Question

Explain tollens reagent with diagram

Answer 1

Tollens’ Reagent is an ammoniacal silver nitrate solution containing the complex ion $[Ag(NH_3)_2]^+$. It is prepared by adding dilute ammonia to AgNO₃ until the brown Ag₂O precipitate just dissolves.
In the Tollens’ test, an aldehyde ($RCHO$) is oxidized to a carboxylate ion ($RCOO^-$) while the silver complex is reduced to metallic silver, producing a reflective “silver mirror”.
Overall reaction:

$$RCHO \;+\; 2\,[Ag(NH_3)_2]^+ \;+\; 3OH^- \;\longrightarrow\; RCOO^- \;+\; 2Ag(s) \;+\; 4NH_3 \;+\; 2H_2O$$

Answer 2

1. Preparation of Tollens’ Reagent

• Start with AgNO₃(aq).
• Add dilute NaOH until a brown Ag₂O precipitate forms.
• Add aqueous NH₃ dropwise until the precipitate dissolves, giving $[Ag(NH_3)_2]^+$.

2. Reaction Mechanism

• Oxidation: Aldehyde $RCHO$ → Carboxylate ion $RCOO^-$.
• Reduction: [Ag(NH_3)_2]^+ → Ag(s) (deposits on the vessel walls as a mirror).

3. Merits of the Test

• Selective for aldehydes (except α-hydroxy ketones).
• Visual: Silver mirror provides clear positive indication.

4. Diagram of the Reaction Flow