Identify the reactions of phenol with bromine in two different media

Phenol is highly reactive toward electrophilic substitution because the lone pair on oxygen donates electron density into the benzene ring.

• In bromine water, phenol gives 2,4,6-tribromophenol readily.
• In non-polar solvent like chloroform, bromination is less extensive and usually gives mainly ortho- and para-bromophenol.

Why does the medium change the product?

The key difference is the nature of the solvent:

• Water is a polar solvent. It helps in stronger polarization of $Br_2$, making bromine more electrophilic.
• Chloroform is much less polar, so $Br_2$ is less polarized and hence less reactive.

Therefore, bromination in water proceeds much more rapidly and further substitution occurs, giving tribromophenol.

Check each option

A: Hyperconjugation in substrate
Incorrect. Hyperconjugation is not the reason for the difference here.

B: Polarity of solvent
Correct. The solvent polarity affects the polarization and electrophilic character of bromine.

C: Free radical formation
Incorrect. Bromination of phenol here is an electrophilic aromatic substitution, not a free-radical reaction.

D: Electromeric effect in the substrate
Incorrect. The major reason for the observed difference is the solvent medium, not a change in electromeric effect of the substrate.

Final answer

The difference in reaction is due to polarity of solvent.

$\boxed{\text{B}}$