1. Activation of the ring: The hydroxyl group (-OH) attached to the benzene ring in phenol is an electron-donating group. It increases the electron density in the ring, particularly at the ortho and para positions. This makes the ring more susceptible to electrophilic attack.
2. Formation of a resonance stabilized intermediate: When bromine reacts with phenol, it forms a bromonium ion intermediate. This intermediate is stabilized by resonance with the lone pair of electrons on the oxygen atom of the hydroxyl group. This resonance stabilization makes the reaction more favorable.
3. Electrophilic attack: The bromonium ion, being a strong electrophile, readily attacks the electron-rich ortho and para positions of the phenol ring. The first bromine atom enters one of these positions, further activating the ring towards electrophilic attack.
4. Successive bromination: After the first bromination, the ring becomes even more activated due to the presence of the bromine substituent. This allows for the second and third bromine atoms to be introduced at the ortho and para positions, leading to trisubstitution.
5. Water as the solvent: Water, being a polar solvent, helps to stabilize the intermediate carbocations formed during the reaction. This further promotes the reaction.
Overall, the combination of the electron-donating effect of the hydroxyl group, the resonance stabilization of the intermediate, and the presence of water as a solvent makes phenol highly susceptible to trisubstitution in aqueous bromine solution.
It's important to note that the reaction can be controlled to obtain mono or di-brominated products by adjusting the reaction conditions. For example, using a cold solution of bromine in a non-polar solvent like carbon tetrachloride can lead to mono-bromination.
