Tollens' reagent

Last updated

Tollens' test for aldehyde: left side positive (silver mirror), right side negative Tollens.JPG
Tollens' test for aldehyde: left side positive (silver mirror), right side negative
Ball-and-stick model of the diamminesilver(I) complex Diamminesilver(I)-3D-balls.png
Ball-and-stick model of the diamminesilver(I) complex

Tollens' reagent (chemical formula ) 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. [1] 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.

Contents

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 ions convert the silver aquo complex form into silver(I) oxide, , which precipitates from the solution as a brown solid:

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:

Alternatively, aqueous ammonia can be added directly to silver nitrate solution. [2] 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, . 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 diamine silver(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 diamine silver(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. [3]

Tollens' reagent can also be used to test for terminal alkynes (). A white precipitate of the acetylide () is formed in this case. Another test relies on reaction of the furfural with phloroglucinol to produce a colored compound with high molar absorptivity. [4] 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.[ citation needed ]

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. [2]

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. [5]

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. [6] [7] 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. [7]

Safety

Aged reagent can be destroyed with dilute acid to prevent the formation of the highly explosive silver nitride. [8]

See also

Related Research Articles

<span class="mw-page-title-main">Acid–base reaction</span> Chemical reaction between an acid and a base

In chemistry, an acid–base reaction is a chemical reaction that occurs between an acid and a base. It can be used to determine pH via titration. Several theoretical frameworks provide alternative conceptions of the reaction mechanisms and their application in solving related problems; these are called the acid–base theories, for example, Brønsted–Lowry acid–base theory.

<span class="mw-page-title-main">Aldehyde</span> Organic compound containing the functional group R−CH=O

In organic chemistry, an aldehyde is an organic compound containing a functional group with the structure R−CH=O. The functional group itself can be referred to as an aldehyde but can also be classified as a formyl group. Aldehydes are a common motif in many chemicals important in technology and biology.

<span class="mw-page-title-main">Base (chemistry)</span> Type of chemical substance

In chemistry, there are three definitions in common use of the word "base": Arrhenius bases, Brønsted bases, and Lewis bases. All definitions agree that bases are substances that react with acids, as originally proposed by G.-F. Rouelle in the mid-18th century.

Benedict's reagent is a chemical reagent and complex mixture of sodium carbonate, sodium citrate, and copper(II) sulfate pentahydrate. It is often used in place of Fehling's solution to detect the presence of reducing sugars. The presence of other reducing substances also gives a positive result. Such tests that use this reagent are called the Benedict's tests. A positive test with Benedict's reagent is shown by a color change from clear blue to brick-red with a precipitate.

<span class="mw-page-title-main">Precipitation (chemistry)</span> Chemical process leading to the settling of an insoluble solid from a solution

In an aqueous solution, precipitation is the process of transforming a dissolved substance into an insoluble solid from a supersaturated solution. The solid formed is called the precipitate. In case of an inorganic chemical reaction leading to precipitation, the chemical reagent causing the solid to form is called the precipitant.

<span class="mw-page-title-main">Hydroxylamine</span> Inorganic compound

Hydroxylamine is an inorganic compound with the chemical formula NH2OH. The compound is in a form of a white hygroscopic crystals. Hydroxylamine is almost always provided and used as an aqueous solution. It is consumed almost exclusively to produce Nylon-6. The oxidation of NH3 to hydroxylamine is a step in biological nitrification.

<span class="mw-page-title-main">Fehling's solution</span> Chemical test for the reducibility of a sugar

In organic chemistry, Fehling's solution is a chemical reagent used to differentiate between water-soluble carbohydrate and ketone functional groups, and as a test for reducing sugars and non-reducing sugars, supplementary to the Tollens' reagent test. The test was developed by German chemist Hermann von Fehling in 1849.

<span class="mw-page-title-main">Potassium tetraiodomercurate(II)</span> Chemical compound

Potassium tetraiodomercurate(II) is an inorganic compound with the chemical formula K2[HgI4]. It consists of potassium cations and tetraiodomercurate(II) anions. It is the active agent in Nessler's reagent, used for detection of ammonia.

Ammonia solution, also known as ammonia water, ammonium hydroxide, ammoniacal liquor, ammonia liquor, aqua ammonia, aqueous ammonia, or (inaccurately) ammonia, is a solution of ammonia in water. It can be denoted by the symbols NH3(aq). Although the name ammonium hydroxide suggests a salt with the composition [NH+
4][OH−
], it is actually impossible to isolate samples of NH4OH. The ions NH+
4 and OH− do not account for a significant fraction of the total amount of ammonia except in extremely dilute solutions.

The Brønsted–Lowry theory (also called proton theory of acids and bases) is an acid–base reaction theory which was first developed by Johannes Nicolaus Brønsted and Thomas Martin Lowry independently in 1923. The basic concept of this theory is that when an acid and a base react with each other, the acid forms its conjugate base, and the base forms its conjugate acid by exchange of a proton (the hydrogen cation, or H+). This theory generalises the Arrhenius theory.

