chemistry

Tollens' reagent

Tollens' reagent is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Tollens' reagent rather than just read about it. In short: Tollens' reagent (chemical formula Ag(NH3)2OH) is a chemical reagent used to distinguish between aldehydes and ketones along with some alpha-hydroxy ketones which can tautomerize into aldehydes. The reagent consists of a solution of silver nitrate, ammonium hydroxide and some sodium hydroxide (to maintain a basic pH of the reagent solution).

Key takeaways

Tollens' reagent belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Tollens' reagent to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Tollens' reagent from memory before moving on to harder problems.

Reference excerpt

Tollens' reagent (chemical formula Ag(NH3)2OH) is a chemical reagent used to distinguish between aldehydes and ketones along with some alpha-hydroxy ketones which can tautomerize into aldehydes. The reagent consists of a solution of silver nitrate, ammonium hydroxide and some sodium hydroxide (to maintain a basic pH of the reagent solution). It was named after its discoverer, the German chemist Bernhard Tollens. A positive test with Tollens' reagent is indicated by the precipitation of elemental silver, often producing a characteristic "silver mirror" on the inner surface of the reaction vessel.

Laboratory preparation This reagent is not commercially available due to its short shelf life, so it must be freshly prepared in the laboratory. One common preparation involves two steps. First a few drops of dilute sodium hydroxide are added to some aqueous 0.1 M silver nitrate. The OH− ions convert the silver aquo complex form into silver(I) oxide, Ag2O, which precipitates from the solution as a brown solid:

2 AgNO3 + 2 NaOH → Ag2O In the next step, sufficient aqueous ammonia is added to dissolve the brown silver(I) oxide. The resulting solution contains the [Ag(NH3)2]+ complexes in the mixture, which is the main component of Tollens' reagent. Sodium hydroxide is reformed:

Ag2O + 4 NH3 + 2 NaNO3 + H2O → 2 [Ag(NH3)2]NO3 + 2 NaOH Alternatively, aqueous ammonia can be added directly to silver nitrate solution. At first, ammonia will induce formation of solid silver oxide, but with additional ammonia, this solid precipitate dissolves to give a clear solution of diamminesilver(I) coordination complex, [Ag(NH3)2]+. Filtering the reagent before use helps to prevent false-positive results.

Uses

Qualitative organic analysis Once the presence of a carbonyl group has been identified using 2,4-dinitrophenylhydrazine (also known as Brady's reagent or 2,4-DNPH or 2,4-DNP), Tollens' reagent can be used to distinguish ketone vs aldehyde. Tollens' reagent gives a negative test for most ketones, with alpha-hydroxy ketones being one exception. The test rests on the premise that aldehydes are more readily oxidized compared with ketones; this is due to the carbonyl-containing carbon in aldehydes having attached hydrogen. The diamminesilver(I) complex in the mixture is an oxidizing agent and is the essential reactant in Tollens' reagent. The test is generally carried out in a test tube in a warm water bath. In a positive test, the diamminesilver(I) complex oxidizes the aldehyde to a carboxylate ion and in the process is reduced to elemental silver and aqueous ammonia. The elemental silver precipitates out of solution, occasionally onto the inner surface of the reaction vessel, giving a characteristic "silver mirror". The carboxylate ion on acidification will give its corresponding carboxylic acid. The carboxylic acid is not directly formed in the first place as the reaction takes place under alkaline conditions. The ionic equations for the overall reaction are shown below; R refers to an alkyl group.

2 [Ag(NH3)2]+ + RCHO + H2O → 2 Ag + 4 NH3 + RCOOH + 2 H+ Tollens' reagent can also be used to test for terminal alkynes (RC2H). A white precipitate of the acetylide (AgC2R) is formed in this case. Another test relies on reaction of the furfural with phloroglucinol to produce a colored compound with high molar absorptivity. It also gives a positive test with hydrazines, hydrazones, α-hydroxy ketones and 1,2-dicarbonyls. Both Tollens' reagent and Fehling's reagent give positive results with formic acid.

Staining In anatomic pathology, ammonical silver nitrate is used in the Fontana–Masson stain, which is a silver stain technique used to detect melanin, argentaffin, and lipofuscin in tissue sections. Melanin and the other chromaffins reduce the silver nitrate to metallic silver.

In silver mirroring Tollens' reagent is also used to apply a silver mirror to glassware; for example the inside of an insulated vacuum flask. The underlying chemical process is called silver mirror reaction. The reducing agent is glucose (an aldehyde) for such applications. Clean glassware is required for a high quality mirror. To increase the speed of deposition, the glass surface may be pre-treated with tin(II) chloride stabilised in hydrochloric acid solution. For applications requiring the highest optical quality, such as in telescope mirrors, the use of tin(II) chloride is problematic, since it creates nanoscale roughness and reduces the reflectivity. Methods to produce telescope mirrors include additional additives to increase adhesion and film resilience, such as in Martin's method, which includes tartaric acid and ethanol.

Safety Aged reagent can be destroyed with dilute acid to prevent the formation of the highly explosive silver nitride.

See also Benedict's reagent Walden reductor (opposite use involving metallic silver)

References

External links Video of experimental process involving Tollens' reagent Tollens' reagent on www.wiu.edu Univ. of Minnesota Organic Chemistry Class Demo Archived 3 March 2016 at the Wayback Machine Result Archived 3 March 2016 at the Wayback Machine

Illustrations

Worked examples

Example 1 — a first encounter with Tollens' reagent

Start with the simplest possible case. Write down what Tollens' reagent claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Tollens' reagent before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Tollens' reagent ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Tollens' reagent

In research: Tollens' reagent appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Tollens' reagent in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Tollens' reagent is common in secondary-school and first-year university syllabi. It links to neighbouring topics Ammine complexes, Analytical reagents, Chemical tests, so understanding it makes those chapters shorter.
In everyday life: Look for Tollens' reagent outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Tollens' reagent in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Tollens' reagent means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Tollens' reagent out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Tollens' reagent in simple terms?

Why does Tollens' reagent matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Tollens' reagent?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Tollens' reagent.