How do you explain the absence of aldehyde group in the pentaccetate of D-glucose?

Explanation

In its cyclic structure, glucose does not contain any free aldehyde group because it exists in the hemiacetal form. When, however, it is dissolved in water, it may undergo ring opening to generate the open chain aldehydic form, thereby indicating the presence of aldehyde group in glucose.
When glucose (hemiacetal) is treated with acetic anhydride, the `-OH` group at `C-1`, along with four other `-OH` groups at `C-2, C-3, C-4 and C-6` are involved in the formation of acetyl derivative. The resulting pentaacetate (having a cyclic structure) does not contain a free `-OH` group at `C-1`, and so it can not produce an open chain aldehydic form. This indicates that glucose pentaacetate does not contain an aldehyde group.