A useful metric when we consider organic formulae is the #"degree of unsaturation...."#
An alkane, general formula #C_nH_(2n+2)#, is said to be FULLY saturated... And each two hydrogens LESS than this formula specifies one degree of saturation. Should there be oxygen in the formula we assess the degree of saturation directly....if there is a nitrogen atom in the formula we subtract #NH# from the formula before the assessment...halogens count for a hydrogen...
And each #"degree of unsaturation"# specifies a #C=C# junction , or another unsaturated bond, #C=N#, or #C=O# OR a ring junction...try it out for ethylene, #H_2C=CH_2# or cyclopropane, #C_3H_6#, or acetylene, with two degrees of unsaturation..
And so for benzene....we gots #C_6H_6#...formally FOUR degrees of unsaturation in #C_6H_6# versus saturated hexane, #C_6H_14#... And of course benzene is an aromatic compound, whereas hexane is aliphatic...and not constrained in a ring.
And as a consequence of this unsaturation, benzene may undergo aromatic electrophilic substitution...
