A saturated, unsubstituted hydrocarbon has a formula of #C_nH_(2n+2)#. Alkanes are said to be #"saturated"#. Try this out with methane, ethane, propane, etc. Each double bond, or ring junction, reduces the hydrogen count by 2, and so introduces #"1 degree of unsaturation"#. #"Cyclohexane"# or #"hexene"# thus have formulae of #C_6H_12#.
Aromatic compounds generally feature the benzene ring, which imposes #"4 degrees of unsaturation"#; i.e. consider the benzene molecule, #C_6H_6#, where hexane would by #C_6H_14#. The extra electron density on the aromatic ring (the six delocalized #pi# electrons) offers peculiar chemisty; distinct from the chemistry of the alkanes.
