Difference Between Reducing and Nonreducing Sugar

Main Difference – Reducing vs Nonreducing Sugar

Sugar is a type of carbohydrate. There are many different types of sugars. Some sugars are simple sugars with a simple structure. They are known as monosaccharides. Some common examples for monosaccharides include glucose, fructose, and galactose.  Some sugars are formed by the bonding of two monosaccharides. They are known as disaccharides. Some common disaccharides are sucrose, maltose, and lactose. What we use as table sugar is obtained from plants such as sugar cane. Sugars can be divided into two groups depending on their chemical behaviors: reducing sugars and nonreducing sugars. The main difference between reducing and nonreducing sugar is that reducing sugars have free aldehyde or ketone groups whereas nonreducing sugars do not have free aldehyde or ketone groups.

Key Areas Covered

1. What is a Reducing Sugar
     – Definition, Chemical Properties
2. What is a Nonreducing Sugar
     – Definition, Chemical Properties
3. What is the Difference Between Reducing and Nonreducing Sugar
      – Comparison of Key Differences

Key Terms: Carbohydrate, Fructose, Galactose, Glucose, Lactose, Maltose, Nonreducing Sugar, Reducing Sugar, Sugar

What is a Reducing Sugar

Reducing sugars are carbohydrates that can act as reducing agents due to the presence of free aldehyde groups or free ketone groups. These are sugars because this group of compounds has a sweet taste as other sugars. All monosaccharides and some disaccharides are reducing sugars.

Figure 1: Common Monosaccharides

Reducing sugars can be oxidized by weak oxidizing agents. In aqueous medium, reducing sugars generate one or more compounds containing an aldehyde group. This is a characteristics property of reducing sugars. Examples of reducing sugars include glucose, fructose, galactose as monosaccharides and lactose, maltose as disaccharides.

The Fehling’s test or the Benedicts test can be used to identify the presence of a reducing sugar in a given sample. The Cu⁺² ions in the Fehling’s reagent or Benedict’s reagent are reduced to Cu₂O. The formation of the Cu₂O precipitate indicates the presence of a reducing sugar.

What is a Nonreducing Sugar

Nonreducing sugars are carbohydrates that cannot act as reducing agents due to the absence of free aldehyde groups or free ketone groups. Some disaccharides and all polysaccharides are reducing sugars. In basic aqueous media, nonreducing sugars do not generate any compounds containing an aldehyde group.

Figure 2: Sucrose is a disaccharide made out of a glucose molecule and a fructose molecule. Sucrose is a nonreducing sugar.

Sucrose is a well-known nonreducing sugar. It is a disaccharide. Nonreducing sugars do not show a positive result for Fehling’s or Benedict tests. This is because Cu⁺² in those test solutions cannot be reduced by the sugar.

Difference Between Reducing and Nonreducing Sugar

Definition

Reducing Sugar: Reducing sugars are carbohydrates that can act as reducing agents due to the presence of free aldehyde groups or free ketone groups.

Nonreducing Sugar: Nonreducing sugars are carbohydrates that cannot act as reducing agents due to the absence of free aldehyde groups or free ketone groups.

Reducing Agents

Reducing Sugar: Reducing sugars are good reducing agents.

Nonreducing Sugar: Nonreducing sugars are not reducing agents.

Chemical Properties

Reducing Sugar: Reducing sugars have free aldehyde or ketone groups.

Nonreducing Sugar: Nonreducing sugar do not have free aldehyde or ketone groups.

Fehling’s Test

Reducing Sugar: Reducing sugars give a positive reaction towards the Fehling’s test.

Nonreducing Sugar: Nonreducing sugar give a negative reaction towards the Fehling’s test.

Examples

Reducing Sugar: All monosaccharides and some disaccharides are reducing sugars.

Nonreducing Sugar: Some disaccharides and all polysaccharides are nonreducing sugars.

Conclusion

Reducing sugars are good reducing agents. Nonreducing sugars are not reducing agents. This is because reducing sugars have free aldehyde or ketone groups whereas nonreducing sugars have no such free groups. This is the main difference between reducing sugar and nonreducing sugar.

References:

1. “Reducing Sugar.” Chemistry LibreTexts, Libretexts, 7 Sept. 2017, Available here.
2. Libretexts. “Nonreducing Sugar.” Chemistry LibreTexts, Libretexts, 21 July 2016, Available here.
3. Hunt, Ian R. Ch25: Reducing sugars, Available here.

Image Courtesy:

1. “217 Five Important Monosaccharides-01” By OpenStax College – Anatomy & Physiology, Connexions Web site, Jun 19, 2013 (CC BY 3.0) via Commons Wikimedia
2. “Figure 03 02 04” By CNX OpenStax (CC BY 4.0) via Commons Wikimedia