Problem 121 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is true? (i) sucrose is a non reducing agent (ii) glucose is oxidized by bromine water (iii) glucose rotates plane polarized light in clockwise direction (iv) fructose is oxidized by bromine water Select the correct answer during the coded given below (a) (i), (ii), (iii) (b) (i), (ii) only (c) (ii), (iii) only (d) (i) (iv) only

Step-by-Step Solution

Answer

The correct answer is (a) (i), (ii), (iii).

Step 1: Analyze Option (i)

Sucrose is a disaccharide composed of glucose and fructose. It is classified as a non-reducing sugar because it does not have a free anomeric carbon atom that can be oxidized. Therefore, it is a non-reducing agent. This statement is true.

Step 2: Analyze Option (ii)

Glucose is a simple sugar that has a free aldehyde group, which allows it to be oxidized by mild oxidizing agents like bromine water. Therefore, glucose can indeed be oxidized by bromine water. This statement is true.

Step 3: Analyze Option (iii)

Glucose is an optically active compound and has the ability to rotate plane polarized light. D-glucose rotates plane polarized light to the right, which means in the clockwise direction. Hence, this statement is true.

Step 4: Analyze Option (iv)

Fructose is a ketohexose, and under typical conditions, it is not oxidized by mild oxidizing agents like bromine water. The oxidation reaction requires more stringent conditions. Therefore, this statement is false.

Step 5: Choose the Correct Answer Based on Analysis

Based on steps 1 to 4, we determine that the true statements are (i), (ii), and (iii). Hence, the correct answer is option (a) (i), (ii), (iii).

Key Concepts

Sugar Chemistry

Sugars are essential organic compounds that play a crucial role in biology and chemistry. They are part of a larger class of molecules known as carbohydrates. Within sugar chemistry, you'll find monosaccharides, which are simple sugars, like glucose and fructose, and disaccharides, like sucrose. Monosaccharides are the building blocks of more complex carbohydrates.

Monosaccharides have functional groups such as aldehydes or ketones and multiple hydroxyl groups. These functional groups influence their chemical reactions and properties. For example, the presence of a free aldehyde or ketone group in a sugar makes it a reducing sugar, capable of participating in reduction-oxidation (redox) reactions. However, when sugars like glucose form a disaccharide like sucrose, the aldehyde group gets involved in a glycosidic bond, often rendering the sugar non-reducing, as seen with sucrose.

In sugar chemistry, the distinction between reducing and non-reducing sugars is essential for understanding the reactivity and applications of sugars in biochemical processes and synthetic chemistry.

Optical Activity

Optical activity is a fascinating property of certain organic compounds, including sugars, where the compound can rotate plane polarized light. This phenomenon results from the presence of chiral centers—carbon atoms bonded to four different substituents—in the molecule.

Each chiral center has two possible configurations, which are mirror images of each other (enantiomers). This chirality is what gives the molecules their optically active nature. For instance, D-glucose, a common sugar in nature, is optically active and rotates plane-polarized light to the right, or in a clockwise direction. This specific rotation can be measured using a polarimeter, an important tool in organic chemistry.

Understanding optical activity is crucial for sugar chemistry, as many biological systems rely on specific enantiomers of sugars. Enzymes, for example, often distinguish between different enantiomers, which influences how sugars are metabolized in the body and used in pharmaceutical applications.

Oxidation Reactions

Oxidation reactions are vital processes in organic chemistry, involving the increase in oxygen or the decrease in hydrogen in a molecule. In sugar chemistry, oxidation commonly affects the aldehyde or ketone groups of monosaccharides.

For instance, glucose, with its free aldehyde group, can readily undergo oxidation. Mild oxidizing agents like bromine water can oxidize glucose to gluconic acid, depicting its reducing nature. In contrast, fructose has a ketone group, which is less susceptible to oxidation by mild agents, requiring stronger conditions for oxidation. This difference in reactivity is a key aspect in distinguishing types of sugars and their ability to undergo redox reactions.

Oxidation reactions help determine the chemical behavior of sugars and are used in various applications, such as the food industry and biochemical assays, to analyze sugar content and activity. Understanding how different sugars react to oxidation agents enables chemists to predict reaction pathways and synthesize desired products efficiently.