Silver mirror formation

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. 

Formation of silver mirror or precipitate of silver indicates reducing agent. (This is often a more sensitive test than I (a) above, and some compounds reduce ammoniacal silver nitrate but are without effect on Fehling s solution.) Given by aldehydes and chloral hydrate formates, lactates and tartrates reducing sugars benzoquinone many amines uric acid. 

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. 

Sometimes the Ag - NHC reagent has a twofold effect (i) transmetallation of the carbene and (ii) oxidation of the metal. The reaction of the dimetal-lic Ag biscarbene 43 with [ (p-cymene)RuCl2 h yields the Ru(II) complex 44 (Scheme 36). However, when the same complex 43 reacts with RuCl2(PPh3)3, a Ru(III) complex is obtained (45) with a CCO tripod coordination of the ligand [140]. In this latter case, the reduction of Ag(I) to Ag(0) is confirmed by the formation of a silver mirror in the reaction vessel. Compound 43 can also react with Cul to afford a square planar NHC - Cu(I) complex [141]. 

An alternative method for carrying out the silver mirror test is the following. Prepare ammoniacal silver nitrate solution by placing 5 ml silver nitrate solution in a thoroughly clean test-tube and add 2-3 drops of dilute sodium hydroxide solution add dilute ammonia solution dropwise until the precipitated silver oxide is almost redissolved (this procedure reduces the danger of the formation of the explosive silver azide, AgN3, to a minimum). Introduce about 0-5 ml neutral tartrate solution. Place the tube in warm water. A silver mirror is formed in a few minutes. 

Tollen s test (Section 19.3) a chemical test for detecting aldehydes by treatment with ammonia-cal silver nitrate. A positive test is signaled by formation of a silver mirror on the walls of the reaction vessel. 

Problem 14.44. An unknown is either 2-pentanone or pentanal. Shaking the unknown with a few drops of Tollens reagent results in the formation of a silver mirror. Identify the unknown and explain. 

The formation of the silver mirror with ammoniacal silver nitrate is the test which distinguishes aldehydes, which give a positive result, from ketones, which have no reaction. Aldehydes are also oxidised by potassium manganate(VII) or acidified potassium dichromate(VI). 

Careful attempts have been made to define the optimum conditions of formation of silver mirrors from silver nitrate and hydrazine sulphate, and the heterogeneous kinetics examined. ... 

An investigation of why hydroxide makes the Tollens silver mirror test for aldehydes more sensitive has focused on thermodynamic versus kinetic factors. Electrochemistry tends to rule out the former the electromotive force (emf) of an appropriate cell changes little with pH. Exploring the kinetics, single electron transfer processes were confirmed by addition of a radical trap (TEMPO), which slowed the reaction. Rate measurements point to the rate of the formation of the anion of the gm-diol (i.e. the hydrate anion) as the key parameter affected by added hydroxide, a factor that also explains how the rapidity of the test varies with the structure of the aldehyde. 

If metal ions in solution are reduced to atoms by an appropriate redudng agent, they normally will combine into colloids or into microcrystalline particles which finely predpitate out. Such processes can occur within seconds, as is known for instance from the formation of silver mirrors on glass by the reduction of silver ions. If the growth of the metal particles in solution can be retarded and if appropriate ligands are additionally offered, it may happen that metal rich ligand stabilized transition metal clusters of uniform rize are formed. 

PROBLEM 9.22 Write an equation for the formation of a silver mirror from formaldehyde and Tollens reagent. 

Tollens s Test Before performing this test, read the Safety Alert and Wrapping It Up sections in Seotion 25.7C. Perform Tollens s test according to the procedure outlined in that seotion using about 0.1 g of the carbohydrate in a clean glass test tube and about 1 ml of Tollens s reagent. The formation of a silver mirror or a black precipitate constitutes a positive test. 

Another method for distinguishing between aldehydes and ketones is Tollens s test. A positive test indicates the presence of an aldehyde function, whereas no reaction occurs with ketones. Tollens s reagent consists of silver-ammonia complex, Ag(NH3)2, in an ammonia solution. This reagent oxidizes both aliphatic and aromatic aldehydes to the corresponding carboxylic acids silver ion is reduced to elemental silver, which is deposited as a silver mirror on the glass wall of a clean test tube. Thus, the formation of the silver mirror or of a precipitate is considered a positive test. Equation 25.12 shows the reaction that occurs. 

To carry out the test, add 0.5 mL of the reagent to 3 drops or 50-100 mg of the unknown compound the formation of a silver mirror or black precipitate constitutes a positive test. The silver deposits in the form of a mirror only on a clean glass surface. A black precipitate, although not as aesthetically pleasing, still constitutes a positive test. If no reaction occurs at room temperature, warm the solution slightly in a beaker of warm water. 

Two chemical ways were used to include metal nanoparticles inclusion in the microcapsule shell. It was photoreduction of silver under of UV-irradiation and chemical reduction of silver by acetaldehyde oxidation (reaction of a "silver mirror"). The polystyrene latex and calcium carbonate were used as a template for the formation of polyelectrolyte shells. 

The comparison of different types of nanoparticles embedded in capsule shells in the terms of interaction with laser light was performed in paper. The main goal was to establish the difference between pre-synthesized silver nanoparticles and the ones fabricated by "silver mirror" reaction during the shell formation process. One type of the capsules was built using calcium carbonate cores of 11.5 m in diameter, and the other one was fabricated on the surface of 10.25 pm polystyrene microspheres. Pre-fabricated nanoparticles were adsorbed from the mixture of silver sol and anionic polyelectrolyte (PSS) solution in 0.5M NaCl. 

Silver Mirror Test for Aldehydes Tollens Reagent. This reaction involves the oxidation of aldehydes to the corresponding carboxylic acid, using an alcoholic solution of silver ammonium hydroxide. A positive test is the formation of a silver mirror, or a black precipitate of finely divided silver ... 

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