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Chapter 23: Problem 20

Identify compounds A through \(\mathrm{C}\) in the synthetic sequence represented by equations (a) through (c). (a) Phenol \(+\mathrm{H}_{2} \mathrm{SO}_{4} \stackrel{\text { heat }}{\longrightarrow}\) Compound \(\mathrm{A}\left(\mathrm{C}_{6} \mathrm{H}_{6} \mathrm{O}_{7} \mathrm{~S}_{2}\right)\) (b) Compound \(\mathrm{A}+\mathrm{Br}_{2} \frac{1 . \mathrm{HO}^{-}}{2 . \mathrm{H}^{+}}\) Compound \(\mathrm{B}\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{BrO}_{7} \mathrm{~S}_{2}\right)\) (c) Compound \(\mathrm{B}+\mathrm{H}_{2} \mathrm{O} \frac{\mathrm{H}^{+}}{\text {heat }}\) Compound \(\mathrm{C}\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{BrO}\right)\)

Short Answer

Expert verified
Compounds A, B, and C are 2,4-disulfophenol, 2,4-disulfo-6-bromophenol, and 6-bromophenol respectively.

Step by step solution

01

Analyzing Reaction (a)

In reaction (a), phenol (C6H5OH) is treated with sulfuric acid (H2SO4) under heat. This generally leads to sulfonation, resulting in the formation of sulfonated phenol. The molecular formula of Compound A, \( \mathrm{C}_{6} \mathrm{H}_{6} \mathrm{O}_{7} \mathrm{S}_{2} \), indicates two sulfates and seven oxygen atoms, which suggests the formation of 2,4-disulfophenol.
02

Analyzing Reaction (b)

In reaction (b), Compound A (2,4-disulfophenol) is reacted with bromine (Br2) in a basic environment and then acidified. Bromination of phenols primarily occurs at the para position. Thus, the product, Compound B, \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{BrO}_{7} \mathrm{S}_{2} \), is likely 2,4-disulfophenol with bromine added at the 6-position, forming 2,4-disulfo-6-bromophenol.
03

Analyzing Reaction (c)

In reaction (c), Compound B is hydrolyzed with water and acid, typically removing the sulfonyl groups from the aromatic ring. The product, Compound C, \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{BrO} \), suggests that sulfonyl groups are removed and 6-bromophenol is formed as a result of the elimination process.

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Key Concepts

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

Phenol
Phenol, a key starting compound in organic synthesis, is an aromatic compound with the formula \( \mathrm{C}_6\mathrm{H}_5\mathrm{OH} \). This molecule is composed of a hydroxyl group \((\mathrm{OH})\) attached to a benzene ring, which plays a crucial role in its reactivity. In the provided exercise, phenol undergoes successive chemical reactions, altering its structure significantly.Phenol is characterized by:
  • A polar hydroxyl group that participates in hydrogen bonding.
  • Aromaticity due to the benzene ring, contributing to its stability and reactivity.
  • Acidic properties, although weaker than typical carboxylic acids.
These properties make phenol a versatile compound in chemical reactions, particularly in electrophilic aromatic substitution processes, as seen in the exercise.
Sulfonation
Sulfonation is a significant reaction for phenols, involving the introduction of one or more sulfonic acid groups \((\mathrm{-SO}_3\mathrm{H})\) into the aromatic ring. In the first step of the exercise, phenol reacts with sulfuric acid \((\mathrm{H}_2\mathrm{SO}_4)\) under heat to undergo sulfonation, forming 2,4-disulfophenol.This reaction includes:
  • Formation of sulfonic acid groups that enhance water solubility.
  • Increased acidity, making the compound more reactive in subsequent steps.
  • Heat provides the energy needed to facilitate the reaction by overcoming activation barriers.
Sulfonated aromatic compounds, like 2,4-disulfophenol, serve as intermediates in further chemical transformations, applicable in various industrial processes.
Bromination
Bromination of phenolic compounds involves the introduction of a bromine atom \((\mathrm{Br})\) onto the benzene ring, a common example of electrophilic aromatic substitution. In step (b) of the exercise, 2,4-disulfophenol is brominated in a basic and then acidic environment, yielding a brominated product, 2,4-disulfo-6-bromophenol.The bromination process is characterized by:
  • Selective substitution, typically favoring the para position relative to the hydroxyl group.
  • An acidic pH can stabilize the reaction intermediate, aiding in product formation.
  • Presence of the sulfonic groups influences the site's reactivity, often enhancing electrophilic attack.
This brominated compound facilitates further reactions by introducing additional sites for chemical modification.
Aromatic Compounds
Aromatic compounds, defined by their stable ring structure and unique electronic configuration, are pivotal in organic chemistry. The aromatic nature of phenol is central to its role in the exercise, allowing for reactions like sulfonation and bromination to occur efficiently.A few important features of aromatic compounds include:
  • Cyclic structures with delocalized \( \pi \) electrons, contributing to stability.
  • Engagement in electrophilic aromatic substitution with various electrophiles.
  • Resonance, which stabilizes intermediates formed during reactions.
The aromatic character of phenol provides the framework for the sequence of transformations, demonstrating the versatility of aromatic compounds in synthesis.

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Most popular questions from this chapter

Which phenol in each of the following pairs is more acidic? Justify your choice. (a) \(2,4,6\) -Trimethylphenol or \(2,4,6\) -trinitrophenol (b) 2,6 -Dichlorophenol or 3,5 -dichlorophenol (c) 3-Nitrophenol or 4-nitrophenol (d) Phenol or 4 -cyanophenol (e) 2,5 -Dinitrophenol or 2,6 -dinitrophenol

Treatment of 3,5 -dimethylphenol with dilute nitric acid, followed by steam distillation of the reaction mixture, gave a compound \(\mathrm{A}\left(\mathrm{C}_{8} \mathrm{H}_{9} \mathrm{NO}_{3}, \mathrm{mp} 66^{\circ} \mathrm{C}\right)\) in \(36 \%\) yield. The nonvolatile residue from the steam distillation gave a compound \(\mathrm{B}\left(\mathrm{C}_{8} \mathrm{H}_{9} \mathrm{NO}_{3}, \mathrm{mp} 108^{\circ} \mathrm{C}\right)\) in \(25 \%\) yield on extraction with chloroform. Identify compounds \(\mathrm{A}\) and \(\mathrm{B}\).

Choose the reaction in each of the following pairs that proceeds at the faster rate. Explain your reasoning. (a) Basic hydrolysis of phenyl acetate or \(m\) -nitrophenyl acetate (b) Basic hydrolysis of \(m\) -nitrophenyl acetate or \(p\) -nitrophenyl acetate (c) Reaction of ethyl bromide with phenol or with the sodium salt of phenol (d) Reaction of ethylene oxide with the sodium salt of phenol or with the sodium salt of \(p\) -nitrophenol (e) Bromination of phenol or phenyl acetate

Write structural formulas for each of the following compounds: (a) Pyrogallol (1,2,3-benzenetriol) (b) o-Benzylphenol (c) 3-Nitro-1-naphthol

Taking advantage of their large differences in \(\mathrm{pK}_{\mathrm{a}}\) values, describe how a mixture of phenol and benzoic acid in diethyl ether solution could be separated. How could a mixture of phenol and cyclohexanol in diethyl ether solution be separated?
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