Consider the following reaction of benzene.
In compound (Q), the percentage of oxygen is ____ %. (Nearest integer)

Solution

Chemistry ma’am · 4 steps

P is a 1,4-diketone, and base with heat always closes a 1,4-diketone into a cyclopentenone.

1. Make P. Anhydrous AlCl₃ turns the acid chloride end into an acylium ion, and benzene is acylated (Friedel-Crafts). P is C₆H₅-CO-CH₂-CH₂-CO-CH₃, 1-phenylpentane-1,4-dione, C₁₁H₁₂O₂.
2. Ring closure. Aqueous NaOH removes a proton from the terminal CH₃, and that carbanion attacks the phenyl ketone carbon. This intramolecular aldol forms a five-membered ring, the only ring size possible here.
3. Dehydration. On heating, the aldol loses H₂O to give the conjugated enone Q, 3-phenylcyclopent-2-en-1-one, C₁₁H₁₀O. Its molar mass is 132 + 10 + 16 = 158 g mol⁻¹.

   % O = 16/158 × 100 = 10.13

4. So. The percentage of oxygen in Q is about 10.1%, which rounds to 10.

The common slip: 18. Using P, C₁₁H₁₂O₂ (or the aldol before it loses water), gives 32/176 × 100 ≈ 18, not 10. Q has already lost H₂O and keeps only one oxygen.

The idea to keep. Count the carbons from the attacking enolate carbon to the target carbonyl. Base-catalysed ring closure prefers 5- or 6-membered rings, and heating removes water to give the conjugated enone.

Answer from the NTA final answer key. Solution by PrepWiser.