Organic Chemistry MCAT Practice Exam

Which of the following sugars is classified as a nonreducing sugar?

1. Maltose

2. Sucrose

3. Lactose

4. Glucose

The correct answer is: Sucrose

A nonreducing sugar is one that does not have a free aldehyde or ketone group, which is crucial for reducing reactions. In the context of disaccharides, this is typically due to how the glycosidic bond forms between the monosaccharides. Sucrose is classified as a nonreducing sugar because it is formed from the linkage of glucose and fructose through a glycosidic bond that involves the anomeric carbons of both monosaccharides. This means that neither glucose nor fructose possesses a free aldehyde or ketone group in the sucrose molecule, preventing it from participating in redox reactions. On the other hand, sugars like maltose, lactose, and glucose are reducing sugars. Maltose consists of two glucose units and has a free anomeric carbon on one of the glucose units that can undergo oxidation. Lactose, made of glucose and galactose, also has a free anomeric carbon from the glucose unit. Glucose, as a monosaccharide, readily has a free aldehyde form, allowing it to act as a reducing agent. Thus, among the sugars listed, sucrose is the only one that is nonreducing because its structure does not allow for a