Aldehyde reaction with Tollens reagent

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... 

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. 

Writing Equations for the Reaction of an Aldehyde and of a Ketone with Tollens Reagent... 

The oxidizing agent in Tollens reagent is Ag, which is reduced to metallic silver. The Tollens test is based on this reaction If Tollens reagent is added to a small amount of an aldehyde in a test tube, the inside of the test tube becomes coated with a shiny mirror of metallic silver. Consequently, if a mirror is not formed when Tollens reagent is added to a compound, it can be concluded that the compound does not have an aldehyde functional group. 

A silver mirror has been deposited in the inside of this flask by the reaction of an aldehyde with Tollens reagent. 

Whilst formylpyrroles behave in many ways as would be expected, some anomalous properties have attracted attention. Thus, 2-formylpyrrole gives an oxime, arylhydrazones and other typical carbonyl derivatives , and condenses with methyl ketones, hippuric acid and other reactive methylene compounds . 168, 169 It does not give the usual aldehyde reactions with Schiff s, Fehling s or Tollen s reagent ", nor does it form a cyanhydrin or take part in the Cannizzaro or Perkin reactions . ... 

Aldoses and ketoses, similar to aldehydes, reduce heated Tollens reagent (see p. 210), Nessler s reagent (p. 211), Fehling s reagent [p. 210 see also (55)], and also Nylander s reagent (an alkaline solution of a bismuth salt) and a solution of ammonium molybdate. The reaction with nitro compounds should also be mentioned here. 

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... 

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.)... 

Aldoses reduce Tollens reagent, as we would expect aldehydes to do. They also reduce Fehling s solution, an alkaline solution of cupric ion complexed with tartrate ion (or Benedict s solution, in which complexing is with citrate ion) the deep-blue color of the solution is discharged, and red cuprous oxide precipitates. These reactions are less useful, however, than we might at first have expected. 

Tollens reagent reacts with some aldehydes in cold conditions and some aldehydes in hot conditions. Metallic silver, produced in the reaction, is deposited on the walls of the vessel (silver mirror) or it precipitates. This reaction is used to detect the presence of an aldehyde or to distinguish between aldehydes and ketones. 

Both aldoses and ketoses are oxidized to aldonic acids by Tollens reagent (Ag, NH3, HO ), so that reagent cannot be used to distinguish between aldoses and ketoses. Recall from Section 20.3, however, that Tollens reagent oxidizes aldehydes but not ketones. Why, then, are ketoses oxidized by Tollens reagent, while ketones are not Ketoses are oxidized because the reaction is carried out under basic conditions, and in a basic solution, ketoses are converted into aldoses by enolization (Section 19.2). For example, the ketose D-fructose is in equilibrium with its enol. However, the enol of D-fructose is also the enol of D-glucose, as well as the enol of D-maimose. Therefore, when the enol reketonizes, all three carbonyl compounds are formed. 

Tollens reagent contains Ag ions dissolved in aqueous ammonia. If the reagent is warmed with an aldehyde, the silver ions are reduced to silver metal and a distinctive silver mirror is deposited on the reaction container. Again, ketones do not react with this reagent. 

Aldehydes and ketones are prepared by the oxidation of primary and secondary alcohols, respectively. Aldehydes can be further oxidized to carboxylic acids, but ketones resist oxidation. Thus, aldehydes are oxidized by Tollens reagent (Ag" ) and Benedict s solution (Cu ), whereas ketones are not. A characteristic reaction of both aldehydes and ketones is the addition of hydrogen to the carbonyl double bond to form alcohols. In a reaction that is very important in sugar chemistry, an alcohol can add across the carbonyl group of an aldehyde to produce a hemiacetal. The substitution reaction of a second alcohol molecule with the hemiacetal produces an acetal. Ketones can undergo similar reactions to form hemiketals and ketals. 

Thus, the chromic acid reagent gives a clear-cut distinction between primary and secondary alcohols and aldehydes on the one hand and tertiary alcohols and ketones on the other. Aldehydes may be distinguished from primary and secondary alcohols by means of Schiff s, Tollens s, Benedict s (Sec. 23.4), and Fehling s tests, and primary and secondary alcohols of lower molar mass may be differentiated on the basis of their rates of reaction with concentrated hydrochloric acid containing zinc chloride—the Lucas reagent (Sec. 25.11B). 

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. 

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. 

Many other such reactions occur with changes by a combination of oxidation-reduction and substitution. When Tollens s reagent, [Ag(NH3)2]", oxidizes aldehydes, the... 

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