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JEE Main · 2020 · 08 Jan 2020 · Shift 1 · Biomolecules

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Which of the following statement is not true for glucose?
  1. (A)Glucose exists in two crystalline forms and
  2. (B)Glucose gives Schiff's test for aldehyde✓ Correct
  3. (C)Glucosse reacts with hydroxylamine to form oxime
  4. (D)The pentaacetate of glucose does not react with hydroxylamine to give oxime

Answer: (B)

Solution

Glucose exists almost entirely in its cyclic hemiacetal form, with far less than a tenth of a percent present as the open-chain aldehyde, so it carries essentially no free aldehyde group and does not respond to Schiff's reagent. It nonetheless reacts with hydroxylamine to form an oxime, because the trace of open-chain aldehyde is continually replenished as it is consumed, whereas glucose pentaacetate has all its hydroxyl groups blocked, cannot reopen to the aldehyde and so forms no oxime.

JEE Main · 2020 · 08 Jan 2020 · Shift 1 · Single correct option

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