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Chapter 5: Problem 90

Phenol is heated with a solution of mixture of \(\mathrm{KBr}\) and \(\mathrm{KBrO}_{3} .\) The major product obtained in the above reaction is : (1) 2-Bromophenol (2) 3-Bromophenol (3) 4-Bromophenol (4) \(2,4,6\)-Tribromophenol

Short Answer

Expert verified
4) 2,4,6-Tribromophenol.

Step by step solution

01

- Identify the Reagents and Conditions

Phenol is treated with a mixture of \(\text{KBr}\) and \(\text{KBrO}_3\). This mixture serves as a brominating agent under heating conditions.
02

- Understand the Mechanism

The mixture of \(\text{KBr}\) and \(\text{KBrO}_3\) generates bromine (Br2) in situ. This bromine reacts with phenol to introduce bromine atoms into the phenol ring.
03

- Identify the Position of Bromination

Phenol has activating groups that direct bromination to the ortho and para positions. Since \(Br_2\) is in excess, the reaction continues leading to multiple brominations.
04

- Predict the Major Product

The result of brominating phenol with excess bromine typically yields \(2,4,6\)-tribromophenol as the major product due to substitution at all ortho and para positions.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Overview of Organic Chemistry
Organic chemistry is the branch of chemistry that studies the structure, properties, and reactions of organic compounds and materials, which primarily contain carbon atoms. This field largely focuses on carbon-containing compounds, their composition, structure, reactivity, and usage in different chemical reactions. Here are some key points to get you started:

  • Organic compounds typically consist of carbon atoms bonded with other elements such as hydrogen, nitrogen, oxygen, and halogens.
  • Organic reactions often involve changes in the functional groups which dictate the chemical behavior of the molecule.
  • Phenol (\text{C\(_6\)H\(_5\)OH}) is an example of an organic compound characterized by a hydroxyl group attached to a benzene ring.
  • Understanding the reactivity and interactions of organic compounds is essential to fields like pharmaceuticals, petrochemicals, and materials science.
Understanding Electrophilic Aromatic Substitution (EAS)
Electrophilic Aromatic Substitution (EAS) is a critical concept for understanding many reactions involving aromatic compounds, such as phenol. In EAS, an electrophile replaces a hydrogen atom on the aromatic ring. Let's break down this process:

  • Aromatic compounds, like benzene, have delocalized π-electrons that make them reactive towards electrophiles.
  • An electrophile is a species that is attracted to electrons and can accept an electron pair.
  • During EAS, an electrophile attacks the aromatic ring, forming a carbocation intermediate. This intermediate is then stabilized by the delocalization of electrons over the ring.
  • A common example of EAS is the bromination of phenol, where bromine (\text{Br\(_2\)}) acts as the electrophile.
  • Phenol has an activating \text{–OH} group which directs the incoming electrophile to the ortho and para positions, increasing the rate of reaction.
This makes EAS a highly useful reaction in organic chemistry for introducing various substituents into aromatic compounds.
Bromination Mechanism in Phenol
The bromination of phenol is a specific example of Electrophilic Aromatic Substitution (EAS). Here’s how it works step-by-step:

Phenol is treated with a mixture of potassium bromide (\text{KBr}) and potassium bromate (\text{KBrO\(_3\)}). This mixture generates bromine (\text{Br\(_2\)}) in situ when heated.
  • Step 1: Bromine Formation - Heat causes the reaction between \text{KBr} and \text{KBrO\(_3\)} to produce bromine.
  • Step 2: Bromine Attack - The bromine (\text{Br\(_2\)}) generated is very reactive and attacks the aromatic ring of phenol. Phenol has an \text{–OH} group, making it more reactive and directing the substitution to ortho and para positions.
  • Step 3: Multiple Bromination - Because bromine is in excess, it continues to substitute at all the ortho and para positions around the phenol ring. This results in the formation of \text{2,4,6-tribromophenol} as the major product.
So, in summary, the reaction of phenol with \text{KBr} and \text{KBrO\(_3\)} produces \text{2,4,6-tribromophenol} due to the activating effects of the \text{–OH} group and the presence of excess bromine.

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