A Silver Mirror

In 1856. Liebig published the following procedure for coaling glass with silver or gold in his Annalen  

The lollowing observations made by Liebig must be taken into account when silver is to be used  

The surface to be coated must he saupulously cleaned otherwise it will acquire spots  

the glass suffice must he equidistant from the base of the vessel so that the level of the liquid is everywhere the same and the silver deposition ts unijorm  

the glass surface must he completely wetted by the sihfcring liquid, and should he washed with alcohol in advance to assure that this will occur more satisfactorily . .. 

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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. 

Reduction of ammoniacal silver nitrate. Place about 5 ml. of AgNOj solution in a thoroughly clean test-tube, and add 2-3 drops of dil. NaOH solution. Add dil. ammonia solution, drop by drop, until the precipitated silver oxide is almost redissolved, then add 2 - 3 drops of formaldehyde or acetaldehyde. A silver mirror is formed. 

Gives a silver mirror with ammoniacal silver nitrate. 

Reduction of ammoniacal silver nitrate. Add a few drops of a neutral solution of a formate to ammoniacal AgNO (see Test 4, p. 342). A silver mirror or more usually a grey precipitate of metallic sih er is produced on boiling. 

Reduction of ammoniacal silver nitrate. Add i drop of dil. NaOH solution to about 5 ml. of AgNO, solution, and add dil. NH solution drop by drop until the silver oxide is almost redissolved. Add AgNO, solution until a faint but permanent precipitate is obtained (see p.525). Then add 0 5 ml. of a neutral tartrate solution. Place the tube in warm water a silver mirror is formed in a few minutes. 

Reduction of ammoniacal silver nitrate. Place 2 ml. of dilute silver nitrate solution in a clean test-tube. Add 1 drop of NaOH solution and then add dil. ammonia drop by drop until the precipitate formed by the NaOH is just not redissolved. Now add 1-2 ml. of glucose solution and place the test-tube in a water-bath at 50-60° a silver mirror is produced in 1 - 2 minutes. 

Does not reduce ammoniacal silver nitrate or Fehling s solution. If, however, the sucrose solution is warmed for some time with the reagent in question, slight hydrolysis to glucose and fructose does take place and reduction then occurs occasionally samples of sucrose will rapidly give a silver mirror, presumably owing to impurities. 

Oxidation, (a) Ammoniacal silver nitrate. To a few ml. of ammoniacal AgNOj (preparation, p. 525), add a few drops of cold aqueous benzo quinone solution a silver mirror or (more generally) a dark precipitate of metallic silver is formed in the cold. 

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. 

Readions. — i. Adda few chops of the acid solution or solution of the calcium salt to a few c.c. of ammonia-silver nitrate and warm in hot water. A silver mirror is deposited. 

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) ... 

FIGURE 19.3 An aldehyde (left) produces a silver mirror with Tollens reagent, but a ketone (right) does not. 

In the presence of a saturated tetraaza macrocycle such as cyclam, disproportionation of Ag(i) occurs to produce a silver mirror and a stable Ag(n) complex of the macrocycle (Kestner Allred, 1972 Barefield Mocella, 1973). In some cases the Ag(n) complexes so formed may then be oxidized further to Ag(m) species either electrochemically or chemically [using nitrosyl (NO+) salts]. 

Lithium containing films were unusual in that the film after curing was damp on the surface with what appeared to be the solvent, DMAC. No other films exhibited this property. The AgNO containing film had the appearance of a silver mirror but the film was exceedingly brittle and "flaky-like". Only two truly flexible films were produced from BTDA + m,m -DABP. These contained Al(acac)3 and NiCl2 6H2O respectively. 

In the case of slow reactions it is inconvenient trying to maintain a sufficiently low temperature throughout the conversion in a normal flask, even with the use of a dry icc-aceione condenser. It is much easier to allow the reaction to proceed in a round- bottomed Dewar flask (fig. 7, a flask with an evacuated space between the two walls). The rate of evaporation is reduced to a minimum (10 g/h or less) if the inner wall is covered with a silver mirror (covering the flask with aluminum foil is a good alternative). An example of a reaction in a Dewar flask is the conversion of Li CH with oxirane to HOCCf CI OLi (Chap, m, exp. 23). The ammoniacal soludon of LiCeCH is first prepared in the usual apparatus and then transferred (by pouring) into the Dewar flask. The oxirane is subsequently added over a short period and a stopper with a drying tube is placed on the flask. After 24 b the soludon is poured into a normal round-bottomed flask and the ammonia is removed by evaporanon. 

Although it is obviously of no practical synthetic significance, mention may be made of Linnemann e observation1087 that sealing propylene oxide in the dark for 4 years in the presence of silver oxide caused deposition of a silver mirrOr and formation of silver acetate (Eq. 430). Formio arid underwent oomplete oxidation to carbon dioxide and water. 

Tollens reagent, which is based on Ag(NH3)2, can be used to test for the presence of aldehydes. The weakly oxidizing system converts aldehydes to carbbkylates and if the reaction is slow and the walls of the vessel are clean, then a silver mirror can often be observed, otherwise a grey or black precipitate results. No oxidation of ketones occurs, except with or-hydroxy ketones, and on the basis of its reaction with sugars, they can be categorized as... 

It has been known for some time that silver(I) porphyrins are generally unstable toward disproportionation to the silver(II) complex, sometimes with deposition of a silver mirror.551... 

Aluminum is unique among the metals because it responds to nearly all of the known finishing processes. It can be finished in the softest, most delicate textures as exemplified by tableware and jewelry. Aluminum can be anodized and dyed to appear like gold. It can be made as specular as a silver mirror and jet black. The metal also can be anodized to an extremely hard, wear- and abrasion-resistant surface that approaches the hardness of a diamond. Aluminum is available in many convenient forms-shapes, sheet, plate, ingot, wire, rod and bar, foil, castings, forgings, powdered metals, and extrusions. 

Treat each with Tollens reagent Only pentanal will give a silver mirror. 

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.)... 

Aldehydes will form a silver mirror or a black precipitate if the test tube is dirty, while ketones will not. 

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

Tollen s test The reagent should be freshly prepared by mixing two solutions (A and B). Solution A is a 10% aqueous AgNOs solution and solution B is a 10% aqueous NaOH solution. When the test is required, 1 ml of solution A and 1 ml of solution B are mixed, and the silver oxide thus formed is dissolved by dropwise addition of 10% aqueous NH4OH. To the clear solution, 10 drops of the compound to be tested are added. A silver mirror is indicative of the presence of an aldehyde. The reagent mixture (A + B) is to be prepared immediately prior to use otherwise, explosive silver fulminate will form. The silver mirror is usually deposited on the walls of the test tube either immediately or after a short warming period in a hot water bath. This is to be disposed of immediately with diluted HN03 (detection limit, 50 mg compounds tested, Q to C6). 

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. 

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