The product Ⓒ in the following sequence of reactions has _______ π bonds.
Correct Answer: 4
Approach Solution - 1
The reaction sequence proceeds as follows:
Step 1: Oxidation of \(\text{C}_6\text{H}_5\text{CH}_2\text{CH}_3\) with \(\text{KMnO}_4\) and \(\text{KOH}\)
The side chain ethyl group of ethylbenzene is oxidized to a carboxylate salt:
\[\text{C}_6\text{H}_5\text{CH}_2\text{CH}_3 \rightarrow [\text{KMnO}_4, \text{KOH}, \Delta] \text{C}_6\text{H}_5\text{COOK} (A).\]
Here, \(A\) is \(\text{C}_6\text{H}_5\text{COOK}\), potassium benzoate.
Step 2: Acidification with \(\text{H}_3\text{O}^+\)
The carboxylate salt is acidified to form benzoic acid:
\[\text{C}_6\text{H}_5\text{COOK} + \text{H}_3\text{O}^+ \rightarrow \text{C}_6\text{H}_5\text{COOH} (B).\]
Here, \(B\) is \(\text{C}_6\text{H}_5\text{COOH}\), benzoic acid.
Step 3: Bromination with \(\text{Br}_2/\text{FeBr}_3\)
Bromination occurs at the para-position of the benzene ring relative to the carboxylic acid group, yielding:
\[\text{C}_6\text{H}_5\text{COOH} \rightarrow [\text{Br}_2, \text{FeBr}_3] p-\text{BrC}_6\text{H}_4\text{COOH} (C).\]
Here, \(C\) is para-bromobenzoic acid (\(p-\text{BrC}_6\text{H}_4\text{COOH}\)).
Step 4: Count the \(\pi\)-bonds in \(C\)
- The benzene ring contributes 3 \(\pi\)-bonds.
- The \(\text{C} = \text{O}\) group in the carboxylic acid contributes 1 \(\pi\)-bond.
Thus, the total number of \(\pi\)-bonds in \(C\) is:
\[\pi\text{-bonds} = 3 + 1 = 4.\]
Final Answer: 4.
Approach Solution -2
The given compound is ethylbenzene (C₆H₅–CH₂–CH₃).
Step 2: Reaction with alkaline KMnO₄ (oxidation step).
Alkaline KMnO₄ oxidizes the entire alkyl side chain of an aromatic ring (no matter its length) to a carboxylic acid group (–COOH).
Thus, product A is benzoic acid (C₆H₅COOH).
Step 3: Acidification (H₃O⁺ treatment).
Acidification just neutralizes the solution and gives pure benzoic acid. Therefore, product B = benzoic acid (C₆H₅COOH).
Step 4: Bromination using Br₂/FeBr₃.
Benzene ring in benzoic acid undergoes electrophilic substitution. Due to the –COOH group being meta-directing, bromination occurs at the meta position, forming 3-bromobenzoic acid.
Product C = 3-bromobenzoic acid.
Step 5: Count the number of π bonds in product C.
- Benzene ring has 3 π bonds.
- The C=O bond in the carboxylic acid group contributes 1 π bond.
Total π bonds = 3 + 1 = 4.
Step 6: Final Answer.
The product C has 4 π bonds.
Final Answer: 4
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