Sucrose is not a reducing sugar because:

• A.) It is chemically stable.
• B.) It contains no free aldehyde or keto group adjacent to a CHOH group.
• C.) It is built up of a fructose unit.
• D.) It is optically active.

Sucrose, commonly known as table sugar, is classified as a non-reducing sugar. The reason for this classification primarily revolves around its chemical structure. Let's break down the options provided to understand why option B is the correct answer.

The Nature of Reducing Sugars

Reducing sugars are characterized by their ability to donate electrons to other molecules, which typically occurs through the presence of a free aldehyde or ketone group. This property allows them to participate in redox reactions, which is a key feature in various biochemical processes.

Analyzing the Options

• A.) It is chemically stable. While sucrose is indeed chemically stable, this characteristic alone does not define whether a sugar is reducing or non-reducing. Stability can apply to both types of sugars.
• B.) It contains no free aldehyde or keto group adjacent to a CHOH group. This is the crux of the matter. Sucrose is a disaccharide composed of glucose and fructose. In its structure, the glycosidic bond between these two monosaccharides prevents the formation of a free aldehyde or ketone group, which is essential for a sugar to be classified as reducing.
• C.) It is built up of a fructose unit. While sucrose does contain a fructose unit, this fact alone does not determine its reducing properties. Fructose itself is a reducing sugar, but when it is part of sucrose, its reducing ability is masked by the glycosidic bond.
• D.) It is optically active. Sucrose is indeed optically active due to its chiral centers, but optical activity does not correlate with the reducing nature of a sugar. Both reducing and non-reducing sugars can exhibit optical activity.

Understanding Sucrose's Structure

To further clarify, let's look at the structure of sucrose. It consists of one glucose molecule and one fructose molecule linked together by an α(1→2) glycosidic bond. This bond connects the anomeric carbon of glucose to the second carbon of fructose, effectively locking the structure in a way that prevents the aldehyde group of glucose from being free. Without this free aldehyde or ketone, sucrose cannot act as a reducing agent.

Real-World Implications

This property of sucrose has practical implications in food chemistry and biochemistry. For example, when sucrose is heated with certain reagents, it does not undergo the Maillard reaction, which is a form of non-enzymatic browning that typically requires reducing sugars. This is why sucrose is often used in recipes where a non-reducing sugar is desired.

In summary, the correct answer is B: Sucrose is not a reducing sugar because it contains no free aldehyde or keto group adjacent to a CHOH group. This structural feature is what distinguishes it from reducing sugars, allowing us to understand its behavior in various chemical contexts.