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: Understanding Sucrose Properties

Sucrose is a disaccharide composed of glucose and fructose. In sucrose, the glycosidic bond between glucose and fructose involves the carbon-1 of glucose and carbon-2 of fructose, which are the reducing ends. Since these carbon atoms are involved in bonding, sucrose does not have a free aldehyde or ketone group and cannot reduce other substances, hence it is a non-reducing sugar.

Step 2: Testing Glucose with Bromine Water

Glucose is an aldose, which means it has a free aldehyde group. Bromine water is a mild oxidizing agent that can oxidize the aldehyde group in glucose to form gluconic acid. Thus, glucose is oxidized by bromine water.

Step 3: Evaluating Glucose Optical Rotation

Glucose is also known as dextrose, which refers to its property of rotating plane-polarized light to the right (clockwise direction). Therefore, glucose does rotate plane-polarized light in a clockwise direction.

Step 4: Testing Fructose with Bromine Water

Fructose is a ketose sugar, meaning it does not have a free aldehyde group like glucose does. Bromine water typically oxidizes aldehyde groups, hence it does not oxidize fructose. Fructose, therefore, will not be oxidized by bromine water.

Step 5: Selecting the Correct Answer

With the above evaluations, the true statements are: (i) sucrose is a non-reducing agent, (ii) glucose is oxidized by bromine water, and (iii) glucose rotates plane polarized light in the clockwise direction. Therefore, the correct response is (a) (i), (ii), (iii).

Key Concepts

Sucrose Properties

Sucrose, commonly known as table sugar, is a fascinating carbohydrate. It's a disaccharide made up of two monosaccharides: glucose and fructose.
In sucrose, these two units are linked together by a glycosidic bond. This bond specifically involves the carbon atoms that would otherwise be free to open up into aldehyde or ketone groups. As a result, sucrose is a non-reducing sugar.

• **Non-Reducing Nature:** The inability to open into a free aldehyde or ketone means sucrose can't donate electrons to other compounds, a process known as reduction.
• **Glycosidic Linkage:** Specifically, this involves the carbon-1 of glucose and carbon-2 of fructose. In this form, neither sugar retains their typical reducing capabilities.

Because of this structure, sucrose isn't able to participate in reactions typical of reducing sugars, such as the Benedict's test. Understanding this property is key in chemistry, especially in determining the characteristics and reactions of sugars.

Glucose Oxidation

Glucose is a simple sugar that plays a vital role in our metabolic system. As an aldose, glucose contains a free aldehyde group which is crucial for its reactions, including oxidation.

• **Bromine Water Reaction:** This is a mild oxidizing agent used to oxidize aldehyde groups in glucose. When glucose reacts with bromine water, the aldehyde group is oxidized to form gluconic acid, confirming its reducing nature.
• **Significance of Oxidation:** This reaction is not only important in carbohydrate chemistry but also in biological pathways where glucose oxidation is a primary source of energy.

The oxidation of glucose is both a chemical and a biological cornerstone, helping in energy production and various biochemical pathways. Observing its reactivity with bromine water is a simple yet profound way to appreciate its oxidative potential.

Optical Activity of Sugars

Sugars are known for their ability to rotate plane-polarized light, a property known as optical activity. Glucose, specifically, exhibits a right-handed or clockwise rotation of plane-polarized light. This distinctive behavior is due to its chiral centers.

• **Dextrose Definition:** Often referred to as dextrose, glucose gets its name from the Latin "dexter," meaning "right," highlighting its clockwise optical rotation.
• **Chirality:** This optical activity arises because of the chiral nature of glucose, which means it can exist in two forms (enantiomers) that are mirror images of each other.

Understanding optical activity is essential for determining the structure and behavior of sugars. Glucose's ability to rotate light is a valuable property used in biochemical detection and differentiation of molecules."