Tollens' Test: The Silver Mirror Test
Tollens' Test is a classic qualitative laboratory test used to distinguish between Aldehydes and Ketones. Aldehydes are readily oxidized by the mild oxidizing agent Tollens' reagent, whereas most ketones are not. A positive test results in the formation of a shiny Silver Mirror on the reaction vessel walls.
1. Preparation of Tollens' Reagent
Tollens' reagent is Ammoniacal Silver Nitrate. It must be prepared fresh in situ because it can form explosive silver nitride over time.
- Add aqueous Sodium Hydroxide ($NaOH$) to Silver Nitrate ($AgNO_3$) to precipitate Silver Oxide ($Ag_2O$).
- Add aqueous Ammonia ($NH_4OH$) dropwise until the brown precipitate just dissolves, forming the complex ion.
Active Species: Diamminesilver(I) ion $[Ag(NH_3)_2]^+$.
2. General Reaction & Mechanism
The complex ion acts as a mild oxidizing agent. It oxidizes aldehydes to carboxylate ions while Silver(I) is reduced to metallic Silver(0).
Redox Change:
- Aldehyde: Oxidized (C goes from +1 to +3).
- Silver: Reduced ($Ag^+ \rightarrow Ag^0$).
3. Scope and Specificity
Who gives a Positive Test?
- All Aldehydes: Both Aliphatic (e.g., Acetaldehyde) and Aromatic (e.g., Benzaldehyde). Note: Fehling's solution fails for aromatic aldehydes.
- Formic Acid ($HCOOH$): It contains a $-CHO$ group structure ($H-C(=O)-OH$). Oxidizes to $CO_2$.
- $\alpha$-Hydroxy Ketones: e.g., Fructose. They tautomerize to form aldehydes in the basic medium.
- Hemiacetals: e.g., Glucose, Maltose (Reducing Sugars). The hemiacetal opens to the free aldehyde form.
- Terminal Alkynes: Form a White Precipitate of Silver Acetylide ($R-C \equiv C^-Ag^+$), not a mirror. This is an acid-base reaction, not redox.
Who gives a Negative Test?
- Ketones: Ordinary ketones (e.g., Acetone, Acetophenone) do not have an oxidizable hydrogen on the carbonyl carbon.
- Non-reducing Sugars: e.g., Sucrose (no free hemiacetal group).
4. Distinguishing Compounds
| Pair | Tollens' Positive | Tollens' Negative |
|---|---|---|
| Aldehyde vs Ketone | Propanal | Propanone |
| Formic vs Acetic Acid | Formic Acid ($HCOOH$) | Acetic Acid ($CH_3COOH$) |
| Glucose vs Sucrose | Glucose (Reducing) | Sucrose (Non-reducing) |
| Aromatic Aldehyde vs Ketone | Benzaldehyde | Acetophenone |
5. Reaction of Formic Acid
Formic acid behaves like an aldehyde because of the $H-C=O$ group.
$$ HCOOH + 2[Ag(NH_3)_2]^+ + 2OH^- \rightarrow CO_2 \uparrow + 2Ag \downarrow + 2H_2O + 4NH_3 $$Tollens' Test Quiz
Test your concepts on Aldehyde detection. 10 MCQs with explanations.
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