When this occurs the sugar undergoes oxidation. Any sugar with a free or potentially free aldehyde group is a reducing sugar. 2 O is a red ppt. The aldehyde in the sugar is oxidized to the acid. The aldehyde is oxidized to the acid under mild oxidation conditions and the primary alcohol is oxidized through the aldehyde to the acid under strong oxidation conditions.
Some sugars are interconvertable via isomerization.
Free aldehyde sugars can undergo enolization to give ketone sugars. Glucose to fructose for example. The PO 4 is frequently the acid. The importance of glycosides in biochem is that this is the bond that holds monosaccharides together as disaccharides, oligos and polysaccharides. Glc 1-4 glc, intermediate in cellulose digestion.