Silver-mirror test

The "silver mirror test" is used to distinguish an aldehyde from a ketone. Tollen s reagent, Ag(NH3)20H, acts as an oxidizing agent. When it is mixed with an aldehyde, the aldehyde oxidizes to the salt of a carboxylic acid. The silver ions in Tollen s reagent are reduced to silver atoms, and coat the glass of the reaction container with solid silver metal. 

Aldehydes are more easily oxidized than ketones. The Tollens silver mirror test is positive for aldehydes and negative for ketones. 

Know the meaning of lithium aluminum hydride, sodium borohydride, Tollens reagent, silver mirror test. 

Use Tollens reagent (the silver mirror test). The pentanal (an aldehyde) will react, whereas 2-pentanone (a ketone) will not. 

Tollens test. Most aldehydes reduce Tollens reagent (ammonia and silver nitrate) to give a precipitate of silver metal. The free silver forms a silver mirror on the sides of the test tube. (This test is sometimes referred to as the silver mirror test.) The aldehyde is oxidized to a carboxylic acid. 

The silver mirror test is best performed as follows. The solution of the tartrate is acidified with dilute nitric acid, excess of silver nitrate solution added and any precipitate present filtered off. Very dilute ammonia solution (approximately 0-02m) is then added to the solution until the precipitate at first formed is nearly redissolved, the solution is filtered, and the filtrate collected in a clean test-tube the latter is then placed in a beaker of boiling water. A brilliant mirror is formed on the sides of the tube after a few minutes. The test-tube may be cleaned either by boiling with chromic acid mixture or by boiling it with a little sodium hydroxide solution, and then rinsing it well with distilled water. 

An alternative method for carrying out the silver mirror test is the following. Prepare ammoniacal silver nitrate solution by placing 5 ml silver nitrate solution in a thoroughly clean test-tube and add 2-3 drops of dilute sodium hydroxide solution add dilute ammonia solution dropwise until the precipitated silver oxide is almost redissolved (this procedure reduces the danger of the formation of the explosive silver azide, AgN3, to a minimum). Introduce about 0-5 ml neutral tartrate solution. Place the tube in warm water. A silver mirror is formed in a few minutes. 

Figure 3. Potential-pH diagram for the aldehyde-acid transformation (solid line). The dashed line gives that for Ag/Ag (0.1 M) when the pH is imposed by NHv The hatched zone corresponds to the only region where the silver mirror test can be performed.

Reducing action of dextrose silver mirror.—Test a dilute solution of glucose with an ammoniacal solution of silver nitrate or with Tollen s reagent (experiment 106d, page 84). 

A redox reaction involving silver is used in a chemical test to determine whether an unknown organic compound is an aldehyde. This test is called a Tollen s test. It is also sometimes called the silver mirror test because a spectacular shiny layer of elemental silver plates out on the inside of a test tube if an aldehyde is present. In this test, a silver nitrate solution is mixed with a solution of the unknown substance, and the mixture is observed to see whether the mirror forms. 

Oxidation of Aldehydes Tollens Silver Mirror Test... 

Aldehydes and ketones are chemically distinguished by oxidation. Aldehydes are easily oxidized and ketones are not. In the Tollens silver mirror test aldehydes are oxidized to carboxylic acids and ketones are not oxidized. A silver mirror plates on the side of the test tube as silver ion is reduced to silver metal. 

Silver metal precipitates from solution and coats the flask, producing a smooth silver mirror, as seen in Figure 14.4. The test is therefore often called the Tollens silver mirror test. The commercial manufacture of silver mirrors uses a similar process. Ketones cannot be oxidized to carboxylic acids and do not react with the Tollens reagent. 

Tollens test (14.4) a test reagent (silver nitrate in ammonium hydroxide) used to distinguish aldehydes and ketones also called the ToUens silver mirror test... 

Tollen s reagent /tol-Sn/ A solution of the complex ion Ag(NH3>2 produced by precipitation of silver oxide from silver nitrate with a few drops of sodium hydroxide solution, and subsequent dissolution of the silver oxide in aqueous ammonia. Tollen s reagent is used in the SILVER-MIRROR TEST for aldehydes, where the Ag ion is reduced to silver metal. It is also a test for alkynes with a triple bond in the Tposi-tion. A yellow precipitate of silver carbide is formed in this case. 

The sample is warmed with the reagent in a test tube. Aldehydes reduce the complex Ag+ ion to metallic silver, forming a bright silver mirror on the inside of the tube (hence the name silver-mirror test). Ketones give a negative result, it is named after Bernhard Tollens (1841-1918). 

We studied the use of Tollens silver mirror test in Section 16.13B. Benedict s solu tion and the related Fehling s solution (which contains a cupric tartrate complex ion) give brick-red precipitates of CUgO when they oxidize an aldose. [In alkaline solution ketoses are converted to aldoses (Section 22.5A), which are then oxidized by the cupric complexes.] Since the solutions of cupric tartrates and citrates are blue, the appearance of a brick-red precipitate is a vivid and unmistakable indication of a positive test. 

An investigation of why hydroxide makes the Tollens silver mirror test for aldehydes more sensitive has focused on thermodynamic versus kinetic factors. Electrochemistry tends to rule out the former the electromotive force (emf) of an appropriate cell changes little with pH. Exploring the kinetics, single electron transfer processes were confirmed by addition of a radical trap (TEMPO), which slowed the reaction. Rate measurements point to the rate of the formation of the anion of the gm-diol (i.e. the hydrate anion) as the key parameter affected by added hydroxide, a factor that also explains how the rapidity of the test varies with the structure of the aldehyde. 

When Tollens reagent is added to an aldehyde, the aldehyde is oxidized to a carboxylic anion, and Ag is reduced to metallic silver. If this reaction is carried out properly, silver precipitates as a smooth, mirrorlike deposit—Whence the name silver-mirror test ... 

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