Discuss the structural elucidation of benzene.

Explanation

(1) From elemental analysis and molecular mass determination,the molecular formula of benzene is found to be `C_(6)H_(6)`.
(2) On the basis of molecular formula benzene is highly unsaturated compound.
(3) Benzene does not show alkene propeties. Benzene is quite stable. It does not decolourise cold aqueous solution of potassium paramanganate.
(4) Benzene shows cyclic structure. (a) Benzene contains three double bonds. Benzene adds three moles of hydrogen in presence of nikel catalyst to form cyclohexane.
`C_(6)H_(6) + 3H_(2) underset(" " Ni" ")overset(180^(@)C)(rarr) C_(6) H_(12)`
(b) All the six hydrgoen atoms in benzene are identical - benzene reacts with bromine in presence of `FeBr_(3)` catalyst to form monobromobenzene.
`C_(6)H_(6) + Br_(2) overset(FeBr_(3)) (rarr) C_(6) H_(5) Br + HBr`
(5) Kekule propose benzene is planar ring structure with alternate single and double bonds.

(6) To overcome this drawback, Kekule suggested that benzene is a mixture of two forms, which are in rapied oscillation.

(7) It is found that the bond length in benzene is same for fill C - C bonds (o.139 nm). this lies between C - C single bond (0.154 nm) and C - C double bond length (0.314 nm).
(8) Resonance structure: Resonance hybrid is more stable than structure 1 and 2 . the stability of benzene due to resonance is so great that `pi` bonds of the molecule will normally resist breaking this explain the stability of benzene.