How does an amino, or an hydroxyl group substituted onto a phenyl ring influence the reactivity of the ring?

1 Answer
Jul 12, 2017

It activates the ring towards electrophilic substitution.......

Explanation:

Consider substitution of #C_6H_5X#, i.e. a substituted phenyl ring by another electrophile. The substituting group, #X#, can either activate (or deactivate) the phenyl ring by pumping electron density into the ring OR withdrawing it.

And so for phenol, or aniline, the substituting group, nitrogen, or oxygen, is MORE electronegative than carbon, however, the lone pairs of electrons on the oxygen or nitrogen atoms are conceived to be delocalized into the carbocycle, and make the ring MORE reactive in electrophilic substitution reactions. Because electron density is donated to the phenyl ring, the #O-H# is somewhat weakened and thus phenol is weakly acidic (the charge density of the phenoxide anion is delocalized over the entire ring, and this entropy effect works to the same end; this effect is probably the more significant).

Note that hydroxyl or amine substitution activates all of the carbons on the ring, however, the ortho and para positions, are PARTICULARLY activated due to the resonance effects, i.e. the lone pair on the oxygen or nitrogen can be delocalized to these positions and make this centre more electron rich.

So practically, if we want to brominate benzene, we would treat #C_6H_6# with bromine in the presence of some Lewis-acid, i.e. #FeBr_3# or something similar. With phenol, the Lewis acid might even be omitted. #"2,4,6-tribromophenol"# would be a conceivable product.