Question

How many of the following are reducing sugars? Glucose Mannose Maltose Lactose Fructose Sucrose Cellulose Starch

Answer 1

5

Answer 2

• Reducing Sugar Definition: A reducing sugar is a sugar that has a free anomeric carbon (a hemiacetal or hemiketal group) that can open to form an aldehyde or ketone group. This group can then be oxidized, thereby reducing other compounds (e.g., Tollen's reagent, Fehling's solution).

Let's analyze each compound:

1. Glucose: A monosaccharide (aldohexose). It has a free hemiacetal group that can open to an aldehyde. (Reducing)
2. Mannose: A monosaccharide (aldohexose), an epimer of glucose. It has a free hemiacetal group that can open to an aldehyde. (Reducing)
3. Maltose: A disaccharide composed of two glucose units linked by an α-1,4-glycosidic bond. One of the glucose units has a free hemiacetal group. (Reducing)
4. Lactose: A disaccharide composed of galactose and glucose linked by a β-1,4-glycosidic bond. The glucose unit has a free hemiacetal group. (Reducing)
5. Fructose: A monosaccharide (ketohexose). Although it's a ketose, in alkaline conditions (like those used in reducing sugar tests), it undergoes tautomerization to form aldoses (glucose and mannose) via an enediol intermediate. These aldoses have free aldehyde groups. (Reducing)
6. Sucrose: A disaccharide composed of glucose and fructose. The anomeric carbon of glucose (C1) is linked to the anomeric carbon of fructose (C2) by an α,β-1,2-glycosidic bond. Both anomeric carbons are involved in the glycosidic linkage, leaving no free hemiacetal or hemiketal groups. (Non-reducing)
7. Cellulose: A polysaccharide of glucose units linked by β-1,4-glycosidic bonds. It is a very large polymer; only the terminal glucose unit at one end of the long chain has a free hemiacetal group. Due to its high molecular weight and the negligible proportion of reducing ends, it is considered non-reducing in practical terms. (Non-reducing)
8. Starch: A polysaccharide of glucose units (amylose and amylopectin) linked primarily by α-1,4-glycosidic bonds. Similar to cellulose, it is a very large polymer with a negligible proportion of free reducing ends. (Non-reducing)

Counting the reducing sugars from the list: Glucose, Mannose, Maltose, Lactose, Fructose. There are 5 reducing sugars.

The final answer is $\boxed{5}$

Explanation of the solution: Reducing sugars possess a free anomeric carbon allowing them to open into an aldehyde or ketone form, capable of reducing other compounds. Monosaccharides (Glucose, Mannose, Fructose) are reducing. Disaccharides like Maltose and Lactose have one free anomeric carbon, making them reducing. Sucrose is non-reducing as both anomeric carbons are involved in the glycosidic bond. Polysaccharides like Cellulose and Starch are considered non-reducing due to the extremely low proportion of free reducing ends relative to their large size. Therefore, Glucose, Mannose, Maltose, Lactose, and Fructose are reducing sugars.