Final answer:
The number of resonance structures for an aromatic ring with an activating substituent depends on the nature of that substituent and the overall molecular structure. Activating groups like hydroxyl can donate electrons, increasing the number of potential resonance forms.
Step-by-step explanation:
The number of resonance structures that can be formed when a substituent on an aromatic ring is an activating group, such as the hydroxyl group (-OH), depends on the nature of the substituent and its position on the aromatic ring. Activating groups, like -OH, donate electrons to the benzene ring, enhancing its electron density and making it more reactive towards electrophiles. As a consequence, these groups increase the number of possible resonance forms because they create more locations on the ring where a positive charge can be stabilized. However, the exact number of resonance structures will also depend on the specific architecture of the molecule, particularly the locations of other substituents if present.
With an activating group such as -OH, the benzene ring can have multiple resonance structures where the lone pair of electrons from the oxygen atom can participate in resonance, delocalizing over the entire ring. This enhances the stability of the molecule and may make it more nucleophilic in the case of nucleophilic aromatic substitutions.