Tollens test, reducing sugars

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. 

Reducing sugar (Section 25.6) A sugar that reduces silver ion in the Tollens test or cupric ion in the Fehling or Benedict tests. 

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.)... 

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°. 

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)... 

Red tetrazolium affords a highly sensitive test for reducing sugars and distinguishes between a-ketols and simple aldehydes more sharply than Fehling s and Tollens tests. 

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

Sugars that give positive tests with Tollens or Benedict s solutions are known as reducing sugars, and all carbohydrates that contain a hemiacetalgroup g)ve positive tests. 

Unlike sucrose, maltose is a reducing sugar, it gives positive tests with Fehlings, Benedicts, and Tollens solutions. Maltose also reacts with phenyUiydrazine to form a monophenylosazone (i.e., it incorporates two molecules of phenylhydrazine). 

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 reduction of metallic salts provides a convenient test for reducing sugars. In alkaline solution, the sugars reduce the salts of copper, silver, mercury, and other metals to the metal or to a suboxide. The well-known Fehling and Tollens solutions are of this character. The sugar and some of the products resulting from isomerization in alkaline solution (see Chapter I) are oxidized to the corresponding acids. The formation of the metal or... 

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... 

FIGURE 16.21 Tollens test for a "reducing" sugar. (From http //static.flickr.com/3283/2316913960 4aca252c22 z.jpg.)... 

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