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

Step-by-Step Solution

Answer

Both pairs are incorrectly matched: sucrose is a disaccharide, not a monosaccharide, and fructose is a keto sugar, not an aldose.

Step 1: Understand the Terminology

Review the definitions for the terms given. A monosaccharide is the simplest form of carbohydrates and cannot be hydrolyzed into smaller sugar units. An aldose sugar is a monosaccharide with an aldehyde group. Sucrose, a disaccharide, consists of glucose and fructose. Fructose is a monosaccharide with a ketone group, making it a keto sugar.

Step 2: Evaluate Each Pair

Analyze each pair to check for correctness. Pair (a) 'sucrose – monosaccharide' is incorrect because sucrose is a disaccharide. Pair (b) 'fructose – aldose sugar' is incorrect because fructose is a keto sugar, not an aldose.

Step 3: Conclusion

Both pairs are incorrect according to their definitions and classifications. However, it is likely the exercise wants us to identify the pair that is incorrect based on common understanding of sugar types. Therefore, focus on the most evidently incorrect information.

Key Concepts

Monosaccharides

Monosaccharides are the simplest form of carbohydrates and serve as the basic building blocks for more complex sugars. They are the smallest units of carbohydrates and cannot be broken down into simpler sugars through hydrolysis. Each monosaccharide is composed of carbon, hydrogen, and oxygen atoms, typically with a formula of \(C_nH_{2n}O_n\).
They are classified based on the number of carbon atoms—trioses (3), tetroses (4), pentoses (5), and hexoses (6) being the most common.
Some key characteristics of monosaccharides include:

• They are reducing sugars, which means they can participate in chemical reactions to transfer electrons.
• They have multiple isomers due to the location of the carbonyl group and the orientation of hydroxyl groups.
• Common examples include glucose, fructose, and galactose.

These sugars play a critical role in metabolism, providing energy through glycolysis and serving as intermediates in various biochemical pathways.

Disaccharides

Disaccharides are carbohydrates composed of two monosaccharide molecules linked together by a glycosidic bond. This bond is formed through a dehydration reaction, where a molecule of water is removed during the joining of two monosaccharides.
They are an integral part of our diet and have different properties and functions based on their constituent monosaccharides.
Examples of common disaccharides include:

• Sucrose: Formed from glucose and fructose, commonly called table sugar, and not a monosaccharide as sometimes mistakenly thought.
• Lactose: Composed of glucose and galactose, found mainly in milk.
• Maltose: Consisting of two glucose molecules, it is found in malted foods and beverages.

Disaccharides must be broken down into their monosaccharide components during digestion for absorption and utilization by the body.

Aldose vs Ketose Sugars

Aldose and ketose sugars are two classifications based on the position of the carbonyl group in the sugar molecule.
An aldose is a sugar where the carbonyl group, a functional group with a carbon double bonded to oxygen, is located at the end of the carbon chain. These sugars contain an aldehyde group and are generally reducing sugars.

Examples and features of aldose sugars include:

• ***Glucose:** An essential energy source for cells is an aldose with six carbon atoms, classifying it as an aldohexose.
• Aldoses can be further classified based on the number of carbon atoms, such as triose, tetrose, or pentose.

A ketose, on the other hand, contains the carbonyl group within the carbon chain and is characterized by a ketone group instead of an aldehyde.

Some important ketoses include:

• ***Fructose:** Known as fruit sugar, fructose is a ketohexose and is mistaken for an aldose sugar when incorrectly identified.
• Ketoses generally have similar metabolic roles to aldoses but differ structurally.

Understanding the distinction between aldose and ketose sugars is crucial for identifying the functions and reactivities of various carbohydrates.