

Evidence in favor of Kekule’s structure:

i)             It explains the addition of three molecules of hydrogen, chlorine, and ozone due to the presence of three double bonds.

ii)            It explains the formation of only one mono-substitution product due to the equivalence of six hydrogen atoms.

iii)           It also explains the formation of di-substitution products:

iv)           X-ray crystallographic studies of the benzene show the presence of hexagonal structure.

v)            The synthesis of benzene from three molecules of acetylene also supports the Kekule structure.