Mutarotation acid/base catalysis

The original experimental evidence for concerted acid-base catalysis of the mutarotation in benzene is now considered unsound133 134 and concerted acid-base catalysis has been difficult to prove for nonenzy-matic reactions in aqueous solution. However, measurements of kinetic isotope effects seem to support Swain and Brown s interpretation.135 Concerted acid-base catalysis by acetic acid and acetate ions may have been observed for the enolization of acetone136 and it may be employed by enzymes.1363... 

Gibbs energy of dissociation, table 293 pKa values of, table 293 strengths of 95-96 Acid -base catalysis 469,486 - 491 concerted 490 of mutarotation 487 Acid - base chemistry... 

When a reaction is susceptible to catalysis by Bronsted acids and bases, the phenomenon is characterized as generalized acid or basic catalysis. One of the earliest reactions discovered to be subject to such generalized acid-base catalysis was the mutarotation of optically active glucose ... 

One of the first reactions to be examined in a reverse micelle was the mutarota-tion of 2,3,4,6-tetramethyl-a-D-glucose in solutions of dodecylammonium salts of carboxylic acids in benzene containing small amounts of water [134], The substrate is relatively hydrophilic, and should be partitioned into the aqueous interiors of the micelles where it is exposed to the ammonium and carboxylate groups of the surfactant which could act as general acid-base catalysts, (Mutarotation in water is subject to general acid-base catalysis,)... 

There is of course ample evidence that acid-base catalysis in solvents of low dielectric constant does not necessarily involve a concerted process. Such a process cannot operate when catalysis is effected by a single acid or base present in an aprotic solvent, and there are many examples of this, including typical prototropic reactions such as the halogenation of acetone, the racemization and inversion of optically active ketones, and the mutarotation of nitrocamphor. Moreover, in the isomerization of mesityl oxide oxalic ester in chlorobenzene, which depends kinetically on the interconversion of two isomeric enols, the velocity in a solution containing both an amine and an acid is no greater than the sum of the velocities for the two catalysts separately, in contrast to the behaviour found by Swain for the mutarotation reaction. 

The depolymerization of dimeric dihydroxyacetone and glycollalde-hyde ° has been studied in aqueous solution, and again shows general acid-base catalysis. Qualitatively similar catalytic behaviour has been observed in the mutarotation of optically active a-keto-esters in alcoholic solution,and in exchange reactions between alcohols and esters, both of which presumably take place by the reversible addition of alcohol to the carbonyl group. 

Thus in a mixture of pyridine and cresol, tetra-O-methylglucose was found to mutarotate, whereas in either pyridine or cresol, mutarotation was insignificant 54h). Lowry, therefore, proposed that the mutarotation of sugars is basically a ternary reaction involving simultaneous acid-base catalysis. The ternary reaction involves the simultaneous transfer of a proton from the acid catalyst to the sugar in the same step that a proton is trans-... 

EXAMPLE The mutarotation of glucose is a case of general acid-base catalysis, with the rate coefficient for the reaction in water having the form... 

Zeffren and Hall (1973) have commented that, since reactions with polar transition states in nonpolar solvents can be accelerated by several orders of magnitude by the presence of low concentrations of salts (Winstein et al., 1959), the rate enhancement of tetramethylglucose mutarotation provided by the presence of acid-base pairs such as phenol and pyridine may be due to formation of ion pairs in benzene solution. Salts which do not act as acids and bases catalyse mutarotation of tetramethylglucose in aprotic solvents (Eastham et al., 1955 Blackall and Eastham, 1955 Pocker, 1960). The efficiency of enzymatic catalysis could arise largely from electrostatic catalysis... 

The idea that simultaneous (i.e., concerted) acid and base catalysis could strongly accelerate reactions was probably introduced by T. M. Lowry in his considerations of general acid-general base catalysis of the mutarotation of glucose (Lowry and Faulkner, 1925). The concept was provided with strong experimental support by C. G. Swain and J. F. Brown (1952a,b), who used... 

General base and general acid catalysis. Base-catalyzed mutarotation might be formulated as follows A hydroxyl ion or some other base attacks the proton on the anomeric -OH group of the sugar, removing it to form an anion and the conjugate acid BH+ (Eq. 9-87,... 

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... 

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. 

The mutarotation reaction exhibits general acid-and-base catalysis, the rate depending on the concentrations of all the acids and bases present. 

The experimental verification of such a complex rate expression is usually carried out by varying the concentrations of a single conjugate acid-base pair at a time. In the mutarotation of glucose in water solution at 18°C, ko is found to be 0.0054, while for acetate ion, /cac- = 0.0265 for phenoxide ion, = 4.4 and for 011 it is 3800. For catalysis by ku is found to be 0.0040 liter/mole-min. It can be seen that the constants cover an exceedingly large range. 

The mutarotation constant, ki + ko is the sum of the constants for the two opposing reactions, and k lki is the equilibrium constant. Lowry and Hudson pointed out and showed that the same value is obtained for ki + kj from the mutarotation of the a and /8 anomers, a situation which has been experimentally confirmed by many others. Hudson found the reaction constant to be independent of the concentration of sugar over a wide range, and dependent on catalysis by both acids and bases, as had also been shown less precisely by Urech and by Levy. The effect of acids, bases, and salts will be considered in more detail in Part II of this review. 

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