Problem 19 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Which of the following statements about lactose is not correct? (a) It does not reduces Tollen's reagent (b) It is a disaccharide. On hydrolysis, it produces two monosaccharides (c) It possess glycosidic linkage (d) All of them

Step-by-Step Solution

Answer

(a) It does not reduce Tollen's reagent is not correct.

Step 1: Understand the Properties of Lactose

Lactose is a disaccharide that, when hydrolyzed, yields two monosaccharides, glucose and galactose. It contains a glycosidic linkage between these two monosaccharide units.

Step 2: Evaluate the Statements

Let's analyze each statement: (a) Lactose is a reducing sugar and can reduce Tollen's reagent, therefore, the statement that it "does not reduce Tollen's reagent" is incorrect. (b) This statement is correct as lactose is indeed a disaccharide that hydrolyzes into glucose and galactose. (c) This statement is also correct because lactose has a glycosidic linkage.

Step 3: Identify the Incorrect Statement

Since statements (b) and (c) about lactose are correct, and (a) is inaccurate, (a) is the incorrect statement. However, option (d) "All of them" suggests that all previous statements are incorrect, which we have proven to be false.

Key Concepts

Disaccharides

Disaccharides are a type of carbohydrate composed of two monosaccharide molecules linked together. A simple example is lactose, which consists of glucose and galactose. These two simple sugars are joined through a specific type of chemical bond called a glycosidic linkage. Disaccharides such as lactose are important in nutrition and energy storage as they must be broken down into their monosaccharide components before being utilized by the body. This process of breaking them down is called hydrolysis, where water is used to cleave the bond connecting the monosaccharides. You'll find disaccharides in many foods, especially those containing milk. Lactose gives milk its slightly sweet flavor. Other examples of disaccharides are sucrose (table sugar) and maltose. Recognizing the type of disaccharide can provide insight into its nutritional properties and potential effects on health. In understanding disaccharides, it's important to note:

• They are formed from two monosaccharides.
• They contain specific glycosidic linkages.
• They are key energy sources and have specific roles in metabolism.

Remember, when you consume foods containing disaccharides, your body enzymatically breaks down these sugars into simpler sugars for energy.

Glycosidic Linkage

Glycosidic linkage is the bond that connects two sugar molecules, formed by the elimination of a water molecule. In lactose, this linkage occurs between the carbon 1 of galactose and the carbon 4 of glucose. This type of covalent bond is essential for the formation of disaccharides and polysaccharides from monosaccharides. The specific nature of a glycosidic linkage affects the properties and reactivity of the sugar compound. Lactose is connected by a β-1,4-glycosidic bond, meaning the bond is between the first carbon of one sugar and the fourth carbon of another, with a specific orientation that impacts the molecule's shape and function. Understanding glycosidic linkages helps in:

• Predicting how sugars will interact with enzymes.
• Determining their digestibility and nutritional value.
• Identifying the differences in structures such as alpha or beta linkages.

When studying carbohydrates, recognizing the type of glycosidic bond can explain why certain sugars act differently under the same conditions. This knowledge is pivotal for fields like food chemistry and pharmaceuticals.

Reducing Sugars

Reducing sugars are sugars that can donate electrons to another chemical, typically through their free aldehyde or ketone groups. This property allows them to react with oxidizing agents such as Tollen's or Benedict's reagent—tests historically used to identify the presence of reducing sugars. Lactose qualifies as a reducing sugar since its glucose component can open to expose a free aldehyde group, which participates in typical redox reactions. In practical terms, the concept of reducing sugars is crucial in various applications:

• The Maillard reaction, which gives browned foods their distinctive flavor.
• Food safety testing, ensuring sugar quality and detection of adulteration.
• Nutritional analysis, as reducing sugars can indicate the glycemic potential of different foods.

Understanding whether a sugar is reducing helps in assessing its behavior in cooking and digestion. In the case of lactose, its capacity to reduce reagents also has implications for lactose intolerant individuals as it impacts digestion and utilization in the body.