There are five hydroxyl groups in the first glucose that could form an acetal with the second. This question reduces to the problem of the number of possible ways in which the hydroxyl groups at these five positions can be linked to the second glucose. The remaining fifth hydroxyl group at C-1 can participate in an acetal bond with the second glucose in four possible ways. Grand total is twenty possible disaccharides. Four of the twenty possibilities are shown below in which the C-4 hydroxyl group of one glucose is linked to the hemiacetal center of the second.
Enzymatic hydrolysis gives sucrose and glucose.
The key to this problem is to figure out where this second glucose molecule is attached to sucrose. In this structure, we see that one hydroxyl group is not methylated. It must be the place where the second glucose is attached in melezitose. Part B. 1.