Final answer:
The phenoxide ion, the base anion of phenol, is a more powerful activating group than neutral phenol because it provides greater resonance stabilization, making the benzene ring more electron-rich and reactive.
Step-by-step explanation:
When comparing the activating effects of phenol and its base anion on a benzene ring, we find that the base anion, phenoxide (ArONa), is a more powerful activating group than the neutral phenol (ArOH). The reason behind this is due to the greater resonance stabilization provided by the phenoxide ion. In phenol, the hydroxyl group (-OH) is already an activating group as it donates electron density into the benzene ring through resonance. However, once phenol loses a proton to become its base anion (phenoxide), the negative charge on the oxygen atom further increases the electron-donating ability through resonance, making the benzene ring even more nucleophilic and hence more reactive towards electrophilic aromatic substitution reactions.
Consequently, phenoxide, the anionic form of phenol, is a stronger activating group because it can more effectively spread the negative charge over the aromatic ring. This effect enhances the electron-rich nature of the benzene ring, making it more susceptible to electrophilic attack. In contrast, the presence of a neutral hydroxyl group provides less electronic enrichment to the ring.