Definition: What is Tollens’ Test?

Tollens’ test, also known as a silver-mirror test, is a qualitative laboratory test that is used to differentiate between an aldehyde and a ketone. It uses the fact that aldehydes can be easily oxidized than ketones. This test cannot distinguish between an aldehyde and an α-hydroxy ketone (e.g., hydroxy acetone) ^[1-4].

Tollens Test

The history of this reaction goes back to 1882 when a work done by German chemist Bernard Tollens was published in a German journal.

Examples of Tollens’ Test

Tollens Test Benzaldehyde

Tollens’ Reagent

Tollens reagent is a mild oxidizing chemical reagent that is used in the Tollens’ test. It is a colorless, basic, and aqueous solution containing silver ions coordinated to ammonia, forming a diaminesilver(I) complex [Ag(NH₃)₂]⁺. Tollens’ reagent is prepared using a two-step procedure. During the process, two events occur. First, the aldehyde gets converted into carbon dioxide. Second, the Ag⁺ ions are reduced to Ag metal, which leaves a shiny appearance inside the test-tube, indicating a positive result ^[1-3].

Tollens Reagent

Mechanism of Tollens Test ^[5]

Tollens Test Mechanism

FAQs

Q.1. Why does fructose reduce Tollens’ reagent?

Ans. In aqueous solution, fructose is enolized and converted into aldehyde in basic medium. As aldehydes generally reduce Tollen’s reagent, thus fructose also reduces Tollen’s reagent.

References

1. Definition and preparation – Chem.libretexts.org
2. Definition and preparation – Byjus.com
3. Definition and preparation – Vedantu.com
4. Definition – Edu.rsc.org
5. Mechanism – Leah4sci.com