Question

What is the nature of glucose-glucose linkage in starch that makes its so susceptible to acid hydrolysis?

• A. Starch is hemiacetal
• B. Starch is acetal
• C. Starch is polymer
• D. Starch contains only few molecules of glucose

Answer 1

Answer: (B)

Since monosaccharides contain a number of hydroxyl groups and an aldehyde or a keto group, therefore, any one of the $OH$ groups (usually $C_4$ or $C_5$ in aldohexoses and $C_5$ or $C_6$ in ketohexoses) may combine with the aldehyde or the keto group to form intramolecular hemiacetal or hemiketal. On the other hand, it is also possible that monosaccharides can link up with themselves to form acetals. For example, starch (polysaccharides). Acetals and hemiacetals get readily hydrolysed by acids to give alcohol.