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JEE Advanced levelSingle correct optionDifficulty 3 of 3bm246

The disaccharide shown is boiled with dilute acid until the glycosidic bond is completely hydrolysed. Which row correctly gives the monosaccharides formed and the behaviour of the original disaccharide towards Fehling's solution?

  1. AD-Galactose and D-glucose; positive
  2. BD-Glucose only; positive
  3. CD-Galactose and D-glucose; negative
  4. DD-Glucose and D-fructose; negative

Answer: A

The non-reducing ring is beta-D-galactopyranose, linked beta-1,4 to a D-glucose unit whose anomeric OH is free. Hydrolysis therefore gives D-galactose and D-glucose. The free hemiacetal in the glucose unit makes the disaccharide (lactose) reducing, so Fehling's test is positive.
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