Question

The difference in the reaction of phenol with bromine in chloroform and bromine in water medium is due to:

• A. Hyperconjugation in substrate
• B. Polarity of solvent
• C. Free radical formation
• D. Electromeric effect of substrate

Answer 1

The Correct Option is B

The reaction of phenol with bromine differs significantly depending on whether the solvent is chloroform or water. This difference can be attributed primarily to the polarity of the solvent, which is given as the correct answer among the options. Let us explore why this is the case by examining the mechanism and nature of phenol reactions in both solvents:

1. Phenol Reaction with Bromine in Chloroform:
   • Chloroform is a non-polar solvent. When phenol reacts with bromine in chloroform, the reaction is less facilitated by the solvent due to its non-polar nature.
   • The substitution of bromine tends to occur at the ortho and para positions, leading to mono-brominated products.
   • This is because the bromine molecule is less ionized in this solvent, resulting in a lesser degree of substitution.
2. Phenol Reaction with Bromine in Water:
   • Water is a highly polar solvent, which facilitates the ionization of bromine into bromonium ions. These ions are much more reactive compared to neutral bromine molecules.
   • As a result, phenol readily undergoes tribromination, where all ortho and para positions are substituted with bromine to give 2,4,6-tribromophenol.
   • The enhanced reactivity is due to the high polarity of water, which stabilizes the transition states through favorable dipole-dipole interactions.

Conclusion: The discrepancy in phenol's reaction with bromine when using different solvents can be attributed to the polarity of the solvent. The polarity affects the extent of ionization and stabilization of intermediates, leading to either mono- or tri-substituted products.

Now let's discuss why other options are incorrect:

• Hyperconjugation in Substrate: Hyperconjugation deals with the delocalization of \(\sigma\)-electrons in alkyl groups attached to a double-bonded system, which is not primarily involved in aromatically stabilized compounds like phenol.
• Free Radical Formation: This is typically involved in reactions like radical halogenations, which are not pertinent to the electrophilic aromatic substitution in phenol with bromine.
• Electromeric Effect of Substrate: While relevant to the instantaneous electron shift in double bonds upon interaction with electrophiles or nucleophiles, the reaction primarily depends on solvent effects in this case.

Thus, the correct answer that justifies the reaction difference in varying solvents for phenol is the polarity of the solvent.