Question

Which molecules will undergo aromatic bromination (`"Br"_2, "FeBr"_3`) the fastest? Why?

1. phenol
2. anisole
3. diphenyl ether
4. acetanilide
5. 4-bromophenol?

Answer 1

Phenol

Explanation:

The `-"OH"` group causes the aromatic ring to be strongly activated. The reaction can undergo at a reasonable rate without the need of `"Fe"` nor `"FeBr"_3` catalyst.

Answer 2

Aromatic bromination is an electrophilic aromatic substitution (EAS) reaction, which will require benzene to act as a nucleophile to acquire an electrophile.

Therefore, any directing groups that activate the ring will make it react more quickly with respect to aromatic bromination.

Activating groups are more electron-donating groups (EDGs), and deactivating groups are more electron-withdrawing groups (EWGs).

• Phenol, having a good EDG, strongly activates the ring.
• Anisole, having an ether substituent, has hyperconjugation between the oxygen's filled `pi` orbital electrons and the methyl's `"C"-"H"` `sigma`-bonding electron pair as a opposing effect to the electron donating behavior of `-"OR"`, which weakens the electron-donating capacity of the substituent, giving anisole a less strongly-activated ring.
• Diphenyl ether, having only one ether group sharing two aromatic rings, only activates one of the rings. Not a great candidate, and worse than anisole.
• Acetanilide, having a decent EWG, is a moderately-activated ring.
• 4-bromophenol has both an `-"OH"` and a `-"Br"` on opposite sides. `-"OH"` is a strongly-activating group (high-lying `sigma` MO), but `-"Br"` is a weakly-deactivating group (due to its filled `pi^"*"`, rather than filled `pi` MOs). These, being para to each other, counteract each other and make the ring overall approximately moderately-activated, depending on the extent of deactivation. Either way, not the best.

Therefore, since phenol contains the most strongly-activated ring towards EAS, it participates fastest in EAS.