Tollens' Reagent | chemca

chemca.in

Home Articles Quizzes About

Reagents

Tollens' Reagent

Ammoniacal Silver Nitrate: The standard test for Aldehydes.

By chemca Team • Updated Jan 2026

Tollens' Reagent is a mild oxidizing agent used primarily to distinguish Aldehydes (both aliphatic and aromatic) from Ketones. It contains the complex ion $[Ag(NH_3)_2]^+$. The formation of a metallic silver layer on the reaction vessel walls gives it the name "Silver Mirror Test".

1. Preparation of Tollens' Reagent

In-Situ Preparation

It is always prepared fresh in the laboratory by mixing Silver Nitrate ($AgNO_3$) and Ammonium Hydroxide ($NH_4OH$).

Step 1: Formation of Silver Oxide precipitate.

$$ 2AgNO_3 + 2NaOH \rightarrow \underset{\text{Brown ppt}}{Ag_2O \downarrow} + 2NaNO_3 + H_2O $$

Step 2: Dissolution of ppt in dilute Ammonia.

$$ Ag_2O + 4NH_4OH \rightarrow 2[Ag(NH_3)_2]^+ OH^- + 3H_2O $$

Active Species: The oxidizing species is the complex ion $[Ag(NH_3)_2]^+$, which acts in an alkaline medium.

2. Reaction with Aldehydes (Silver Mirror Test)

General Reaction

Tollens' reagent oxidizes aldehydes to carboxylate ions while being reduced to metallic silver.

$$ R-CHO + 2[Ag(NH_3)_2]^+ + 3OH^- \xrightarrow{\Delta} \underset{\text{Carboxylate}}{R-COO^-} + \underset{\text{Silver Mirror}}{2Ag \downarrow} + 4NH_3 + 2H_2O $$

Scope:

• Aliphatic Aldehydes: Acetaldehyde, Propanal, Glucose, etc. give positive test.
• Aromatic Aldehydes: Benzaldehyde reduces Tollens' reagent (unlike Fehling's solution).
• Ketones: Normal ketones (Acetone, Acetophenone) do NOT react.

3. Reaction with $\alpha$-Hydroxy Ketones

Exceptions to the Rule

Although ketones generally don't react, $\alpha$-Hydroxy Ketones give a positive Tollens' test because they tautomerize to form an aldehyde intermediate in alkaline medium.

$$ \underset{\alpha\text{-Hydroxy Ketone}}{R-CO-CH(OH)-R} \overset{OH^-}{\rightleftharpoons} \text{Enediol} \rightleftharpoons R-CH(OH)-CO-R \text{ (Aldehyde-like)} $$

Common Example: Fructose
Fructose is a keto-hexose but it reduces Tollens' reagent due to the presence of the $\alpha$-hydroxy ketone group. Such sugars are called Reducing Sugars.

4. Reaction with Other Compounds

Specific Cases

A. Formic Acid ($HCOOH$):
It contains an aldehyde-like group ($-CHO$) and thus reduces Tollens' reagent to $CO_2$.

$$ HCOOH + 2[Ag(NH_3)_2]^+ + 2OH^- \rightarrow CO_2 \uparrow + 2Ag \downarrow + 4NH_3 + 2H_2O $$

B. Terminal Alkynes:
They do NOT reduce Ag+ to Ag. Instead, they form a White Precipitate of Silver Acetylide (Acid-Base reaction).

$$ R-C \equiv C-H + [Ag(NH_3)_2]^+ \rightarrow \underset{\text{White ppt}}{R-C \equiv C^-Ag^+} + NH_4^+ + NH_3 $$

C. Hydroxylamine ($NH_2OH$) & Hydrazine ($N_2H_4$):
These are strong reducing agents and reduce Tollens' reagent to Silver.

5. Quick Comparison

Compound	Tollens' Test	Observation
Acetaldehyde ($CH_3CHO$)	Positive	Silver Mirror / Grey ppt
Benzaldehyde ($PhCHO$)	Positive	Silver Mirror
Acetone ($CH_3COCH_3$)	Negative	No Reaction
Glucose (Aldose)	Positive	Silver Mirror
Fructose (Ketose)	Positive	Silver Mirror
Sucrose (Disaccharide)	Negative	Non-reducing sugar
Acetylene ($HC \equiv CH$)	White Ppt	Forms Silver Acetylide

Knowledge Check

Test your understanding of Tollens' Reagent

© 2026 chemca.in. All rights reserved.

Optimized for learning Organic Chemistry.

Quiz Revision Notes Downloads Class Notes Video Lectures