Tollen s reagent

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. 

Nitro compounds and their reduction products. Tertiary nitro compounds (these are generally aromatic) are reduced by zinc and ammonium chloride solution to the corresponding hydroxylamines, which may be detected by their reducing action upon an ammoniacal solution of silver nitrate or Tollen s reagent ... 

It must be remembered, however, that nitroso, azoxy and azo compounds (which are usually more highly coloured than nitro compounds) may bo reduced by zinc powder to the corresponding hydroxylamino, hydrazo and hydrazine compounds respectively, all of which reduco Tollen s reagent in the cold. 

Tmta — see Acetic acid, trimethylenediaminetetra-Tollen s reagent, 5, 780 Tolman s cone angle, 2, 1015 Toluene acetoxylation... 

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

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 ... 

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. 

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. 

Glucose also will reduce Fehling s solution [Cu(II) — Cu(I)] and Tollen s reagent [Ag(I)—> Ag(0)] and, for this reason, is classified as a reducing sugar.4... 

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 reagent is prepared as follows Dissolve 3 g of silver nitrate in 30 ml of water (solution A) and 3g of sodium hydroxide in 30 ml of water (solution B). When the reagent is required, mix equal volumes (say, 1 ml) of solutions A and B 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. 

The chemical properties of monosaccharides are further complicated by the fact that they can exhibit tautomerism in aqueous basic solutions (Figure 1.15). This means that after a short time a basic aqueous solution of a monosaccharide will also contain a mixture of monosaccharides that will exhibit their characteristic chemical properties. For example, a solution of fructose will produce a silver mirror when treated with an ammoniacal solution of silver nitrate (Tol-len s reagent). This is because under basic conditions fructose undergoes tautomerism to glucose, whose structure contains an aldehyde group, which reduces Tollen s reagent to metallic silver. 

Chemically deposited films on frosted glass slides provide a more facile and reproducible approach to SERS substrates. One such approach [31], which has proven to be very successful, involves the initial preparation of Tollen s reagent. The reagent is prepared by adding about 10 drops of fresh 5% sodium hydroxide solution to 10 mL of 2 to 3% silver nitrate solution, whereupon a dark-brown AgOH precipitate is formed. This step is followed by the dropwise addition of concentrated NH4OH, at which point the... 

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