Question 13

**Working through the sequence** - **X**: 2‑bromobenzaldehyde protected as its ethylene glycol acetal. - **Step 1 (Na, dry ether; then H₃O⁺)**: Wurtz–Fittig type aryl–aryl coupling of two molecules of X, followed by acetal hydrolysis → **P = biphenyl‑2,2′‑dicarbaldehyde**, C₁₄H₁₀O₂, M = 210 ✓. 16 mol X → 8 mol P (100 %). - **Step 2 (NaOH, Δ; H₃O⁺)**: intramolecular Cannizzaro → **Q = 2′‑(hydroxymethyl)biphenyl‑2‑carboxylic acid**. 50 % → 4 mol. - **Step 3 (NaOH/CaO, Δ)**: soda‑lime decarboxylation → **R = biphenyl‑2‑ylmethanol** (Ar–CH₂OH). 50 % → 2 mol. - **Step 4 (PBr₃)**: **T = 2‑(bromomethyl)biphenyl** (Ar–CH₂Br). 50 % → 1 mol. - **Step 5 (NaH, R)**: Williamson ether synthesis, alkoxide of R attacks T → **S = bis(biphenyl‑2‑ylmethyl) ether**, Ar–CH₂–O–CH₂–Ar, C₂₆H₂₂O. T (1 mol) is limiting; 50 % → 0.5 mol S. **Molar mass of S**: 26(12) + 22(1) + 16 = 312 + 22 + 16 = 350 g/mol **Mass of S** = 0.5 × 350 = **175 g**

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