JEE Advanced 2022 Paper 2 Q18 Chemistry Biomolecules & Polymers Carbohydrates Hard

JEE Advanced 2022 Paper 2 · Q18 · Carbohydrates

Treatment of D-glucose with aqueous NaOH results in a mixture of monosaccharides, which are:

  1. A. (A) Three aldohexoses drawn as Fischer projections: first sugar with configuration HO–H, HO–H, H–OH, H–OH (top-to-bottom, with CHO at the top, CH$_2$OH at the bottom); second sugar with H–OH, HO–H, HO–H, H–OH; and third sugar with HO–H, HO–H, HO–H, H–OH (all three aldoses).
  2. B. (B) One ketohexose (CH$_2$OH-C(=O)-... with HO–H, H–OH, H–OH below the C=O) AND two aldohexoses: HO–H, H–OH, H–OH, H–OH and HO–H, HO–H, HO–H, H–OH (i.e. one fructose-type ketose + two aldose epimers).
  3. C. (C) One aldohexose with H–OH, HO–H, H–OH, H–OH (D-glucose); one ketohexose (D-fructose-like with CH$_2$OH-C(=O)-HO–H, H–OH, H–OH); and one aldohexose with HO–H, HO–H, H–OH, H–OH (D-mannose — the C-2 epimer of D-glucose). [The Lobry de Bruyn–Van Ekenstein product mixture: glucose + fructose + mannose.]
  4. D. (D) Three aldohexoses: H–OH, HO–H, HO–H, H–OH; H–OH, HO–H, H–OH, H–OH; HO–H, HO–H, H–OH, H–OH (no ketose included).
Reveal answer + step-by-step solution

Correct answer:C

Solution

Treatment of D-glucose with dilute aqueous NaOH effects the Lobry de Bruyn–Van Ekenstein rearrangement: an enediol intermediate is formed at C1–C2, which can re-tautomerise to give:

(i) D-glucose itself (aldose, CHO at C1, OH on C2-right Fischer) (ii) D-fructose (ketose, CH$_2$OH-CO- at C1–C2) (iii) D-mannose (aldose, C-2 epimer of D-glucose; the H/OH at C2 are swapped relative to glucose).

The equilibrium mixture is therefore glucose + fructose + mannose. The set must contain one ketohexose (fructose) and two aldohexoses that are C-2 epimers (glucose and mannose).

Option (A): three aldoses, no ketose — WRONG. Option (B): one ketose + two aldoses, but the aldoses shown are not the correct glucose/mannose pair — WRONG. Option (C): glucose + fructose + mannose — CORRECT. Option (D): three aldoses, no ketose — WRONG.

Answer: (C).

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