Transformations

Abstract

In the last chapter, we briefly discussed mutarotation. If one dissolves either pure α-D-glucopyranose ([α] ²⁰_D = + 112°) or pure ß-D-glucopyranose ([α] ²⁰_D = + 19°) in water, a complex series of reactions take place to give a mixture of products that are in equilibrium. The [α] ²⁰_D of this mixture is +52.7° and represents the resultant optical rotation of five different compounds: 37% α-D-glucopyranose, 67% ß-D-glucopyranose, 0.5% α-D-glucofuranose, 0.5% ß-D-glucofuranose, and 0.002% of the open-chain free aldehyde [1]. The structures of the five forms of D-glucose in solution at equilibrium are shown in Figure 3.1. The process of mutarotation gives this equilibrium mixture if the starting compound is any one of the five forms. The four ring structures are transformed into each of the other ring structures through the open-chain form, until the equilibrium amounts are obtained. The process is slow, taking many hours to reach equilibrium in distilled water at 20°C. Both acid and base can catalyze the transformations. Alkali is the better of the two catalysts. Dilute alkali (pH 10) catalyzes the transformation approximately 5,000 times faster than an equivalent amount of acid (pH 4). The transformation is also catalyzed by 2-hydroxypyridine and by the enzyme mutarotase, which is produced by several fungi such as Penicillium notatum and Aspergillus niger and found in some animal tissues. Catalysis by 2-hydroxypyridine was the first reported example of concerted acid-base catalysis. 2-Hydroxypyridine has acidic (pyridinium ion) and base (phenoxylate ion) groups rigidly held in a favorable position for effecting catalysis. 2-Hydroxypyridine is approximately 7,000 times more effective as a catalyst than the hydroxide ion at pH 10.