Simple aliphatic molecules with unconjugated double bonds react quickly with bromine or chlorine to form saturated, dihalogenated molecules:

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 another.)

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.