Ketoses oxidation

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. 

Aldoses are reducing sugars because they possess an aldehyde function m then-open chain form Ketoses are also reducing sugars Under the conditions of the test ketoses equilibrate with aldoses by way of enediol intermediates and the aldoses are oxidized by the reagent... 

The parallel synthesis of furans from a-hydroxycarbonyl compounds is frequently conducted using aldoses or ketoses as readily available sources of this functional grouping, especially as the resulting polyhydroxyalkyl side-chain can be removed easily by oxidative degradation (Schemes 67d and 67e) 56MI30300). 

Although the Tollens reaction is a useful test for reducing sugars, it doesn t give good yields of aldonic acid products because the alkaline conditions cause decomposition of the carbohydrate. For preparative purposes, a buffered solution of aqueous Br2 is a better oxidant. The reaction is specific for aldoses ketoses are not oxidized by aqueous Br2. 

However, the pattern is complicated by several factors. The sugar molecules to be hydrogenated mutarotate in aqueous solutions thus coexisting as acyclic aldehydes and ketoses and as cyclic pyranoses and furanoses and reaction kinetics are complicated and involve side reactions, such as isomerization, hydrolysis, and oxidative dehydrogenation reactions. Moreover, catalysts deactivate and external and internal mass transfer limitations interfere with the kinetics, particularly under industrial circumstances. 

In aqueous solution the rate law for oxidation of simple a-ketols, such as acetoin and benzoin and various aldoses and ketoses o. sog-sioj markedly different from that of Wiberg and Nigh (vide supra), viz. 

The faster cupric ion oxidation of ketoses compared with aldoses is due to the more strongly reducing nature of the a-hydroxyketone group. Pre-treatment... 

Enediol formation is also rate limiting in the oxidation of aldoses and ketoses by alkaline ferricyanide , the rate expression being... 

Direct Oxidation of Ketose Sugars. The primary alcoholic group at Cl adjacent to the keto group at C2 of a ketose sugar such as D-fructose (XXI) or l-sorbose, is more sensitive to oxidation than the other primary alcoholic group at C6 or secondary alcoholic groups at C3, C4, and C5. 

Oxidation of Isopropylidene Derivatives of Ketose Sugars. Whenever isopropylidene derivatives can be obtained in which all groups except the primary alcoholic grouping at Cl are blocked this method is undoubtedly the best. It may be illustrated by reference to the preparation of 2-keto-D-gluconic acid from D-fructose.14... 

Measurement of the acidity produced in the periodate oxidation of carbohydrates is confined mainly to the determination of formic acid. Only from unsubstituted ketoses is the formation of such acids as glyoxylic and glycolic to be expected in carbohydrate chemistry.274... 

Carbon dioxide is not a common oxidation product in periodate work, but it does appear in the oxidation of ketoses,49 a-keto acids,14,39 and a-hydroxy acids,14 39 and it is often a product23 141 of overoxidation. Carbon dioxide analyses have been carried out using the Plantefol apparatus,49 the Warburg apparatus,14 23 and the Van Slyke-Neill mano-metric apparatus,39 and by absorption in standard sodium hydroxide141 followed by back-titration with acid. A most convenient method is the very old, barium hydroxide absorption scheme.16 The carbon dioxide is swept from the reaction mixture into a saturated, filtered barium hydroxide solution by means of a stream of pure nitrogen. The precipitated barium carbonate is filtered, dried, and weighed. This method is essentially a terminal assay. The manometric methods permit kinetic measurements, but involve use of much more complicated apparatus. 

Furan Derivatives from Ketoses and Their Oxidation Products ... 

Bertrand A microbiological process for oxidizing aldoses to ketoses. Used in the manufacture of ascorbic acid. 

In clinical chemistry however, these systems have not been differentiated as yet. Since the oxidation of an alcohol corresponds to the reduction of a ketose or an aldose, the designations ketose reductase and aldose reductase, respectively, were suggested (H4, W14). In this paper however, the enzyme or enzyme system will be named polyol or sorbitol dehydrogenase (SDH), although the latter expression does not characterize exacdy the enzyme s function in a general biochemical sense. But sorbitol or fructose have been commonly used as substrates in clinical chemical investigations. 

Oxidizing the aldehyde group present in aldoses is easy oxidizing the carbonyl group in a ketose is far more difficult. The susceptibility (or lack thereof) to simple oxidation is a useful method of distinguishing between aldoses and ketoses. The next sections explore the various types of monosaccharide oxidation reactions that can occur. 

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