1. Molecular formula:
Elemental analysis and molecular weight determination have proved that the molecular formula of benzene is C6H6. This indicates that benzene is a highly unsaturated compound.
2. Straight chain structure is not possible:
Benzene could be constructed as a straight chain but it not feasible since it does not show the properties of alkenes or alkynes.
For example, it does not decolorise the bromine water in CCl4
3. Evidence of cyclic structure:
(1) In the presence of Nickel, benzene reacts with hydrogen to give cyclohexane, a six membered ring. This proves that benzene is a hexagonal molecule with three double bonds.
(2) Benzene reacts with bromine in the presence of iron to give substituted C6H5Br. No isomers of C6H5Br was identified. On further reaction with bromine three isomeric disubstituted products C6H4Br2 are formed. On this basis Kekule proposed that benzene consists of ring of carbon atoms with alternate single and double bonds.
4. Resonance description of benzene:
The phenomenon in which two or more structures can be written for a substance which has identical position ofatomsis called resonance. The actual structure of the molecule is said to be a resonance hybrid of various possible alternative structures. In benzene. Kekule’s structure (I) and (II) represented the resonance structures and structure (III) is the resonance hybrid of structure I and II.
5. Spectroscopic measurements:
X-ray and electron dîflìaction studies indicated that all carbon-carbon bonds are of equal length which is in between that of a single bond (1 .45Å ) and that of a double bond (1.34Å ).
6. Molecular orbital structure :
(1). Benzene is a flat hexagonal molecule with all carbons and hydrogen lying in the same plane with a bond angle of 120º. Each carbon atom has sp2 hybrid orbitais of carbon, overlap with each other and with s-orbitals of six hydrogen atoms forming six sigma (σ) CH bonds and six sigma (σ) C – C bonds.
(2). All the σ-bonds in benzene lies in one plane with bond angle of 120º. Each C-atom in benzene possess an unhybridised p-orbital containing one electron. The lateral overlap of their p-orbitais produces 3π-bond, the six electrons of the p-orbitais cover all the six Catoms and are said to be delocalised. Due to this delocalisation, strong π-bond is formed which makes the molecule stable.
7. Representation of benzene:
Hence, there are three ways is which benzene can be represented.
