Intermediate Topic 805 of 1102 Organic Chemistry
Tollens' Test
Silver mirror forms with aldehydes, distinguishing them from ketones.
In-depth explanation of Tollens' Test
Tollens' test uses ammoniacal silver nitrate, [Ag(NH3)2]+, which is reduced by aldehydes. The aldehyde is oxidized to a carboxylate, and the silver ion is reduced to metallic silver, which deposits on the inside of a clean test tube as a silver mirror. Ketones do not react, so the test distinguishes aldehydes from ketones. The reagent also gives a positive result with alpha-hydroxy ketones and with reducing sugars, which can isomerize to aldehydes in base.
Going deeper: Tollens' Test
Key ideas: Reagent is ammoniacal silver nitrate, [Ag(NH3)2]+, freshly prepared and mildly basic. Aldehydes reduce Ag+ to metallic silver, forming a silver mirror on warming, while being oxidised to carboxylate: RCHO plus 2 Ag(NH3)2+ plus 3 OH- gives RCOO- plus 2 Ag plus 4 NH3 plus 2 H2O. Ketones do not respond, except alpha-hydroxy ketones (sugars); terminal alkynes give silver acetylide precipitates.
Common mistakes and connections: Students store the Tollens reagent, but it must be prepared fresh since it forms explosive compounds on standing. Another error is using dirty glassware that gives no mirror. Links to reducing sugars, the Fehling test and oxidation of aldehydes.
Visualization of Tollens' Test
Aldehydes reduce Ag+ to metallic silver, giving a mirror, while ketones do not react: a simple test to tell them apart. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Glucose, a reducing sugar, forms a silver mirror on the inside of the test tube when warmed with Tollens' reagent.
Frequently asked questions about Tollens' Test
What is Tollens' Test?
Silver mirror forms with aldehydes, distinguishing them from ketones.
Is Tollens' Test a beginner, intermediate or advanced topic?
Tollens' Test is a intermediate topic in Organic Chemistry. It builds on the vocabulary of beginner topics in this area.
What should I learn before Tollens' Test?
Study these earlier topics in Organic Chemistry first: Nucleophilic Addition to Carbonyls, Aldol Condensation, Cannizzaro Reaction.
What comes after Tollens' Test?
The next topics in Organic Chemistry are Fehling's Test and Grignard Reagents.
Can I practise Tollens' Test with quizzes?
Yes. The interactive lesson includes a self-check question on Tollens' Test, and the Organic Chemistry quiz draws on every topic in this area.
Where Tollens' Test fits in Organic Chemistry
Tollens' Test is topic 77 of 148 in Organic Chemistry.