Ketones Tollens’ reaction

When aliphatic nitro compounds are used instead of aldehydes or ketones, no reduction occurs, and the reaction is essentially a Knoevenagel reaction, though it is usually also called a Tollens reaction ... 

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. 

In Tollens reaction an aldehyde or ketone containing an a hydrogen is treated with formaldehyde in the presence of Ca(OH)2 or a similar base. The first step is a mixed aldol reaction (6-39). 

Optimum conditions (Table VI) for ring-hydroxymethylation of 1 mole phenol 2 moles of CH20, 1 mole of NaOH, 0.8-1 NaOH concentration, 3 to 4 days at room temperature for Tollens reaction (1 mole of model ketone) 8 moles of CH20, 2 moles of NaOH, 2N NaOH concentration, 5 days at room temperature. 

When aliphatic nitro compounds are used instead of aldehydes or ketones, no reduction occurs, and the reaction has been referred to as a Tollens reaction (see 16-43). However, the classical condensation of an aliphatic nitro compound with an aldehyde or ketone is usually called the Henry reaction or the Kamlet reaction, and is essentially a nitro aldol reaction. A variety of conditions have been reported, including the use of a silica catalyst, Mg—A1 hydrotalcite, a tetraalkylam-monium hydroxide,proazaphosphatranes, " or an ionic liquid.A solvent free Henry reaction was reported in which a nitroalkane and an aldehyde were reacted on KOH powder. Potassium phosphate has been used with nitromethane and aryl aldehydes. The Henry reaction has been done using ZnEt2 and 20%... 

In the Cannizzaro reaction, the hydride ion that is being used to effect this reduction may come from an aldehyde that lacks an a-hydrogen atom, e.g. methanal or benzaldehyde. The receiving molecule may be a second molecule of the same aldehyde or a different one. The reaction requires a strong base, and the rate law is found to depend on the square of the concentration of the aldehyde and either the concentration or the square of the concentration of the base used. Overall a carboxylate anion and an alcohol are formed from two molecules of the aldehyde(s). The reaction may occur intramolecularly, i.e. a-ketoaldehydes give the a-hydroxycarboxylic acids on treatment with hydroxide ions. A variation of this process is called the Tollens reaction. In this case, a ketone or aldehyde that contains an a-hydrogen is treated with formaldehyde in the presence of Ca(OH)2. 

Tollens reaction The reaction of an aldehyde or ketone that has an a-hydrogen with formaldehyde, in the presence of Ca(OH)2 to give initially a mixed aldol product this is then usually reduced by... 

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. 

Ketones CH, COCH3 + Ag+ (in Tollens reagent) gives no reaction... 

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. 

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... 

The aldehyde or ketone group of monosaccharides can undergo an intramolecular reaction with one of its own hydroxyl groups to form a cyclic, hemiacetal or hemiketal structure, respectively (Fig. 20). In aqueous solutions, this cyclic structure actually predominates. The open-chain aldehyde or ketone form of monosaccharides is in equilibrium with the cyclic form, but the open structure exists less than 0.5% of the time in aqueous environments. It is the open form that reduces Fehling s or Tollen s... 

Oxidation by Tollens reagent is useful chiefly for detecting aldehydes, and in particular for differentiating them from ketones (see Sec. 19.17). The reaction is of value in synthesis in those cases where aldehydes are more readily available than the corresponding acids in particular, for the synthesis of unsaturated acids from the unsaturated aldehydes obtained from the aldol condensation (Sec. 21.6), where advantage is taken of the fact that Tollens reagent does not attack carbon-carbon double bonds. 

The Tollens condensation is a crossed aldolization employing formaldehyde and an aldehyde or a ketone in the presence of a weak base. Since formaldehyde has no -hydrogen atoms, it is an extremely reactive carbanion acceptor (p. 155). Consequently, reaction occurs readily with carbanions and leads to polymethylol derivatives 6... 

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. 

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

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. 

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 ... 

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). 

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