The benzene molecule is unsaturated but the double bonds present inside the benzene ring are delocalized due to bond resonance (pi structure). This makes the double bonds of benzene much less reactive then more discreet double bonds (as in ethylene). This structure makes it behave more like a saturated compound, preferring substitution reactions over addition reactions. It is resistant to addition reactions across the double bond because such a reaction reduces the resonance stabilization energy. However, when reactions do occur, resonance stability is almost always re-established
