Problem 118 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following pairs is incorrectly matched? (a) sucrose 1\. monosaccharide (b) fructose 2\. aldose sugar (c) glucose 3\. mutarotation (d) sucrose 4\. reducing sugar (a) 2,3 and 4 (b) 1,2 and 3 (c) 1,2 and 4 (d) 2 and 4 only

Step-by-Step Solution

Answer

Choice (c) 1, 2, and 4 are incorrectly matched.

Step 1: Understanding the Problem

We need to find which pairs are incorrect. Each choice includes a compound and its characterization. We'll examine each pair to determine if the match is correct.

Step 2: Analyze Option (a)

The pair 'sucrose – monosaccharide' is given. Sucrose is actually a disaccharide, composed of glucose and fructose. Thus, the pairing is incorrect.

Step 3: Analyze Option (b)

The pair 'fructose – aldose sugar' is given. Fructose is a ketohexose, not an aldose, so this pairing is incorrect because aldoses contain an aldehyde group, whereas fructose contains a ketone group.

Step 4: Analyze Option (c)

The pair 'glucose – mutarotation' is given. Glucose can undergo mutarotation, which is the change in optical rotation that occurs when an alpha form of glucose converts into the beta form, and vice versa. This is a correct pairing.

Step 5: Analyze Option (d)

The pair 'sucrose – reducing sugar' is given. Sucrose is a non-reducing sugar because it does not have a free aldehyde or ketone group. Thus, this pairing is incorrect.

Step 6: Determine Incorrectly Matched Pair

From the analysis: (a) sucrose is not a monosaccharide, (b) fructose is not an aldose, and (d) sucrose is not a reducing sugar. All these matches are incorrect.

Step 7: Match Incorrectly to Given Choices

Identify if these incorrect pairings match with the given answer choices. (a) Incorrect, (b) Incorrect, (d) Incorrect. Therefore, options 1, 2, and 4 are incorrect. Thus, the answer is (c) 1,2 and 4.

Key Concepts

Monosaccharides

Monosaccharides are the simplest form of carbohydrates and are often referred to as simple sugars. They are the building blocks for more complex carbohydrates, such as disaccharides and polysaccharides. These single sugar molecules have the general formula of \({C_nH_{2n}O_n}\), where \( n \) is usually 3 to 7.

Common examples of monosaccharides include glucose, fructose, and galactose. These sugars are categorized based on the number of carbon atoms they possess:

• Triose (3 carbons)
• Tetrose (4 carbons)
• Pentose (5 carbons)
• Hexose (6 carbons)

Additionally, they are further classified by the type of functional group they contain:

• Aldoses have an aldehyde group (e.g., glucose).
• Ketoses have a ketone group (e.g., fructose).

Monosaccharides play a crucial role in metabolism, providing energy and serving as building blocks for other essential molecules.

Disaccharides

Disaccharides are carbohydrates composed of two monosaccharide units linked by a glycosidic bond. Common disaccharides include sucrose, lactose, and maltose, each having unique monosaccharide components and bond types.

For instance, sucrose, which many of us know as table sugar, consists of one glucose molecule and one fructose molecule. It forms a glycosidic linkage between carbon 1 of glucose and carbon 2 of fructose, resulting in the release of a water molecule—a process called dehydration synthesis.

• Sucrose: Glucose + Fructose
• Lactose: Glucose + Galactose
• Maltose: Glucose + Glucose

Disaccharides vary in their digestion and absorption rates. Enzymes such as sucrase, lactase, and maltase are responsible for breaking down these sugars into their monosaccharide components in the digestive tract.

Mutarotation

Mutarotation is a phenomenon observed in carbohydrates, specifically those that contain a hemiacetal or hemiketal group, like glucose. It refers to the change in the optical rotation that occurs when an alpha form of a sugar converts into its beta form, and vice versa.

The process occurs because these sugars exist in equilibrium between their open-chain form and different cyclic forms. When dissolved in water, the open-chain form will interconvert to cyclic forms until equilibrium is achieved.

• Alpha (1): Hydroxyl group on carbon 1 is down
• Beta (2): Hydroxyl group on carbon 1 is up

The rate of mutarotation can be influenced by various factors such as the type of solvent, concentration, temperature, and presence of acids or bases. This property is essential for the reactivity and function of carbohydrates in biological systems.

Reducing Sugar

Reducing sugars are carbohydrates that have the ability to act as reducing agents because they possess a free aldehyde or ketone group. These sugars can donate electrons to other molecules, reducing them in the process.

To identify a reducing sugar, the sugar must be in its open-chain form. Common reducing sugars include glucose, fructose, lactose, and maltose. Sucrose, however, is an exception as it is a non-reducing sugar due to its glycosidic bond formation, which locks its anomeric carbons.

• Reducing sugars react with Benedict's reagent to form a red or orange precipitate.
• In the food industry, the reducing properties of sugars can influence browning reactions.

Understanding which sugars are reducing or non-reducing is crucial in both chemical analysis and many industrial applications.