Question

Which of the following test is not given by aldehydic group present in glucose?

• A. Tollen's test
• B. Schiff's test
• C. Fehling test
• D. None of them

Hint:

Glucose is a reducing sugar and gives Tollen's and Fehling's tests due to its ability to revert to the open-chain form, but Schiff's test requires a readily free aldehyde group, which glucose does not provide effectively.

Answer 1

The Correct Option is B

Step 1: Recall the nature of glucose.
Glucose is an aldohexose (contains an aldehyde group at C-1). However, in aqueous solution, glucose predominantly exists in cyclic hemiacetal form (pyranose or furanose), in which the aldehyde group is not free but can open to show reducing properties.

Step 2: Tests for aldehyde group.
- Tollen's test: Glucose gives Tollen's test (silver mirror test) because it is a reducing sugar.
- Fehling's test: Glucose also gives Fehling's test, producing a red precipitate of Cu$_2$O.
- Schiff's test: This test is generally not given by glucose. Schiff's reagent detects free aldehydes that immediately react to give a magenta colour. Since glucose mainly exists in cyclic form and its aldehyde group is not freely available, it gives only a very weak or negative Schiff's test.

Step 3: Conclusion.
Thus, among the given options, glucose does not give Schiff's test effectively.
\[ \boxed{\text{Schiff's test (B)}} \]