Problem 1 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which one of the following statements is not true? [Main Sep. 06, 2020 (II)] (a) Lactose contains \(\alpha\)-glycosidic linkage between \(\mathrm{C}_{1}\) of galactose and \(\mathrm{C}_{4}\) of glucose. (b) Lactose is a reducing sugar and it gives Fehling's test. (c) Lactose \(\left(\mathrm{C}_{11} \mathrm{H}_{22} \mathrm{O}_{11}\right)\) is a disaccharide and it contains 8 hydroxyl groups. (d) On acid hydrolysis, lactose gives one molecule of \(\mathrm{D}(+)\)-glucose and one molecule of D(+)-galactose

Step-by-Step Solution

Answer

Statement (a) is not true.

Step 1: Analyze Each Statement

We need to determine which statement about lactose is not true by reviewing each option. Lactose is known to have a β-glycosidic linkage, not an α-glycosidic linkage as stated in option (a). Therefore, statement (a) may be incorrect. Statement (b) is in line with the fact that lactose is a reducing sugar as it has a free hemiacetal group. Statement (c) includes a chemical formula but incorrectly states the count of hydroxyl groups. Lactose actually has 8 hydroxyl groups, confirming statement (c) is likely correct. Statement (d) accurately describes the result of acid hydrolysis of lactose.

Step 2: Determine the Key Facts

Let's verify some key facts about lactose: (1) Lactose is a disaccharide formed from β-glycosidic linkage between galactose and glucose, not α-glycosidic. (2) Lactose is a reducing sugar, thus it tests positive with Fehling's test. (3) Lactose's chemical composition contains 8 hydroxyl groups. (4) Acid hydrolysis breaks lactose into one glucose and one galactose molecule.

Step 3: Evaluate the Statements

Based on the identified facts, evaluate the correctness of each statement: - Statement (a) is incorrect because it describes an α-glycosidic linkage instead of a β-glycosidic linkage. - Statement (b) is correct as lactose is a reducing sugar. - Statement (c) is correct regarding the chemical formula and hydroxyl groups. - Statement (d) is accurate about the products of acid hydrolysis.

Key Concepts

Beta-glycosidic linkage

In the world of carbohydrates, linkages play a crucial role in determining the structure and function of sugar molecules. One common type of linkage found in disaccharides like lactose is the **beta-glycosidic linkage**. This occurs between the first carbon (C1) of one sugar molecule, typically in the beta configuration, and another carbon (C4) of a different sugar molecule.

In lactose, this linkage specifically joins galactose to glucose, forming a bridge through an oxygen atom. Unlike an alpha-glycosidic linkage, where the OH group on the anomeric carbon is downward, the beta configuration has this OH group pointing upwards.

• Provides stability to the structure of lactose
• Influences the digestibility and breakdown of the molecule
• Is crucial for the recognition of enzymes that break down lactose

The proper understanding of beta-glycosidic linkages helps in differentiating the properties of lactose from other sugars, especially in biological and enzymatic reactions.

Reducing sugars

In chemistry, sugars are categorized based on their ability to act as reducing agents. **Reducing sugars** are capable of donating electrons, thereby participating in oxidation-reduction (redox) reactions. A key feature of reducing sugars is the presence of a free aldehyde group or a free hemiacetal group in their structure.
Lactose, the sugar found in milk, is classified as a reducing sugar. This is because it has a free hemiacetal group at the anomeric carbon of its glucose unit. The presence of this group enables lactose to interact with various reagents, undergoing change in redox reactions.

• Can be detected using specific chemical tests, such as Fehling's test
• React with other molecules, forming new bonds and compounds
• Play a role in biological systems, such as energy release and metabolism

Understanding reducing sugars is essential in fields ranging from food chemistry to metabolism, revealing the dynamic properties of these carbohydrates.

Fehling's test

One of the classic tests in organic chemistry for identifying reducing sugars is **Fehling's test**. This test allows chemists to detect the presence of aldehyde groups in solutions, indirectly identifying reducing sugars like lactose.
In Fehling's test:

• The sugar solution is mixed with Fehling’s reagent, which contains copper(II) ions and isa blue solution
• On heating, a red precipitate forms if a reducing sugar is present, indicating the reduction of copper(II) to copper(I)
• It's a useful qualitative test to reveal whether sugars like lactose can be oxidized

Fehling's test not only highlights the basic chemical reactions of reducing sugars but is also a vital tool in biochemistry and food science, providing insight into the composition of various natural substances.

Disaccharides

Disaccharides are carbohydrates consisting of two monosaccharide units linked together. **Lactose** is a prime example of a disaccharide, composed of the monosaccharides glucose and galactose. The linkage in lactose is a beta-glycosidic bond, distinct for its involvement in the formation of this sugar.

• Generally readily soluble in water
• Serve an important role in nutrition and energy supply
• Each disaccharide has unique properties depending on its constituent sugars

In the human diet, disaccharides like lactose are broken down during digestion to release the simpler sugars, aiding in efficient absorption and utilization of energy. Knowing the structure and behavior of disaccharides helps in understanding carbohydrate metabolism and potential dietary issues, such as lactose intolerance.

Acid hydrolysis of carbohydrates

**Acid hydrolysis** is a process where acidic conditions are used to break down more complex carbohydrates into simpler sugars. This chemical reaction involves the cleavage of glycosidic linkages in disaccharides and polysaccharides.
When applied to lactose, acid hydrolysis results in the breakdown of its beta-glycosidic linkage, yielding one molecule each of glucose and galactose.

• Involves the use of acid (usually hydrochloric acid) and heat
• Converts complex sugars to monosaccharides
• Used in laboratory settings to analyze carbohydrate composition

Understanding acid hydrolysis allows for greater insight into the digestion and conversion of carbohydrates in both scientific and nutritional contexts, highlighting its importance in analytical chemistry and food sciences.