Question

What is Tollen's reagent? Write the chemical equation for the reactions of this reagent with glucose and fructose.

Hint:

Remember: Tollen's tests for \(-\)CHO. Ketoses like fructose still give a positive test because base-induced enediol formation converts them to aldoses first.

Answer 1

Tollen's reagent: Freshly prepared ammoniacal silver nitrate containing the complex ion \([\mathrm{Ag(NH_3)_2}]^+\) (often written as \([\mathrm{Ag(NH_3)_2}]\mathrm{OH}\)). It oxidises aldehydes to carboxylates giving a silver mirror.
With glucose (an aldose):
\[ \mathrm{C_6H_{12}O_6} + 2\,[\mathrm{Ag(NH_3)_2}]^+ + 3\,\mathrm{OH^-} \;\longrightarrow\; \mathrm{C_6H_{11}O_6^-} + 2\,\mathrm{Ag(s)} + 4\,\mathrm{NH_3} + 2\,\mathrm{H_2O} \] (on acidification \(\mathrm{C_6H_{11}O_6^-} \rightarrow \mathrm{C_6H_{12}O_7}\), gluconic acid).
With fructose (a ketose):
In alkaline medium fructose \(\rightleftharpoons\) enediol \(\rightleftharpoons\) glucose/mannose, hence it also reduces Tollen's reagent: \[ \mathrm{C_6H_{12}O_6\ (fructose)} + 2\,[\mathrm{Ag(NH_3)_2}]^+ + 3\,\mathrm{OH^-} \;\longrightarrow\; \mathrm{C_6H_{11}O_6^-} + 2\,\mathrm{Ag(s)} + 4\,\mathrm{NH_3} + 2\,\mathrm{H_2O} \]