Question

Which of the following reactions is not explained by the open chain structure of glucose?

• A. Glucose on prolonged heating with HI forms n-hexane.
• B. Glucose reacts with hydroxylamine to form an oxime.
• C. Glucose gets oxidized to gluconic acid on reaction with bromine water.
• D. Glucose exists in two different crystalline forms, alpha ($\alpha$) and beta ($\beta$).

Hint:

Anomers (alpha and beta forms) and the lack of reaction with Schiff's reagent strongly necessitate the cyclic hemiacetal structure of glucose.

Answer 1

The Correct Option is D

Step 1: Open-chain structure and its explanatory scope.
The Fischer open-chain structure of glucose (a 6-carbon aldehyde sugar) correctly predicts reactions involving the aldehyde ($-CHO$) and hydroxyl ($-OH$) groups.
Step 2: Options explained by the open chain.
(A) Heating with HI gives n-hexane, confirming the 6-carbon straight chain. (B) Reaction with hydroxylamine giving an oxime confirms the free $-CHO$ group. (C) Oxidation by bromine water to gluconic acid also confirms the aldehyde.
Step 3: Anomers require the cyclic structure.
Glucose exists as two crystalline forms: $\alpha$-D-glucose (m.p. 146°C) and $\beta$-D-glucose (m.p. 150°C). These anomers arise when the open chain cyclises to a six-membered ring, creating a new chiral centre (anomeric carbon) at C1.
Step 4: Open chain cannot explain anomers.
The open-chain structure shows only one form with a single aldehyde at C1. It cannot account for the existence of two distinct crystalline anomers. Option (D) is the answer.
\[ \boxed{(D)} \]