Question: 76. The reagents with which glucose does not react to give the corresponding tests/products are
A. Tollen’s reagent
B. Schiff’s reagent
C. HCN
D. \mathrm{NH}_{2} \mathrm{OH}
E. \mathrm{NaHSO}_{3}
Choose the correct options from the given below :
(1) B and C
(2) A and D
(3) B and E
(4) E and D
Answer: Option (3)
Explanation:
Glucose is an aldohexose, but in aqueous solution it exists predominantly in the cyclic hemiacetal form rather than the free aldehyde form.
Tollen’s reagent gives a positive test with glucose because glucose can open up to its aldehyde form
and reduce \mathrm{Ag}^{+} to metallic silver.
Schiff’s reagent gives a positive test only with free aldehydes.
Since glucose exists mainly in its cyclic form and does not have a free aldehyde group in solution,
it does not give Schiff’s test.
Hydrogen cyanide reacts with the carbonyl group to form cyanohydrin.
Glucose can open its ring structure and react with HCN, so this reaction occurs.
\mathrm{NH}_{2} \mathrm{OH} reacts with the aldehyde group to form an oxime.
Glucose can react with hydroxylamine due to ring opening, hence this reaction occurs.
\mathrm{NaHSO}_{3} forms bisulphite addition products with free aldehydes and ketones.
Glucose does not form a stable bisulphite addition product because its aldehyde group is not freely available.
Therefore, glucose does not react with Schiff’s reagent and \mathrm{NaHSO}_{3}.