Tollens’ silver mirror test

Aldehydes are more easily oxidized than ketones. The Tollens silver mirror test is positive for aldehydes and negative for ketones. 

Oxidation of Aldehydes Tollens Silver Mirror Test... 

Aldehydes and ketones are chemically distinguished by oxidation. Aldehydes are easily oxidized and ketones are not. In the Tollens silver mirror test aldehydes are oxidized to carboxylic acids and ketones are not oxidized. A silver mirror plates on the side of the test tube as silver ion is reduced to silver metal. 

Silver metal precipitates from solution and coats the flask, producing a smooth silver mirror, as seen in Figure 14.4. The test is therefore often called the Tollens silver mirror test. The commercial manufacture of silver mirrors uses a similar process. Ketones cannot be oxidized to carboxylic acids and do not react with the Tollens reagent. 

We studied the use of Tollens silver mirror test in Section 16.13B. Benedict s solu tion and the related Fehling s solution (which contains a cupric tartrate complex ion) give brick-red precipitates of CUgO when they oxidize an aldose. [In alkaline solution ketoses are converted to aldoses (Section 22.5A), which are then oxidized by the cupric complexes.] Since the solutions of cupric tartrates and citrates are blue, the appearance of a brick-red precipitate is a vivid and unmistakable indication of a positive test. 

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. 

A laboratory test that distinguishes aldehydes from ketones takes advantage of their different ease of oxidation. In the Tollens silver mirror test, the silver-ammonia complex ion is reduced by aldehydes (but not by ketones) to metallic silver. The equation for the reaction may be written as follows ... 

But how are the atoms arranged What functional groups are present And what about the carbon skeleton Is it acyclic or cyclic Are there branches and where are they located Are benzene rings present All of these questions and more had to be answered by chemical means. Reactions, such as ozonolysis (Sec. 3.17.b) or saponification (Sec. 10.13), could be used to convert complex molecules to simpler ones whose structures were easier to determine. To identify functional groups, various chemical tests could be applied (such as the bromine or permanganate tests for unsaturation or the Tollens silver mirror test for an aldehyde group). 

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. 

Know the meaning of lithium aluminum hydride, sodium borohydride, Tollens reagent, silver mirror test. 

Use Tollens reagent (the silver mirror test). The pentanal (an aldehyde) will react, whereas 2-pentanone (a ketone) will not. 

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. 

Reducing action of dextrose silver mirror.—Test a dilute solution of glucose with an ammoniacal solution of silver nitrate or with Tollen s reagent (experiment 106d, page 84). 

A redox reaction involving silver is used in a chemical test to determine whether an unknown organic compound is an aldehyde. This test is called a Tollen s test. It is also sometimes called the silver mirror test because a spectacular shiny layer of elemental silver plates out on the inside of a test tube if an aldehyde is present. In this test, a silver nitrate solution is mixed with a solution of the unknown substance, and the mixture is observed to see whether the mirror forms. 

Tollens test (14.4) a test reagent (silver nitrate in ammonium hydroxide) used to distinguish aldehydes and ketones also called the ToUens silver mirror test... 

Tollen s reagent /tol-Sn/ A solution of the complex ion Ag(NH3>2 produced by precipitation of silver oxide from silver nitrate with a few drops of sodium hydroxide solution, and subsequent dissolution of the silver oxide in aqueous ammonia. Tollen s reagent is used in the SILVER-MIRROR TEST for aldehydes, where the Ag ion is reduced to silver metal. It is also a test for alkynes with a triple bond in the Tposi-tion. A yellow precipitate of silver carbide is formed in this case. 

The sample is warmed with the reagent in a test tube. Aldehydes reduce the complex Ag+ ion to metallic silver, forming a bright silver mirror on the inside of the tube (hence the name silver-mirror test). Ketones give a negative result, it is named after Bernhard Tollens (1841-1918). 

When Tollens reagent is added to an aldehyde, the aldehyde is oxidized to a carboxylic anion, and Ag is reduced to metallic silver. If this reaction is carried out properly, silver precipitates as a smooth, mirrorlike deposit—Whence the name silver-mirror test ... 

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 ... 

TMS tetramethylsilane. An inert, volatile liquid used as a reference in NMR, given a chemical shift of zero. Tollens reagent an aqueous solution of silver nitrate in excess ammonia solution, sometimes called ammoniacal silver nitrate solution. It is used to distinguish between aldehydes and ketones. It gives a positive silver mirror test when warmed with aldehydes, but no change is observed with ketones, triple covalent bond three shared pairs of electrons bonding two atoms together. 

---