Question

Write the structural formula and I.U.P.A.C. name of D-glucose. How will you prove the presence of aldehyde group in glucose molecule?

Hint:

Glucose is an aldohexose; aldehyde group is confirmed by classical silver mirror and Fehling’s tests.

Answer 1

Step 1: Structural formula.
The open-chain form of D-glucose: \[ CH_2OH - CHOH - CHOH - CHOH - CHOH - CHO \] It has 6 carbons, 5 hydroxyl groups, and 1 aldehyde group.
Step 2: IUPAC name.
As an aldohexose, the correct name is
2,3,4,5,6-pentahydroxyhexanal. In stereochemical form, it is (2R,3S,4R,5R).
Step 3: Test for aldehyde group.
-
Fehling’s Test: On heating with Fehling’s solution, glucose reduces Cu\(^{2+}\) to red precipitate of Cu\(_2\)O. -
Tollen’s Test: On heating with ammoniacal silver nitrate, glucose reduces Ag\(^{+}\) to metallic silver (silver mirror).
Step 4: Conclusion.
Thus, D-glucose contains an aldehyde group confirmed by positive Fehling’s and Tollen’s tests.