Tollen’s reaction

Dissolve 0-5 g. of the substance in 10 ml. of 50 per cent, alcohol, add 0-5 g. of solid ammonium chloride and about 0 -5 g. of zinc powder. Heat the mixture to boiling, and allow the ensuing chemical reaction to proceed for 5 minutes. Filter from the excess of zinc powder, and teat the filtrate with Tollen s reagent Section 111,70, (i). An immediate black or grey precipitate or a silver mirror indicates the presence of a hydroxyl-amine formed by reduction of the nitro compound. Alternatively, the filtrate may be warmed with Fehling s solution, when cuprous oxide will be precipitated if a hydroxylamine is present. Make certain that the original compound does not aflfect the reagent used. 

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. 

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. 

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. 

Aldehydes can also be oxidized selectively in the presence of other functional groups using silver (I) oxide (Ag20) in aqueous ammonium hydroxide (Tollen s reagent). Since ketones have no H on the carbonyl carbon, they do not undergo this oxidation reaction. 

Thoria catalyst, 735 Tischenko reactions, 318 Tollen s ammoniacal silver nitrate reagent, 330, 1061, 1074 p-Tolualdehyde, 689, 697 o-Toluamide, 798 Toluene, 510, 516, 615 purification of, 173, 174 separation from benzene, 231 p-Toluenesulphinic acid, 821, 826 o-Toluenesulphonamide, 821, 824 p-Toluenesulphonamide, 820, 823 p-Toluenesulphonates, 422, 437, 650, 684 ... 

The aldehyde or ketone group of monosaccharides can undergo an intramolecular reaction with one of its own hydroxyl groups to form a cyclic, hemiacetal or hemiketal structure, respectively (Fig. 20). In aqueous solutions, this cyclic structure actually predominates. The open-chain aldehyde or ketone form of monosaccharides is in equilibrium with the cyclic form, but the open structure exists less than 0.5% of the time in aqueous environments. It is the open form that reduces Fehling s or Tollen s... 

Ammoniacal silver nitrate solution (Totten s solution). Aldehydes alone reduce Tollen s reagent and produce a silver mirror on the inside of the test tube. Add 2-3 drops (or 0.05 g) of the compound to 2-3 ml of Tollen s solution contained in a clean test tube (the latter is preferably cleaned with hot nitric acid). If no reaction appears to take place in the cold, warm in a beaker of hot water. (CAUTION After the test, pour the contents of the test tube into the sink and wash the test tube with dilute nitric acid. Any silver fulminate present, which is highly explosive when dry, will thus be destroyed.)... 

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 ... 

To 2.0 mL of 5% silver nitrate solution, add 1.0 mL of 10% sodium hydroxide in a test tube. To the gray precipitate of silver oxide, AgjO, add 0.5 mL of a 2.8% ammonia solution (10 mL of concentrated ammonium hydroxide diluted to 100 mL). Stopper the tube and shake it. Repeat the process until almost all of the precipitate dissolves (3.0 mL of ammonia at most) then dilute the solution to 10 mL. Empty the test tubes of sodium hydroxide solution, rinse them, and add 1 mL of Tollen s reagent to each. Add one drop (no more) of the substance to be tested by allowing it to run down the inside of the inclined test tube. Set the tubes aside for a few minutes without agitating the contents. If no reaction occurs, warm the mixture briefly on a water bath. As a known aldehyde try one drop of a 0.1 M solution of glucose. A more typical aldehyde to test is benzaldehyde. 

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. 

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