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Hydroxyl groups and Benzene
- Thread starter tpwelie34
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It all boils down to the role benzene plays in each mechanism.
In electrophillic aromatic substitution, the benzene acts as the nucleophile and attacks an electrophile. The lone pairs on the OH group makes benzene more negative and therefore a better nucleophile hence "activating the benzene ring"
Conversely, when dealing with nucleophilic aromatic substitution, the benzene plays the role of the electrophile. Therefore the OH group in this scenario would "deactivate" the benzene because the lone pairs on the oxygen is making the benzene more negative which is not how you make an electrophile more susceptible to a nucleophilic attack.
In general, deactivating / activating groups for electrophilic aromatic substitution plays an opposite role in nucleophilic aromatic substitution.
One last thing, the directing rules for EAS and NAS are opposite. If you draw out the resonance structure and remember whether you want your benzene to be a nucleophile or electrophile, it'll all make sense.
🙂
In electrophillic aromatic substitution, the benzene acts as the nucleophile and attacks an electrophile. The lone pairs on the OH group makes benzene more negative and therefore a better nucleophile hence "activating the benzene ring"
Conversely, when dealing with nucleophilic aromatic substitution, the benzene plays the role of the electrophile. Therefore the OH group in this scenario would "deactivate" the benzene because the lone pairs on the oxygen is making the benzene more negative which is not how you make an electrophile more susceptible to a nucleophilic attack.
In general, deactivating / activating groups for electrophilic aromatic substitution plays an opposite role in nucleophilic aromatic substitution.
One last thing, the directing rules for EAS and NAS are opposite. If you draw out the resonance structure and remember whether you want your benzene to be a nucleophile or electrophile, it'll all make sense.
🙂
