Tollens’ test

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

Thymine, electrostatic potential map of, 1104 structure of, 1101 Thyroxine, biosynthesis of, 551 structure of. 1020 TIme-of-flight (TOP) mass spectrometry, 417-418 Titration curve, alanine, 1023 TMS, see Tetramethylsilane see Trimethylsilyl ether Tollens reagent, 701 Tollens test, 992... 

Problem 15.53 Compound (A), C HgCI, is hydrolyzed to compound (B), C4HgO, which gives an oxime and a negative Tollens test. What is the structure of (A) <... 

Phenylhydrazone formation indicates a carbonyl compound. Since the negative Tollens test rules out an aldehyde, (A) must be a ketone. A negative iodoform test rules out the CH,C=0 group, and the reduction product, pentane, establishes the C s to be in a continuous chain. The compound is CHjCHjCOCHjCH,. 

Problem 15.55 A compound (C,HgOj) is reduced to pentane. With HjNOH it forms a dioxime and also gives positive iodoform and Tollens tests. Deduce its structure. 4... 

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°. 

Why does the Tollens test produce solid silver with aldehydes and not with ketones ... 

Instead of using the Tollens test, a student uses nmr spectroscopy to distinguish an aldehyde from a ketone. What should the student expect to find ... 

The molecular formula, C4H10O, tells us that compound A is saturated. The fact that A can be oxidized with PCC to an aldehyde (note that B gives a positive Tollens test) tells us that A is a primary alcohol. We can draw only two structures for A that are consistent with these data ... 

The appearance of a silver mirror in a Tollens test indicates the presence of ... 

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. 

A Tollens test is usually done on a small scale, but it can also create a silver mirror on a large object. 

Silver ion, Ag, oxidizes aldehydes selectively in a convenient functional-group test for aldehydes. The Tollens test involves adding a solution of silver-ammonia complex (the Tollens reagent) to the unknown compound. If an aldehyde is present, its oxidation reduces silver ion to metallic silver in the form of a black suspension or a silver mirror deposited on the inside of the container. Simple hydrocarbons, ethers, ketones, and even alcohols do not react with the Tollens reagent. 

Which of the following compounds would give a positive Tollens test (Remember that the Tollens test involves mild basic aqueous conditions.)... 

Tollens Test Tollens test detects aldehydes, which react with Tollens reagent to give carboxylate ions and metallic silver, often in the form of a silver mirror on the inside of the container. 

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)... 

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