Tollens’ reagent = ammoniacal AgNO₃

It contains the complex ion:$$\boxed{[Ag(NH_3)_2]^+}$$

It is mainly used to identify aldehydes.

1. Basic reaction

For an aldehyde:$$RCHO + 2[Ag(NH_3)_2]^+ + 3OH^- \rightarrow RCOO^- + 2Ag\downarrow +4NH_3+2H_2O$$

Observation: Silver mirror is formed.

Aldehyde → oxidised to carboxylate
Ag⁺ → reduced to metallic Ag

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2. Which compounds give Tollens’ test?

Compound	Tollens’ test
Aldehyde, R–CHO	✅ Positive
Formic acid, HCOOH	✅ Positive
Formate ion, HCOO⁻	✅ Positive
α-hydroxy aldehyde	✅ Positive
Reducing sugars	✅ Positive
Ketones, R–CO–R	❌ Usually negative
Most carboxylic acids	❌ Negative
Esters	❌ Negative

3. Important JEE trick

Formic acid is an exception.$$\boxed{HCOOH \xrightarrow{\text{Tollens}} CO_2 + Ag}$$

Because formic acid can be oxidised to $CO_2$, it reduces Ag⁺.

So:$$\boxed{\text{Aldehydes + HCOOH + formates → Tollens positive}}$$

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4. Why ketones generally don’t react?

Ketones do not readily oxidise under the mild conditions of Tollens’ reagent.$$R-CO-R’ \quad \xrightarrow{\text{Tollens}} \quad \text{No reaction}$$

But beware: some compounds containing a ketone group can give a positive test if they can tautomerise/rearrange to an aldehyde-type reducing species.

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5. Glucose gives Tollens’ test

Glucose predominantly exists in cyclic form, but it can establish equilibrium with its open-chain aldehyde form:$$\text{Glucose} \rightleftharpoons \text{open-chain glucose}$$

The open-chain form reduces Tollens’ reagent.

Hence:$$\boxed{\text{Glucose → positive Tollens’ test}}$$

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6. Common JEE/NEET comparison

$$\boxed{\text{Tollens’ test → Aldehyde}}$$$$\boxed{\text{Fehling’s test → Aliphatic aldehydes mainly}}$$$$\boxed{\text{Iodoform test → } CH_3CO- \text{ or } CH_3CH(OH)-}$$

One-line memory

“Tollens = Silver mirror = Aldehyde oxidation.”