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Question
How do you explain the absence of the aldehyde group in the pentaacetate of D-glucose?
Solution
The cyclic hemiacetal form of glucose contains an OH group at C-l which gets hydrolysed in aqueous solution to produce an open-chain aldehydic form, which then reacts with NH2OH to form the corresponding oxime. Thus, glucose contains an aldehydic group. However, when glucose is reacted with acetic anhydride, the OH group at C-l along with the other OH groups at C-2, C-3, C-4 and C-6 forms a pentaacetate. Since the pentaacetate of glucose does not contain a free OH group at C-l, it cannot get hydrolysed in aqueous solution to produce an open-chain aldehydic form and hence glucose pentaacetate does not react with NH2OH to form glucose oxime. Hence, glucose pentaacetate does not contain the aldehdye group.
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