Simple aliphatic molecules with unconjugated double bonds react
quickly with bromine or chlorine to form saturated, dihalogenated
If the Kekule' structure for benzene were correct, one would expect
the same rapid halogenation reaction:
This is not what happens at all. The reaction is slow, and only
results in the substitution of first one and then two bromine atoms
for hydrogens around the ring, with the delocalized ring structure
remaining intact. (Ortho-, meta-, and para-dibromobenzene, with
the substituted bromine atoms adjacent, separated by one carbon,
and diametrically across the ring, are structural isomers of one
The reason why the ring delocalization is not affected, of course,
is that delocalization confers 40 kcal mole-1
of extra stability on the molecule. Part or all of this 40 kcal
mole-1 would have to be supplied
from an outside source to eliminate some of the delocalization in
a chemical reaction. Hence aromatic molecules have a slowness to
react that is more like the behaviour of alkanes than alkenes.
As with the straight-chain hydrocarbons, so with aromatic molecules.
The easiest chemical derivatives to prepare are the chlorides and
bromides. These are the gateways to the great variety of organic
compounds that are the subject of the next chapter.