Question

Write chemical equation of two chemical properties of glucose.

Hint:

Both Fehling's and Tollens' tests are classical tests for aldehydes. Glucose, despite existing mostly in cyclic form, gives these reactions due to the presence of a small amount of open-chain aldehyde structure in equilibrium.

Answer 1

Property 1: Reduction of Fehling's solution.
Glucose is an aldohexose containing an –CHO group. It reduces Fehling's solution (alkaline Cu$^{2+}$ ions) to red cuprous oxide (Cu$_2$O).
\[ \mathrm{R{-}CHO + 2\,Cu^{2+} + 5\,OH^- \;\longrightarrow\; R{-}COO^- + Cu_2O\downarrow + 3\,H_2O} \] Property 2: Reduction of Tollens' reagent.
Glucose reduces Tollens' reagent (ammoniacal silver nitrate) to metallic silver (silver mirror).
\[ \mathrm{R{-}CHO + 2\,[Ag(NH_3)_2]^+ + 3\,OH^- \;\longrightarrow\; R{-}COO^- + 2\,Ag\downarrow + 4\,NH_3 + 2\,H_2O} \] Note:
These reactions confirm that glucose contains a free aldehyde (–CHO) group in its open-chain structure.