Question

What is the difference between aliphatic and aromatic compounds?

Answer 1

Well, an aromatic compound has `"4 degrees of unsaturation"`....What does this mean?

Explanation:

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.