Reactants:
* Phenol (C6H5OH): An aromatic compound with a hydroxyl group (-OH) attached to the benzene ring.
* Bromine water (Br2(aq)): A solution of bromine (Br2) in water.
Products:
* 2,4,6-Tribromophenol (C6H2Br3OH): A white solid formed by the substitution of three bromine atoms onto the phenol ring.
* Hydrogen bromide (HBr): A colorless gas, usually dissolved in water to form hydrobromic acid.
Reaction Mechanism:
1. Electrophilic attack: Bromine (Br2) is a weak electrophile. However, in the presence of water, it gets polarized, making one bromine atom more electrophilic. This electrophile attacks the electron-rich benzene ring of phenol.
2. Formation of a carbocation: The attack leads to the formation of a positively charged intermediate called a carbocation.
3. Loss of a proton: The carbocation is unstable and loses a proton (H+) from the carbon adjacent to the site of bromination. This restores aromaticity to the ring.
4. Further bromination: The bromine atom on the phenol ring activates the ortho and para positions for further electrophilic attack by bromine. As a result, the reaction continues, leading to the formation of 2,4,6-tribromophenol.
Observations:
* Color change: The yellowish-brown bromine water rapidly decolorizes as the reaction proceeds.
* White precipitate: A white precipitate of 2,4,6-tribromophenol is formed.
Reaction equation:
C6H5OH + 3Br2(aq) → C6H2Br3OH + 3HBr
Important points:
* The reaction is highly influenced by the presence of the hydroxyl group on the phenol ring. The electron-donating effect of the hydroxyl group makes the benzene ring more electron-rich and susceptible to electrophilic attack.
* This reaction is also used as a test for the presence of phenol. If bromine water decolorizes rapidly and a white precipitate is formed, it confirms the presence of phenol.
* The reaction is often carried out in the presence of a catalyst like iron (III) bromide (FeBr3). The catalyst helps to polarize the bromine molecule and facilitate the electrophilic attack.
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