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

Concept: Aldehydes are readily oxidised by mild oxidising agents. Tollen's reagent and Fehling's solution are gentle oxidisers used to detect the aldehyde group; they oxidise the aldehyde to the corresponding carboxylate ion while they themselves get reduced. Ketones do not respond to these tests.

Step 1: Reaction with Tollen's reagent. Tollen's reagent is an ammoniacal solution of silver nitrate, i.e. it contains the diammine silver(I) ion \([Ag(NH_3)_2]^+\). On warming with an aldehyde, silver ion is reduced to metallic silver, which deposits as a shiny silver mirror on the inside of the tube. The aldehyde is oxidised to a carboxylate ion.
\[ RCHO + 2[Ag(NH_3)_2]^+ + 3OH^- \rightarrow RCOO^- + 2Ag\downarrow + 4NH_3 + 2H_2O \]
Step 2: Reaction with Fehling's reagent. Fehling's solution is a deep blue alkaline solution of a copper(II) complex (Fehling A = copper sulphate, Fehling B = sodium potassium tartrate in NaOH). On heating with an aliphatic aldehyde the blue \(Cu^{2+}\) is reduced to a red-brown precipitate of cuprous oxide \(Cu_2O\), while the aldehyde is oxidised to the carboxylate.
\[ RCHO + 2Cu^{2+} + 5OH^- \rightarrow RCOO^- + Cu_2O\downarrow + 3H_2O \]
Result: A silver mirror confirms Tollen's test and a red-brown \(Cu_2O\) precipitate confirms Fehling's test; both prove the presence of the aldehyde group.