Q.Assertion: Phenol forms 2,4,6-tribromophenol on treatment with bromine in carbon disulphide at 273 K.
Reason: Bromine polarises in carbon disulphide.

Concept: Electrophilic Aromatic Substitution -- phenol is highly activated toward bromination by the -OH group, but whether the reaction stops at mono- or goes all the way to tri-substitution depends on how strongly the solvent polarises Br2.

Reasoning:

1. In carbon disulphide (CS2) -- a non-polar solvent -- bromine is only weakly polarised, so at 273 K phenol undergoes controlled monobromination (mainly p-bromophenol), not tribromination.
2. 2,4,6-Tribromophenol forms only when phenol is treated with bromine in a strongly polarising, protic medium such as bromine water, at room temperature. …

1. Bromination in a polar solvent

In water (bromine water), phenol's strongly activated ring reacts with Br2 almost instantly at all three activated positions (2,4,6), because water strongly polarises the Br-Br bond, generating an effective electrophile:

C6H5OH + 3 Br2 (in H2O) -> 2,4,6-tribromophenol (precipitate) + 3 HBr

2. Bromination in carbon disulphide (a non-polar solvent)

CS2 is non-polar and cannot polarise the Br-Br bond nearly as effectively as water can. Bromine therefore stays largely as molecular Br2, a comparatively mild electrophile. At 273 K, this gives only monobromination, predominantly p-bromophenol (with some o-bromophenol), not the fully tri-substituted product.

3. Evaluating the assertion and the reason …

Method: Conceptual Analysis of Assertion-Reason Questions

Relevant Concept: Electrophilic Aromatic Substitution in Phenol — Effect of Solvent and Temperature

Steps:

1. Analyse the Assertion

   • Phenol reacts with bromine in carbon disulphide ($CS_2$) at 273 K to give 2,4,6-tribromophenol.
   • This is correct. In non-polar solvents like $CS_2$ or $CCl_4$, bromination occurs without further substitution beyond the tribromo product, because the reaction is controlled and does not proceed to ring oxidation.

2. Analyse the Reason

   • The reason states: "Bromine polarises in carbon disulphide."
   • This is incorrect. Bromine does not polarise significantly in $CS_2$ — $CS_2$ is a non-polar solvent. …

Here’s a breakdown of the common mistakes students make on this question and how to avoid them.

Mistake 1: Confusing the solvent’s role (polar vs. non-polar)

• The mistake: Students think that because bromine ($Br_2$) is a non-polar molecule, it cannot react unless the solvent “polarises” it. They assume carbon disulphide ($CS_2$) is a polar solvent that polarises bromine, making the Reason statement correct.
• Why it’s wrong: Carbon disulphide is non-polar. In a non-polar solvent, bromine does not get polarised. The reaction with phenol in $CS_2$ at 273 K stops at monobromination (forming p-bromophenol), not tribromination. The Reason statement is factually incorrect.
• How to avoid: Memorise the solvent effect:
  • Aqueous bromine water (polar medium) → tribromination (2,4,6-tribromophenol).
  • Non-polar solvent like $CS_2$, $CCl_4$, or $CHCl_3$ at low temperature → monobromination (mainly para product).

Mistake 2: Assuming the Assertion is correct because “phenol + bromine” always gives tribromophenol

• The mistake: Students recall that phenol reacts with bromine to give a white precipitate of 2,4,6-tribromophenol, so they mark the Assertion as correct without checking the solvent condition.
• Why it’s wrong: The Assertion specifically says “bromine in carbon disulphide at 273 K”. Under these conditions, the product is not 2,4,6-tribromophenol. The Assertion is therefore incorrect.
• How to avoid: Always read the reaction conditions (solvent, temperature) before confirming a product. For phenol bromination:
  • Aqueous/Bromine water → tribromophenol.
  • Non-polar solvent, low temp → monobromophenol.

Mistake 3: Misinterpreting the Assertion-Reason relationship

• The mistake: Students think that if the Reason is wrong, the Assertion must be correct (or vice versa). They choose option (C) “Assertion correct, Reason wrong” without verifying the Assertion.
• Why it’s wrong: Both statements need independent verification. Here, the Assertion is wrong (product is not tribromophenol under given conditions) and the Reason is also wrong ($CS_2$ does not polarise bromine). So the correct option is (D).
• How to avoid: Treat Assertion and Reason as two separate true/false statements. First, decide if each is true or false. Then, check if the Reason explains the Assertion (only if both are true). Use this checklist:
  1. Is Assertion true? → No.
  2. Is Reason true? → No.
  3. Therefore, option (D) is correct.

Mistake 4: Forgetting that $CS_2$ is non-polar …