Glucose’s pentaacetate does not react with hydroxylamine, indicating the absence of a free —CHO group. This contradicts the open-chain structure (I) and suggests a different configuration.
What evidence from the reaction with its pentaacetate supports the absence of a free —CHO group in glucose?
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The reaction of glucose with acetic anhydride in the presence of pyridine to form its pentaacetate provides evidence for the absence of a free aldehyde (—CHO) group. In this reaction, glucose undergoes acetylation, and each hydroxyl group can be acetylated. The absence of a free aldehyde is indicated by the inability of the pentaacetate derivative to react with agents that specifically test for aldehydes. Since all hydroxyl groups in glucose participate in the formation of acetate esters, the pentaacetate derivative does not react as an aldehyde, supporting the conclusion that the original aldehyde group has been derivatized.