Tollen’s test

For years, Tollen s Reagent (Ag (NH3)20H-) was used in the identification of aldehydes. Aldehydes reacted with Tollen s reagent to deposit silver metal on the walls of the reaction vessel, forming a mirror. An example of a positive Tollen s test is in Figure 10-33. 

Glycosides aren t susceptible to simple oxidation via Fehling s or Tollen s test (we explain these tests in greater detail later in this chapter). 

Carbohydrates such as aldoses that undergo oxidation with metal ions are referred to as reducing sugars. Both copper(II) ions and silver ions are capable of oxidizing aldoses. Oxidation by copper(II) ions is the basis for Fehling s test and Benedict s test, whereas oxidation by silver ions is the key to Tollen s test. (Note These tests work for any sugar with a hemiacetal, but they don t work on acetals or ketals.)... 

Tollen s test may result in the simple oxidation to a carboxylic acid, or it may cause fragmentation of the carbon backbone similar to the oxidation reaction seen with periodic acid (see the later related section). Figure 16-8 shows the general reaction that occurs when you conduct Tollen s test. 

Tollen s test The reagent should be freshly prepared by mixing two solutions (A and B). Solution A is a 10% aqueous AgNOs solution and solution B is a 10% aqueous NaOH solution. When the test is required, 1 ml of solution A and 1 ml of solution B are mixed, and the silver oxide thus formed is dissolved by dropwise addition of 10% aqueous NH4OH. To the clear solution, 10 drops of the compound to be tested are added. A silver mirror is indicative of the presence of an aldehyde. The reagent mixture (A + B) is to be prepared immediately prior to use otherwise, explosive silver fulminate will form. The silver mirror is usually deposited on the walls of the test tube either immediately or after a short warming period in a hot water bath. This is to be disposed of immediately with diluted HN03 (detection limit, 50 mg compounds tested, Q to C6). 

Aldehydes Alkanes Benedict s test Dichromate test 2,4-Dinitrophenylhydrazine (2,4-DNP) Fehling test Iodoform test Oxime Permanganate test Semicarbazone Tollen s test No test Positive for all aldehydes Positive for all aldehydes Positive for all aldehydes (and ketones) Positive for all aldehydes Positive only for acetaldehyde Positive for all aldehydes (and ketones) Positive for all aldehydes Positive for all aldehydes (and ketones) Positive for all aldehydes... 

Tollen s test (Section 19.3) a chemical test for detecting aldehydes by treatment with ammonia-cal silver nitrate. A positive test is signaled by formation of a silver mirror on the walls of the reaction vessel. 

Tollen s test. Glucose and other aldoses are oxidized by an aqueous solution of a silver-ammonia complex. What are the reaction products ... 

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. 

Tollen s test is a characteristic criteria for detecting aldehydes by oxidation. The final products include a silver mirror which can be observed as a sign of the aldehyde s presence. If the compound is not an aldehyde, there is no reaction. The reaction mechanism is shown below. 

Tollen s test, described previously, can be used to distinguish between the aldehyde and -the ether. Also, oxime formation with hydroxylamine, and Schiff test with the fuchsin-aldehyde reagent to fom a characteristic magenta color can be used. 

Therefore, to distinguish between n-propyl ether and diethyl acetal, one can add acid to both compounds and then perform either Tollen s test or Schiff test both tests are described in previous problems. The appearance of a silver mirror can identify it as acetaldehyde, that is, the compound is diethyl acetal. 

To distinguish between dioxane and trioxane, one can decyclize trioxane back into formaldehydes by adding acid and heating and then perform the Tollen s test or Schiff test. 

Tollen s test A chemical test for aldehydes by oxidation to a carboxylic acid and formation of a mirror-like silver precipitate. 

Aldonic acids Carboxylic acid products of Fehling s and Tollen s tests or bromine oxidation on sugars. Aldose A sugar based on aldehydes—so that a three-carbon chain would be an aldotriose, for example. Aliphatic compound A nonaromatic compound. 

Reducing sugars Sugars identifiable by Fehling s and Tollen s tests, oxidation to carboxylic acid. 

The majority of qualitative tests for sugars depend on the reducing characteristics of free monosaccharides. Fehling s solution, which contains cupric ions, is reduced to red CU2O by aldoses or ketoses. Similarly Tollen s test involves the reduction of Ag(NH2)2 to metallic silver. 

Tollens s test is designed to distinguish between aldehydes and ketones. A complete discussion of this test together with an experimental procedure is included with the classification tests for aldehydes in Section 25.7C. 

Tollens s Test Before performing this test, read the Safety Alert and Wrapping It Up sections in Seotion 25.7C. Perform Tollens s test according to the procedure outlined in that seotion using about 0.1 g of the carbohydrate in a clean glass test tube and about 1 ml of Tollens s reagent. The formation of a silver mirror or a black precipitate constitutes a positive test. 

The arylhydrazines will give a positive test for either an aldehyde or a ketone. Schiff s and Tollens s tests given in Parts B and C, respectively, provide methods for distinguishing between these two types of compounds. 

Another method for distinguishing between aldehydes and ketones is Tollens s test. A positive test indicates the presence of an aldehyde function, whereas no reaction occurs with ketones. Tollens s reagent consists of silver-ammonia complex, Ag(NH3)2, in an ammonia solution. This reagent oxidizes both aliphatic and aromatic aldehydes to the corresponding carboxylic acids silver ion is reduced to elemental silver, which is deposited as a silver mirror on the glass wall of a clean test tube. Thus, the formation of the silver mirror or of a precipitate is considered a positive test. Equation 25.12 shows the reaction that occurs. 

The second is Tollens s test, in which a solution of silver ion (such as in AgNOs) precipitates a silver mirror on exposure to an aldehyde. 

Results of Fehling s and Tollens s Tests on Aldoses and Ketoses... 

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