How many of the following facts about glucose could NOT be explained by its open chain structure?
(a) Glucose does not give Schiff’s test.
(b) Glucose reacts with hydroxylamine to form an oxime.
(c) Glucose does not form the hydrogensulphite addition product with NaHSO3.
(d) The pentaacetate of glucose does not react with hydroxylamine.
(e) Glucose adds a molecule of hydrogen cyanide to give a cyanohydrin.
(f) Glucose exists in two different crystalline forms, α and β.
(g) Glucose is oxidised to gluconic acid by bromine water.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 4
The open chain structure has a free –CHO group, so it predicts every reaction of an aldehyde. Facts that show the –CHO group is not free, or that glucose has two forms, need the cyclic hemiacetal structure, in which the –OH at C-5 has added to the –CHO group.
Not explained by the open chain structure:
(a) No Schiff’s test (an aldehyde would give the test)
(c) No hydrogensulphite addition product (a free aldehyde group would add NaHSO3)
(d) Pentaacetate does not react with hydroxylamine (no free –CHO group is left)
(f) Two crystalline forms, α and β (the two cyclic anomers, which differ at C1)
Explained by the open chain structure:
(b) Oxime with hydroxylamine — shows the carbonyl group of the open chain.
(e) Cyanohydrin with HCN — another reaction of the carbonyl group.
(g) Gluconic acid with bromine water — the free –CHO group is oxidised to –COOH.
Not explained by the open chain structure: (a), (c), (d) and (f) = 4.
NCERT (2026-27 edition):
10.1.2.1 Glucose – “The structure (I) of glucose explained most of its properties but the following reactions and facts could not be explained by this structure.”
“1. Despite having the aldehyde group, glucose does not give Schiff’s test and it does not form the hydrogensulphite addition product with NaHSO3.”
“2. The pentaacetate of glucose does not react with hydroxylamine indicating the absence of free —CHO group.”
“3. Glucose is found to exist in two different crystalline forms which are named as α and β.”
“3. Glucose reacts with hydroxylamine to form an oxime and adds a molecule of hydrogen cyanide to give cyanohydrin. These reactions confirm the presence of a carbonyl group (>C = O) in glucose.”
“4. Glucose gets oxidised to six carbon carboxylic acid (gluconic acid) on reaction with a mild oxidising agent like bromine water.”