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

Explanation

Glucose, as we know is an aldohexose and it is expected to give the characteristic reactions of the aldehydic group e.g., action with `NH_(2)OH,HCN`, Tollen's regent, Fehling regant etc. However, the pentaacetyl glucose formed by the acylation of glucose with acetic anhydride does not give these reactions. This means that the aldehydic group is either absent or is not available in the pentaacetyl derivative has. It is therefore, not free or available to take part in these reactions.