Tollens oxidation

Silver oxide, AgjO Oxidizes primary alcohols in aqueous ammonia solution to yield aldehydes (Tollens oxidation Section 19.3). 

Chapter 19—Aldehydes and Ketones Nucleophilic Addition Reactions. The Tollens oxidation has been deleted, and the mechanism of the Wittig reaction has been updated. 

Tollens reagent An ammoniacal solution of silver oxide which is used as a lest for aldehydes, which, unlike ketones, cause the deposition of a silver mirror. 

It is preferable to use Tollen s ammoniacal silver nitrate reagent, which is prepared as follows Dissolve 3 g. of silver nitrate in 30 ml. of water (solution A) and 3 g. of sodium hydroxide in 30 ml. of water (solution B). When the reagent is requir, mix equal volumes (say, 1 ml.) of solutions A and JB in a clean test-tube, and add dilute ammonia solution drop by drop until the silver oxide is just dissolved. Great care must be taken in the preparation and use of this reagent, which must not be heated. Only a small volume should be prepared just before use, any residue washed down the sink with a large quantity of water, and the test-tubes rinsed with dilute nitric acid. 

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. 

I Oxidation of a primary alcohol or an aldehyde yields a carboxylic acid (Sections 17.7 and 19.3). Primary alcohols are often oxidized with C1O3 in aqueous acid, and aldehydes are oxidized with either acidic Cr03 or basic silver oxide (Tollens reagent). 

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. 

To identify the specific aldehyde that is actually involved in the light-emitting reaction of living luminous bacteria, Shimomura et al. (1974a) extracted and purified the aldehyde from 40 g each of the bacterial cells of P. phosphoreum, Achromobacter (Vibrio or Photobacterium) fischeri, and an aldehydeless mutant of A. fischeri. The aldehyde fractions were purified, and then oxidized with Tollens reagent (silver oxide dissolved in ammonia) to convert the CHO group into the COOH group. Then the acids obtained were analyzed by mass spectrometry. The results indicated that P. phosphoreum had contained a mixture of aldehydes dodecanal (5%), tetradecanal (63%) and hexadecanal (30%), as shown in Table 2.2. Thus, tetradecanal was clearly predominant in... 

Dichromate oxidation of secondary alcohols produces ketones in good yield, with little additional oxidation. For example, CH,CH2CH(OH)CH3 can be oxidized to CH CH2COCH3. The difference between the ease of oxidation of aldehydes and that of ketones is used to distinguish them. Aldehydes can reduce silver ions to form a silver mirror—a coating of silver on test-tube walls—with Tollens reagent, a solution of Ag1" ions in aqueous ammonia (Fig. 19.3) ... 

Oxidize aldehyde to RCOOH Ag20,0H" or Ag(NH3)2+ Tollen s reagent, ketone is not oxidized... 

This mixture of ammoniacal silver oxide and sodium hydroxide solution is potentially dangerous, because if kept for a few horns it deposits an explosive precipitate. This danger was described by Tollens in 1882 but is not generally known now. Prepare the reagent just before use, in the tube to be used for the test, and discard... 

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. 

At the beginning of section 2.2, you learned about Tollen s reagent. Tollen s reagent oxidizes aldehydes to produce the salt of a carboxylic acid. How does this reagent work as a test to distinguish between aldehydes and ketones ... 

Tollen s aldehyde test analychem A test that uses an ammonlacal solution of silver oxides to test for aldehydes and ketones. tal anz al da.hTd. test) tolnaftate org chem C19H17NOS An agricultural fungicide It Is also used medically as an antifungal agent. tol naf.tat ... 

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. 

Tollens reagent, AgCNHjlj , is a specific oxidant for CHO->COOH. 

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. 

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°. 

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