In electrophilic aromatic substitution reactions, why is the the hydroxyl group an o,p-director?

Question

Zoe Alexander · Accepted Answer

The hydroxyl group is an electron-donating group, so it activates the ring. i.e. it donates electron density into the ring, instead of withdrawing it from the ring.

Consider the resonance structures of phenoxide. We would have:

You can see that only the 2, 4, and 6 positions can support the electron density from the hydroxyl group.

In electrophilic aromatic substitution (EAS), benzene acts as a nucleophile upon an electrophile.

So, if you had #"Br"_2# as an electrophile, with #"FeBr"_3# as the catalyst, then the benzene ring can act as a nucleophile in the EAS mechanism as follows:

Nicholas Antonuccio · Answer

undefined The hydroxyl group is an o,p-director in electrophilic aromatic substitution reactions because it can donate electron density through resonance, stabilizing positive charges on ortho and para positions of the aromatic ring. This occurs because the lone pair of electrons on the oxygen atom in the hydroxyl group can delocalize into the aromatic ring, increasing electron density at the ortho and para positions and making them more reactive towards electrophilic attack.