Organic 2 Review Problem 157

Organic 2 Review Problem 157 - PREPARATIONS 141...

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PREPARATIONS 141 composition, the material darkening in colour immediately before melting. a- and £-Pentacetylglucose. C 6 H 7 O(O-COCHg) 5 . When glucose is heated with an excess of acetic anhydride in the presence of a catalyst, all five hydroxyl groups are acetylated, and the resulting pen- tacetyl-glucose can clearly exist in two isomeric forms, corresponding to the a and p forms of glucose itself (see Formulae I and II, p. 135). When zinc chloride is used as a catalyst, the a-pentacetylglucose is formed, and when sodium acetate is used, the [i-pentacetylglucose is produced. It is probable that the product in each case contains traces of the other isomeride, which however can be readily eliminated by recrystallisation. (A) a-PENTACETYLGLUCOSE. Required: Zinc chloride, ca. i g.; acetic anhydride, 25 ml.; glucose, 5 g. From a clean, dry stick of zinc chloride, break off a piece weighing roughly i g., crush it quickly into coarse fragments in a mortar, and then add it to 25 ml. of acetic anhydride contained
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