When pentaacetate of glucose is made to react with hydroxylamine, it does not react indicating the absence of free —CHO group.
In aqueous solution, the cyclic structure (have -OH group at C-1 ) should get converted to open chain structure which has an aldehyde group at C-1. It then should react with hydroxylamine and give glucose oxime. But such case is not observed. This suggests that in aqueous solution open chain structure doesn't exist and as a result, the aldehyde group is absent in pentaacetate of glucose.