<span class="mw-page-title-main">Reducing sugar</span> Sugars that contain free OH group at the anomeric carbon atom

A reducing sugar is any sugar that is capable of acting as a reducing agent. In an alkaline solution, a reducing sugar forms some aldehyde or ketone, which allows it to act as a reducing agent, for example in Benedict's reagent. In such a reaction, the sugar becomes a carboxylic acid.

<span class="mw-page-title-main">Silver chloride</span> Chemical compound with the formula AgCl

Silver chloride is an inorganic chemical compound with the chemical formula AgCl. This white crystalline solid is well known for its low solubility in water and its sensitivity to light. Upon illumination or heating, silver chloride converts to silver, which is signaled by grey to black or purplish coloration in some samples. AgCl occurs naturally as the mineral chlorargyrite.

Classical qualitative inorganic analysis is a method of analytical chemistry which seeks to find the elemental composition of inorganic compounds. It is mainly focused on detecting ions in an aqueous solution, therefore materials in other forms may need to be brought to this state before using standard methods. The solution is then treated with various reagents to test for reactions characteristic of certain ions, which may cause color change, precipitation and other visible changes.

<span class="mw-page-title-main">Chemical test</span> Procedure for identifying or quantifying a chemical compound or group

In chemistry, a chemical test is a qualitative or quantitative procedure designed to identify, quantify, or characterise a chemical compound or chemical group.

The sodium fusion test, or Lassaigne's test, is used in elemental analysis for the qualitative determination of the presence of foreign elements, namely halogens, nitrogen, and sulfur, in an organic compound. It was developed by J. L. Lassaigne.

<span class="mw-page-title-main">Metal ammine complex</span>

In coordination chemistry, metal ammine complexes are metal complexes containing at least one ammonia ligand. "Ammine" is spelled this way for historical reasons; in contrast, alkyl or aryl bearing ligands are spelt with a single "m". Almost all metal ions bind ammonia as a ligand, but the most prevalent examples of ammine complexes are for Cr(III), Co(III), Ni(II), Cu(II) as well as several platinum group metals.

A salt metathesis reaction is a chemical process involving the exchange of bonds between two reacting chemical species which results in the creation of products with similar or identical bonding affiliations. This reaction is represented by the general scheme:

<span class="mw-page-title-main">Silver oxide</span> Chemical compound

Silver oxide is the chemical compound with the formula Ag2O. It is a fine black or dark brown powder that is used to prepare other silver compounds.

<span class="mw-page-title-main">Silver carbonate</span> Chemical compound

Silver carbonate is the chemical compound with the formula Ag2CO3. This salt is yellow but typical samples are grayish due to the presence of elemental silver. It is poorly soluble in water, like most transition metal carbonates.

<span class="mw-page-title-main">Iron(III) nitrate</span> Chemical compound

Iron(III) nitrate, or ferric nitrate, is the name used for a series of inorganic compounds with the formula Fe(NO3)3.(H2O)n. Most common is the nonahydrate Fe(NO3)3.(H2O)9. The hydrates are all pale colored, water-soluble paramagnetic salts.

References

  1. ↑ Tollens, B. (1882). "Ueber ammon-alkalische Silberlösung als Reagens auf Aldehyd" [On an ammonical alkaline silver solution as a reagent for aldehydes](PDF). Berichte der Deutschen Chemischen Gesellschaft (in German). 15 (2): 1635–1639. doi:10.1002/cber.18820150243.
  2. 1 2 Webpath website http://library.med.utah.edu/WebPath/HISTHTML/MANUALS/FONTANA.PDF. Retrieved 4 February 2009
  3. ↑ "Oxidation of aldehydes and ketones". chemguide.co.uk. Retrieved 31 January 2010.
  4. ↑ Oshitna, K.; Tollens, B. (1901). "Ueber Spectral-reactionen des Methylfurfurols". Ber. Dtsch. Chem. Ges. 34 (2): 1425. doi:10.1002/cber.19010340212.
  5. ↑ Hart, M. (1992). Manual of scientific glassblowing. St. Helens, Merseyside [England]: British Society of Scientific Glassblowers. ISBN   0-9518216-0-1.
  6. ↑ N. Chitvoranund1; S. Jiemsirilers; D.P. Kashima (2013). "Effects of surface treatments on adhesion of silver film on glass substrate fabricated by electroless plating". Journal of the Australian Ceramic Society. 49: 62–69.{{cite journal}}: CS1 maint: numeric names: authors list (link)
  7. 1 2 C. Heber D. (February 1911). "Methods of Silvering Mirrors". Publications of the Astronomical Society of the Pacific. 23 (135): 15–19. Bibcode:1911PASP...23...13C. doi: 10.1086/122040 . hdl: 2027/mdp.39015018047608 .
  8. ↑ Svehla, G.; Vogel, Arthur Anton (1996). Vogel's Qualitative Inorganic Analysis. New York: Longman. ISBN   0-582-21866-7.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.