The -OH group in phenol activates the benzene ring, allowing halogenation to occur at low temperatures in solvents like CHCl₃ or CS₂, forming monobromophenols. Phenol’s high reactivity is attributed to the activating effect of the -OH group, eliminating the need for a Lewis acid like FeBr₃.
How does the presence of the -OH group in phenol affect the halogenation reaction with bromine in solvents of low polarity, and why does phenol not require a Lewis acid like FeBr₃ for bromine polarisation?
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The -OH group in phenol enhances its reactivity in halogenation reactions with bromine in solvents of low polarity. Phenol’s oxygen donates electron density through resonance, activating the ring and making it more nucleophilic. This facilitates electrophilic attack by bromine, leading to bromination. Unlike benzene, phenol doesn’t require a Lewis acid like FeBr₃ for bromine polarization. The oxygen lone pairs in phenol can directly interact with bromine, facilitating bromine’s attack on the ring. The -OH group’s electron-donating nature increases the nucleophilicity of the ring, promoting halogenation without the need for an additional Lewis acid catalyst.