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JEE Advanced 2024 · Paper 1, Chemistry question 10

Complete reaction of acetaldehyde with excess formaldehyde, upon heating with conc. NaOH solution, gives P and Q. Compound P does not give Tollens' test, whereas Q on acidification gives positive Tollens' test. Treatment of P with excess cyclohexanone in the presence of catalytic amount of p-toluenesulfonic acid (PTSA) gives product R.

Sum of the number of methylene groups (−CHX2−\ce{-CH2-}) and oxygen atoms in R is _____.

Answer and worked solution

Official answer: 18

Aldol. Three successive aldol additions of HCHO to CHX3CHO\ce{CH3CHO} give (HOCHX2)X3C−CHO\ce{(HOCH2)3C-CHO}.

Crossed Cannizzaro with excess HCHO gives pentaerythritol C(CHX2OH)X4\ce{C(CH2OH)4} (P, no Tollens' test) and HCOONa\ce{HCOONa} (Q; formic acid reduces Tollens' reagent).

Ketalisation. P with excess cyclohexanone/PTSA forms a bis-ketal: two 1,3-dioxane rings spiro-fused to the central carbon and to two cyclohexane rings.

Count in R:

  • CHX2\ce{CH2} groups: 4 (OCHX2\ce{OCH2} of the core) + 2×52\times5 (cyclohexane rings, whose sixth carbon is the ketal carbon) = 14.
  • O atoms: 4.

Sum =18= 18.

Topics:Aldol and related condensationsNamed reactionsAlcohols
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