Animated Solution for Chemistry - Organic Chemistry: Which of the following statement(s) is(are) true ?

The study of biomolecules is a fascinating journey into the chemical foundations of life. This question from JEE Advanced tests your fundamental understanding of carbohydrates, specifically focusing on the reactions of glucose, the structural nuances of its cyclic forms, and the properties of disaccharides like sucrose. Let's break down each statement to uncover the truth.

The Oxidation of Glucose

The first statement claims that oxidizing glucose with bromine water yields glutamic acid. To evaluate this, we need to recall the chemical properties of glucose. Glucose contains an aldehyde group (`$-CHO$`) at one end and a primary alcohol group (`$-C{H}_{2}OH$`) at the other, along with secondary alcohol groups in between.

Bromine water (`$B{r}_2/H_{2}O$`) is known to be a mild oxidizing agent. When it reacts with glucose, it selectively oxidizes the more reactive aldehyde group into a carboxylic acid group (`$-COOH$`), leaving the rest of the molecule intact.

The resulting product is a six-carbon sugar acid called gluconic acid. Glutamic acid, on the other hand, is an amino acid—a completely different class of biomolecules containing an amino group (`$-N{H}_2$`). Therefore, statement (A) is fundamentally incorrect.

Cyclic Structures and Anomers

Statement B discusses the cyclic hemiacetal forms of D-(+)-glucose. In aqueous solution, the open-chain form of glucose is in equilibrium with two cyclic forms. These cyclic structures are formed when the hydroxyl group on the fifth carbon (`$C-5$`) attacks the carbonyl carbon (`$C-1$`), creating a six-membered ring known as a pyranose ring.

Because the carbonyl carbon (`$C-1$`) is planar, the attack can happen from either side, leading to two distinct spatial arrangements of the newly formed hydroxyl group at `$C-1$`. These two isomers are designated as $\alpha$-D-glucose and $\beta$-D-glucose.

Isomers that differ in configuration exclusively at the hemiacetal (or acetal) carbon—which is `$C-1$` in aldoses like glucose—are specifically termed anomers. Thus, statement (B) is a perfectly accurate description of this phenomenon.

The Inversion of Sucrose

Next, we look at the hydrolysis of sucrose. Sucrose is a common disaccharide composed of one unit of glucose and one unit of fructose. In its pure form, sucrose is dextrorotatory, meaning it rotates plane-polarized light to the right, with a specific rotation of `$+52.5^{\circ }$`.

When sucrose is hydrolyzed (broken down by water, often catalyzed by an acid or the enzyme invertase), it yields an equimolar mixture of D-(+)-glucose and D-(-)-fructose. Here is where the magic happens: while the glucose produced is dextrorotatory (`$+52.5^{\circ }$`), the fructose produced is strongly laevorotatory (`$-92.4^{\circ }$`).

Because the magnitude of the laevorotation of fructose is greater than the dextrorotation of glucose, the net rotation of the resulting mixture is laevorotatory. This change in the sign of optical rotation from positive to negative during hydrolysis is famously known as the inversion of cane sugar. Therefore, statement (C) is absolutely true.

The Definition of Monosaccharides

The final statement touches upon the very definition of monosaccharides. Carbohydrates are broadly classified based on their behavior upon hydrolysis.

Monosaccharides are the simplest units of carbohydrates. By definition, they are polyhydroxy aldehydes or ketones that cannot be hydrolyzed into simpler carbohydrate molecules. If you try to hydrolyze glucose or fructose, you won't get smaller sugar molecules; you would just break down the molecule entirely.

Disaccharides (like sucrose) and polysaccharides (like starch or cellulose) can be hydrolyzed into their constituent monosaccharides, but the monosaccharides themselves are the end of the line for hydrolysis. Hence, statement (D) is true.

After a thorough chemical analysis of each option, we find that statements (B), (C), and (D) are factually correct, while statement (A) presents a common trap by confusing gluconic acid with glutamic acid. This problem beautifully illustrates the importance of precise terminology and a solid grasp of fundamental organic reactions in biochemistry.