When forming the disaccharide maltose from two glucose monosaccharides, A) water is eliminated B) a hemiacetal is converted to an acetal C) the resulting dissacharide is no longer a reducing sugar D) Both A and B E) A, B, and C above.
The Correct Answer and Explanation is:
The correct answer is E) A, B, and C above.
Explanation:
Maltose is a disaccharide formed from two glucose molecules. The reaction involves a glycosidic bond between the two glucose units, and this process can be broken down as follows:
- Water is eliminated (Choice A):
In the formation of maltose, the reaction is a condensation reaction (also known as a dehydration synthesis), in which a water molecule is eliminated. Each glucose molecule has a hydroxyl group (-OH) on its anomeric carbon (the carbon that was part of the aldehyde group in the open-chain form). When two glucose molecules link together, the hydroxyl group of one glucose reacts with the hydroxyl group of the other, and a water molecule (H₂O) is released. This is a hallmark of a condensation reaction. - A hemiacetal is converted to an acetal (Choice B):
Glucose, as an aldose sugar, exists in equilibrium between its open-chain and cyclic forms. In its cyclic form, glucose forms a hemiacetal. When two glucose molecules react to form maltose, the hemiacetal group on one glucose reacts with the hydroxyl group of another glucose molecule, forming a glycosidic bond. This bond converts the hemiacetal into an acetal at the anomeric carbon. The acetal linkage involves the bond between the anomeric carbon of one glucose and the hydroxyl group on another glucose molecule, replacing the original hydroxyl group at the anomeric center with a bond to another glucose unit. - The resulting disaccharide is no longer a reducing sugar (Choice C):
A reducing sugar contains a free aldehyde or ketone group that can be oxidized. In the case of maltose, the glucose unit involved in the glycosidic bond no longer has a free anomeric carbon (because it is involved in the acetal linkage), which means it cannot act as a reducing agent. Therefore, maltose is no longer classified as a reducing sugar, because the anomeric carbon of the second glucose molecule is involved in the acetal linkage, leaving no free aldehyde or ketone group available for reduction.
Conclusion:
All of the statements are correct because the process of forming maltose from two glucose molecules involves water elimination (A), the conversion of a hemiacetal to an acetal (B), and results in a non-reducing disaccharide (C). Therefore, the correct answer is E) A, B, and C above.