Mutarotation, glucose

A third possible type of catalysis requires that a base and an acid act synchronously to effect the breaking and formation of bonds in a single step. Thus, tetramethyl-glucose mutarotates very slowly in benzene containing either pyridine (a base) or phenol (an acid). However, when both pyridine and phenol are present, mutarota-tion is rapid. This suggested to Swain and Brown132 a concerted mechanism (Eq. 9-92) in which both an acid and a base participate. 

A possibility that was proposed quite early for the glucose mutarotation, and that could conceivably be of importance for other reactions, is simultaneous catalysis by an acid and a base. It will be recalled from Section 8.1 that hydration requires addition of a proton at one site and removal of a proton from another. If both these processes were to occur in one step, either by means of separate acid and base molecules acting together or by action of a single molecule containing both an acidic and a basic center, we would designate the process as a concerted acid and base catalysis (Equation 8.39).60 Swain found that the rate of... 

These specific mixed solvents were chosen for this study, since solvent systems of this type have been used in similar studies such as the study of glucose mutarotation kinetics and equilibria in 50 mass % THF-H20... 

Catalysts other than the template (6) can be found for this system. The action of bifunctional catalysts in acylation reactions and glucose mutarotation... 

Section 23.8 Hemiacetal forms of carbohydrates are interconvertible in water. The equilibrium mixture can contain a and p anomers of furanose and pyranose forms. The change from one form to the equilibrium mixture is accompanied by a change in optical rotation called mutarotation. For D-glucose, mutarotation can be described in terms of the interconversion of a-pyranose and P-pyranose forms by way of the open-chain form. 

This is the typical approach for studying the Glucose Mutarotation problem (Guerasimov, 1995 Martinez-Luaces, 2009e), but not the best one for this article devoted to inverse problems. In order to get a different point of view, we shall write the mechanism ... 

When dissolved in water, the a-glucose mutarotated downward and the 7-glucose upwards to the same constant specific rotation of 52.5°. Tanret s jS-glucose exhibited no mutarotation and later was considered to be a mixture of the two other forms in their equilibrium proportions. The name of j3-glucose is now given to the form which he named as 7-glucose. The common form is the a-isomer. 

The high efficiency of this combined attack is related to the existence of biflinc-tional catalysts of prototropic enolization. A molecule of these catalysts contains both acidic and basic groups. For example, 2-hydroxypyridine is such a catalyst. It catalyzes glucose mutarotation 7000-fold more efficiently than a mixture of phenol and pyridine (298 K, H2O, catalyst concentration 0.001 M). Keto acids, P-diketones, and pyrazole have a similar catalytic effect. The transformation scheme is the following ... 

The rate constant of glucose mutarotation catalyzed by acids (A) is expressed according to the following equation k = kt) + Ua [A, where ko stands for a first order rate constant in the absence of acids. In the presence of acetic acid, the following results were obtained ... 

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