When glucose is treated with acetic anhydride (CH3CO)2O in the presence of pyridine or few drops of conc. H2SO4, it undergoes oxidation to give glucose penta acetate, indicating the presence of five –OH groups, in which one group is a primary 1° –OH group and four groups are secondary 2° –OH groups. When glucose is oxidized with concentrated HNO3, it gets oxidized to form saccharic acid, a dicarboxylic acid. This reaction proves that primary –OH group gets oxidized to –COOH but only in the presence of strong acids, the secondary –OH group gets oxidized.