Sugar, reactions mutarotation

The reversible reactions are initiated by an equilibrium between neutral and ionized forms of the monosaccharides (see Fig. 6). The oxyanion at the anomeric carbon weakens the ring C-O bond and allows mutarotation and isomerization via an acyclic enediol intermediate. This reaction is responsible for the sometimes reported occurrence of D-mannose in alkaline mixtures of sucrose and invert sugar, the three reducing sugars are in equilibrium via the enediol intermediate. The mechanism of isomerization, known as the Lobry de Bruyn-... 

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. 

Acetals and ketals are also called glycosides. Acetals and ketals (glycosides) are not in equilibrium with any open chain form. Only hemi-acetals and hemiketal s can exist in equilibrium with an open chain form. Acetals and ketals do not undergo mutarotation or show any of the reactions specific to the aldehyde or ketone groups. For example, they cannot be oxidized easily to form sugar acids. As an acetal, the carbonyl group is effectively protected. 

Many reactions that are promoted by enzymes can also be catalyzed by acids or bases or by both. An example is mutarotation, the reversible interconversion of the a- and (i-anomeric forms of sugars (Eqs. 4-1 and 9-86). This reaction is catalyzed by a specific mutarotase and also by inorganic acids and bases. 

The question of the detailed composition of sugar solutions in various solvents together with the chemistry of the mutarotation reaction has continued to attract attention through the years. In their classical review, Mutarotation of Sugars in Solution, completed in 1969, Isbell and Pig-man have cited references to more than 320 papers and books (5, 6). The appearance of this volume justifies Isbells faith that carbohydrate chemistry continues to have a significant impact on the development of new concepts permeating all branches of chemistry and biochemistry (7). ... 

The results obtained are shown in Table III (see p. 26). The mutarotation of the product indicated, in each case, that the /3-D-modification predominated for the acetyl sugars formed from reaction with water. The data show that the reaction was only slightly affected by change of solvent or of temperature, and was free of orthoester formation. These facts, and the almost exclusive formation of 1,2-trans products from the 1,2-cis-bromide, was interpreted as evidence that the reaction proceeds by way of the Sw2 mechanism. It was suggested that the increase of O-acetyl-a-D-glucoside formed at 50° (over that at 20°) may be due to racemization through carbonium-ion formation. 

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