Q.14 The major product formed in the following reaction is
Question Overview
Phenol is treated with bromine (Br₂) in carbon disulfide (CS₂) at a low temperature (< 5°C). You are asked to identify the major product formed in this electrophilic aromatic substitution reaction.
Key Concepts Involved
1. Activating Effect of –OH Group
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The –OH group in phenol is a strong activating group.
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It donates electron density into the benzene ring via +R (resonance) effect.
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This makes the ortho (o-) and para (p-) positions highly reactive toward electrophiles like Br⁺.
2. Role of Solvent (CS₂)
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CS₂ is a non-polar solvent, so:
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It reduces the reactivity of bromine.
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Prevents excessive bromination (unlike aqueous bromine).
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Hence, controlled mono-bromination occurs.
3. Effect of Low Temperature (< 5°C)
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Low temperature further slows down the reaction, favoring selective substitution rather than multiple substitutions.
Analysis of Each Option
Option (A): Ortho-bromophenol
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Substitution at ortho position is possible.
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However, steric hindrance between –OH and Br reduces its formation.
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❌ Formed in smaller amount.
Option (B): Para-bromophenol ✅ (Correct Answer)
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Para position is:
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Activated by –OH group
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Less sterically hindered than ortho
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Under Br₂/CS₂ at low temperature, para-bromophenol is the major product.
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✔️ This is the most stable and favored product.
Option (C): 2,4,6-Tribromophenol
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Forms only when bromination is done in:
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Aqueous medium
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Or at higher temperature
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❌ Not formed under controlled CS₂ conditions.
Option (D): Di-bromophenol
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Requires stronger conditions or prolonged reaction time.
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❌ Not favored at low temperature in CS₂.
Final Answer
The major product is para-bromophenol → Option (B).
SEO-Optimized Introduction
Understanding the major product of bromination of phenol is a common and important question in organic chemistry, especially for competitive exams. When phenol reacts with bromine in a non-polar solvent like CS₂ at low temperature, selective substitution occurs. This article explains the reaction mechanism, solvent effects, temperature control, and why para-bromophenol is the dominant product.
Keywords Used Naturally
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electrophilic substitution in phenol
