Question

Write the chemical equations when an aldehyde reacts with the following: (i) Tollen's reagent (ii) Fehling's reagent. (1+1=2)

Hint:

Both are mild oxidisers of the aldehyde group: Tollen's gives a silver mirror (Ag) and Fehling's gives a red precipitate of Cu₂O, while the aldehyde becomes a carboxylate.

Answer 1

Step 1: What the two reagents are. Both are soft oxidising reagents that pick out the aldehyde group. Tollen's reagent supplies \([Ag(NH_3)_2]^+\) in an alkaline medium, and Fehling's solution supplies a complexed \(Cu^{2+}\) ion in an alkaline (tartrate) medium.

Step 2: Electron book-keeping. In each test the aldehyde carbon goes from the +1 oxidation state (in \(RCHO\)) to +3 (in \(RCOO^-\)), losing electrons. Those electrons reduce the metal ion: \(Ag^+\) falls to \(Ag^0\) (silver mirror) and \(Cu^{2+}\) falls to \(Cu^{+}\) (as \(Cu_2O\)).

Step 3: Balanced equations.
Tollen's: \(RCHO + 2[Ag(NH_3)_2]^+ + 3OH^- \rightarrow RCOO^- + 2Ag\downarrow + 4NH_3 + 2H_2O\)
Fehling's: \(RCHO + 2Cu^{2+} + 5OH^- \rightarrow RCOO^- + Cu_2O\downarrow + 3H_2O\)

Step 4: Observation. Tollen's gives a bright silver mirror on the glass; Fehling's gives a brick-red precipitate of \(Cu_2O\). Aromatic aldehydes such as benzaldehyde give the Tollen's test but usually fail the Fehling's test, and ketones give neither, so these reactions distinguish aldehydes from ketones.