Aldose Tollens’ test

Oxidation of the aldehyde group of an aldose to form a carboxylic acid or carboxylic acid anion is often used analytically to determine the amount of reducing sugar. The Benedict and Fehling methods measure the amount of reducing sugar present in a fluid. In these reactions, the oxidant, Cu2+, is reduced to Cu+. Cu+ precipitates as Cu20, which can be measured in a variety of ways. In the Tollens test, Ag+ is reduced to Ag°. 

Tollens test cannot distinguish between aldoses and ketoses because the basic Tollens reagent promotes enediol rearrangements. Under basic conditions, the open-chain form of a ketose can isomerize to an aldose, which reacts to give a positive Tollens test. 

Any sugar that gives a positive Tollens test. Both ketoses and aldoses (in their hemiacetal forms) give positive Tollens tests, (p. 1119)... 

A test for reducing sugars, employing the same silver-ammonia complex used as a test for aldehydes. A positive test gives a silver precipitate, often in the form of a silver mirror. Tollens reagent is basic, and it promotes enediol rearrangements that interconvert ketoses and aldoses. Therefore, both aldoses and ketoses give positive Tollens tests if they are in their hemiacetal forms, in equilibrium with open-chain carbonyl structures, (p. 1118)... 

Although the Tollens reaction is a useful test for reducing sugars, it doesn t give good yields of aldonic acid products because the alkaline conditions cause decomposition of the carbohydrate. For preparative purposes, a buffered solution of aqueous Br2 is a better oxidant. The reaction is specific for aldoses ketoses are not oxidized by aqueous Br2. 

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

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. 

Benedicts reagent (an alkaline solution containing a cupric citrate complex ion) and Tollens solution [Ag (NH3)20H] oxidize and thus give positive tests with aldoses and ketoses. The tests are positive even though aldoses and ketoses exist primarily as cyclic hemiacetals. 

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. 

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. 

The Tollens reagent indicates the presence of an aldose or ketose when the surface of the reaction flask is coated with metaUic silver (looking like a mirror), while the other two tests indicate the presence of an aldose or ketose with the formation of a reddish precipitate (CU2O). These three reactions are used only as tests to obtain structural information about unknown carbohydrates but are not efficient as preparative methods when sufficient quantities of the aldonic acid are desired. A carbohydrate that tests positively for any of these three tests is said to be a reducing sugar because the carbohydrate can reduce the oxidizing agent. 

Historically, the oxidation of an aldose with either Ag" " in aqueous ammonia (called Tollens reagent) or Cu " with aqueous sodliun citrate (Benedict s reagent) formed the basis of simple tests for what are called reducing sugars. (Reducing... 

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

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