JEE Advanced Chemistry Question (2026) — Solution

Step-by-step solution

Step 1: The starting material is m-xylene (1,3-dimethylbenzene). It undergoes Friedel-Crafts acylation with chloroacetyl chloride ( ClCH _2 COCl ) in the presence of anhydrous AlCl _3 to form 2-chloro-1-(2,4-dimethylphenyl)ethanone. Step 2: The chloro ketone reacts with NaI (Finkelstein reaction) to form 2-iodo-1-(2,4-dimethylphenyl)ethanone. Step 3: This iodo compound undergoes Williamson ether synthesis with sodium 3-nitrophenoxide to form compound J . The molecular formula of J is C _ 16 H _ 15 NO _4. Step 4: Compound J is reduced by NaBH _4, which converts the ketone group to a secondary alcohol. Subsequent reaction with PBr _3 replaces the hydroxyl group with a bromine atom, yielding compound K . The molecular formula of K is C _ 16 H _ 16 BrNO _3. Molar mass of K = (16 12) + (16 1) + 80 + 14 + (3 16) = 350 g/mol , which matches the given data. Step 5: Compound K reacts with sodium thiophenoxide ( PhSNa ) in an S _ N 2 reaction, replacing the bromide with a phenylthio group ( -SPh ) to form compound M . The molecular formula of M is C _ 16 H _ 16 ( SC _6 H _5) NO _3 C _ 22 H _ 21 NO _3 S . Molar mass of M = 350 - 80 ( for Br ) + 109 ( for SPh ) = 379 g/mol . Step 6: Calculate the moles of compound M used in the Carius method. Moles of M = 3.79 g 379 g/mol = 0.01 mol . Step 7: Since each molecule of M contains exactly one sulphur atom, the moles of sulphur present is 0.01 mol . In the Carius method, all sulphur is quantitatively converted to barium sulphate ( BaSO _4). Moles of BaSO _4 formed = 0.01 mol . Step 8: Calculate the mass of BaSO _4 formed. Molar mass of BaSO _4 = 137 ( Ba ) + 32 ( S ) + 4 16 ( O ) = 233 g/mol . Mass of BaSO _4 = 0.01 mol 233 g/mol = 2.33 g . Answer: 2.33