Formulas Fischer-Tollens

The aldohexose D-glucose in the open-chain Fischer formula (I), the Fischer-Tollens hemiacetal ring formula (II), and the Haworth formula (III). The numbering system is shown. 

When the hydroxyl groups of glucose are esterified by treatment with acetic anhydride and a catalyst, two isomeric pentaacetates are formed. Similarly, isomeric methyl glucosides are formed by treatment of glucose with methanol and hydrogen chloride. The existence of two glucosides 17) and two pentaacetates 18) cannot be predicted on the basis of the aldehyde formula, a conclusion stated by Fischer in the case of the methyl glucosides, which he discovered, and even earlier by Colley and Tollens. 

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