Oxidation of aldehydes (Tollens test): practice and mechanism
The silver(I) ion oxidises the aldehyde to the carboxylic acid and is itself reduced to silver (silver mirror). Ketones do not react.
Example
Aldehydes are easily oxidised to carboxylic acids, ketones are not. The silver(I) ion is reduced to metallic silver (silver mirror), which is why the Tollens test tells aldehydes from ketones.
Mechanism step by step
- 1
In ammoniacal solution silver is present as [Ag(NH₃)₂]⁺.
- 2
The aldehyde (or its hydrate) gives up electrons and is oxidised to the carboxylate.
- 3
Ag⁺ is reduced to metallic silver, which deposits as a mirror on the glass; acidification gives the acid.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- That would be a reduction (alcohol). Ag⁺ is an oxidant: it takes electrons and the aldehyde is oxidised.
- No ester forms here; there is no alcohol in the reagent. The aldehyde is oxidised to the carboxylic acid (first as carboxylate).
Often confused with: Reduction with sodium borohydride · Grignard reaction
Frequently asked questions
Why is the test useful?
It tells aldehydes (silver mirror) from ketones (no reaction), and also detects reducing sugars.
Are there similar tests?
Fehling's test: Cu²⁺ is reduced to red Cu₂O, also only with aldehydes.