Which one of the following reactions is NOT possible?
The Correct Option is B
Approach Solution - 1
Step 1: Analyze each reaction
1. Reaction (1):
The reaction involves the cleavage of the ether bond (\(\text{C} - \text{OCH}_3\)) by \(\text{HBr}\), producing phenol (\(\text{C}_6\text{H}_5 - \text{OH}\)). This reaction is possible due to the nucleophilic substitution mechanism.
2. Reaction (2):
The reaction involves the conversion of phenol (\(\text{C}_6\text{H}_5 - \text{OH}\)) to chlorobenzene (\(\text{C}_6\text{H}_5 - \text{Cl}\)) by \(\text{HCl}\). However, this reaction is \textbf{NOT} possible because the hydroxyl group in phenol is directly attached to the benzene ring, and it does not undergo nucleophilic substitution to form \(\text{C}_6\text{H}_5 - \text{Cl}\). The lone pair on oxygen in phenol makes the \(-\text{OH}\) group resistant to substitution by \(\text{HCl}\).
3. Reaction (3):
The reaction involves the hydrolysis of chlorobenzene (\(\text{C}_6\text{H}_5 - \text{Cl}\)) under high temperature and pressure in the presence of \(\text{NaOH}\). This reaction is possible via nucleophilic aromatic substitution, producing phenol (\(\text{C}_6\text{H}_5 - \text{OH}\)).
4. Reaction (4):
The reaction involves the electrophilic substitution of anisole (\(\text{C}_6\text{H}_5 - \text{OCH}_3\)) with chlorine in the presence of \(\text{AlCl}_3\). This reaction is possible, producing a mixture of ortho and para substituted products.
Step 2: Conclusion
Among the given reactions, only Reaction (2) is not possible because phenol does not undergo nucleophilic substitution with \(\text{HCl}\) to form chlorobenzene.
Final Answer: (2).
Approach Solution -2
The reaction shown is:
\[ \text{C}_6\text{H}_5\text{OH} \xrightarrow{\text{HCl}} \text{C}_6\text{H}_5\text{Cl} \]
That is, phenol reacting with hydrogen chloride to form chlorobenzene.
Step 2: Understand the reactivity of phenol.
Phenol contains an –OH group directly attached to the benzene ring. The C–O bond in phenol has a partial double-bond character due to resonance between the lone pair of oxygen and the aromatic π-electrons. This makes the C–O bond very strong and difficult to break.
Hence, phenol does not react with HCl to form chlorobenzene.
Step 3: Explain why this reaction is NOT possible.
For such a substitution to occur, the –OH group must be converted into a good leaving group. However, in phenol, this is not possible under normal acidic conditions because the C–O bond is stabilized by resonance.
Therefore, HCl cannot replace the –OH group in phenol directly to form chlorobenzene.
Step 4: Correct Answer.
The reaction shown in the image (phenol + HCl → chlorobenzene) is NOT possible.
Final Answer: Option 2
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