JEE Main 2026 Jan 23 Shift 2, Chemistry Q25: Properties and Important Reactions of Aldehydes & Ketones

JEE Main2026Jan 23, Shift 2Chemistry

In compound (Q), the percentage of oxygen is _____%. (Nearest integer)

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Solution

Step 1 - Friedel-Crafts acylation: Benzene attacks the acyl chloride end of $C H_{3} - C O - C H_{2} - C H_{2} - C O - C l$ in presence of anhydrous $A l C l_{3}$ . The acyl group attaches to the ring, giving the diketone (P): $C_{6} H_{5} - C O - C H_{2} - C H_{2} - C O - C H_{3}$ (1-phenyl-1,4-pentanedione).

Step 2 - Intramolecular aldol condensation: Aq. $N a O H$ , $Δ$ , drives an intramolecular aldol/dehydration to form a five-membered cyclopentenone ring fused with the phenyl substituent: 2-phenyl-2-cyclopentenone (Q).

Molecular formula of Q: $C_{11} H_{10} O$ , molar mass $= 11 (12) + 10 (1) + 16 = 132 + 10 + 16 = 158$ .

$% O = \frac{16}{158} \times 100 \approx 10.12%$ .

Nearest integer: $10 %$ .

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JEE Main 2026 Jan 23 Shift 2, Chemistry Q25: Properties and Important Reactions of Aldehydes & Ketones

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