(N/A) Benzene was isolated by Michael Faraday in $1825$. The empirical formula of benzene is $CH$,the molecular mass is $78$,and the molecular formula of benzene is $C_{6}H_{6}$.
$\rightarrow$ Benzene possesses a high degree of unsaturation. This molecular formula did not account for its relationship to corresponding alkanes,alkenes,and alkynes. Due to its unique properties and unusual stability,it took several years to assign its structure.
$\rightarrow$ On the addition of three moles of $H_{2}$,it forms $C_{6}H_{12}$; with three moles of $Cl_{2}$,it forms $C_{6}H_{6}Cl_{6}$. Also,benzene was found to be a stable molecule and forms a triozonide,which indicates the presence of three double bonds.
$-$ Benzene was further found to produce one and only one monosubstituted derivative,which indicated that all six carbon and six hydrogen atoms of benzene are identical.
On the basis of this observation,August Kekulé in $1865$ proposed the following structure for benzene,having a cyclic arrangement of six carbon atoms with alternate single and double bonds and one hydrogen atom attached to each carbon atom.
$-$ The Kekulé structure indicates the possibility of two isomeric $1,2-$dibromobenzenes. In one of the isomers,the bromine atoms are attached to the doubly bonded carbon atoms,whereas in the other,they are attached to the singly bonded carbons.
$-$ However,benzene was found to form only one ortho-disubstituted product. This problem was overcome by Kekulé by suggesting the concept of the oscillating nature of double bonds in benzene.