Mutarotation, of « -D-glucose

Capon and Walker, 1971. The reference intermolecular reaction is the mutarotation of D-glucose, catalysed by a carboxylic acid of pK, 4.3... 

Measurement. Rotations were measured with a Rudolph Model 200 photoelectric polarimeter operating from a sodium vapor lamp. Water was circulated through 20-cm. tubes with glass end plates, from a bath maintained at 25.0° 0.1°. The runs on the mutarotation of D-glucose showed excellent first-order dependence over two half lives. In studies on the mutarotation of glucosamine hydrochloride in the presence of varying concentrations of sodium hydroxide and metal salt, it was necessary to add the alkali first, then the metal salt, to avoid precipitation. 

Fig. 2-8. Mutarotation of D-glucose. 1, a-D-Glucopyranose 2, /3-D-glucopyranose 3, a-D-glucofuranose 4, /3-D-glucofuranose 5, the open-chain aldehyde form.

A reaction exhibiting general acid catalysis, the ester hydrolysis (XXXVIII), has been discussed in Sect. 2.3. The present section deals with a classic reaction which is subject to both general acid and base catalysis in homogeneous media, the mutarotation of D-glucose. 

Los and Simpson reported that the catalytic constant for the mutarotation of D-glucose is strongly dependent on the D-glucose concentration, and they suggested that this effect is occasioned by the incursion of a concerted mechanism for the formation of the open-chain form (see XLI — XLII). Such a mechanism was proposed by Swain and Brown ... 

Increase of hydrostatic pressure is reported to increase the rate of mutarotation, and, at 2500 atm, the mutarotation of D-glucose is 3.4 times that at atmospheric pressure. From such data, Sander calculated that the decrease in volume of a-D-glucose in the activated state is in the range of 12.5 to 21.2 ml per mole. 

Problem 34.21 From what you learned in Secs. 19.8 and 19.15, suggest a mechanism for the acid-catalyzed mutarotation of D-(+)-glucose. 

A number of other reactions in this general area such as the mutarotation of D-(+)-glucose and aldose-ketose isomerizations are also subject to catalysis by metal ions. 

Fig. 2.7 Mutarotation of D-glucose (the arrow in intermediate, open-chain structure denotes bond rotation).

To evaluate the catalytic coefficients for a specific reaction, the rate expression, equation 9, is set up for the known catalysts. Then, a series of measurements is made under such conditions that certain terms in the equation are negligible and the contribution of the catalyst under study becomes important. Under these conditions, the effect of the concentration of the catalyst on the rate constant may be determined. For example, to determine the catalytic coefficients for the mutarotation of D-glucose in an aqueous sodium acetate-acetic acid solution, the following equation is set up ... 

Some values of kHl0 and km0 reported in the literature for the mutarotation of D-glucose are summarized in Table VII. 

Catalytic Constants for Water and Hydronium Ions in the Mutarotation of D-Glucose°... 

Catalytic Constants189 for the Mutarotation of D-Glucose by Weak Acids and Their Anions at 18°... 

The coefficient for the water molecule acting as a proton donor (that is, as an acid catalyst) may be calculated from equation 12 by using data for the mutarotation of D-glucose at 18°, with acetic acid as the reference point. From equation 15,... 

Comparison of Various Types of Acid and Base Catalysts in the Mutarotation of D-Glucose ... 

In these experiments, the contributions of the D-glucosate ion to the total rate-constant ranged from 18 to 41 %, depending on the concentration of the sugar. In a subsequent investigation,230 the study was extended, and catalytic coefficients and heats of activation were determined for catalysis of the mutarotation of D-glucose in water by each of three acids and four bases. 

A possible variation of the concerted mechanism includes a dimeric water molecule as a bifunctional catalyst (see Fig. 10). Another concept concerning the role of water molecules has been developed by Schmid.254 He suggested an activated complex of the type (BH . .. H20. .. G ), where B is a base, and G is a D-glucosyloxy anion. In the hydrogen-ion-catalyzed mutarotation of D-glucose in aqueous solutions, H20 acts as a base to remove H30 , and the activated complex is D-glucose ...H20...H30 . 

---