Problem 5 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which of the following statement is not true for glucose? [Main Jan. 08, 2020 (I)] (a) Glucose exists in two crystalline forms \(\square\) and \(\square\) (b) Glucose gives Schiff's test for aldehyde (c) Glucose reacts with hydroxylamine to form oxime The pentaacetate of glucose does not (d) react with hydroxylamine to give oxime

Step-by-Step Solution

Answer

Option (d) is not true for glucose.

Step 1: Analyzing Option (a)

Glucose is known to exist in two crystalline forms, commonly referred to as alpha (α) and beta (β) glucopyranose forms. This is due to the different orientations of the -OH group on the first carbon atom. Thus, option (a) is true.

Step 2: Examining Option (b)

Schiff's test is a chemical test used to detect aldehydes. Free or open-chain glucose, which is an aldehyde, can typically give positive results in Schiff's test. Therefore, option (b) is also true.

Step 3: Investigating Option (c)

Hydroxylamine can react with aldehydes and ketones to form oximes. Since glucose has an aldehyde group in its open-chain form, it can react with hydroxylamine to form an oxime. Therefore, option (c) is true.

Step 4: Evaluating Option (d)

Glucose pentaacetate has all its hydroxyl groups acetylated, which prevents the formation of the free aldehyde group. Without the free aldehyde group, the glucose pentaacetate cannot react with hydroxylamine to form an oxime. As a result, option (d) is not true.

Key Concepts

Chemical Reactions of Glucose

Glucose is a fundamental sugar in biology and chemistry, comprising numerous functional groups, primarily aldehyde and hydroxyl (alcohol) groups. These groups enable glucose to undergo a wide range of chemical reactions.
One typical reaction is with hydroxylamine. Here, glucose reacts to form an oxime, a reaction typical of aldehydes and ketones due to the presence of its aldehyde group in the open-chain structure.
Another significant reaction is the acetylation of glucose, forming its pentaacetate. By acetylating all available hydroxyl groups, the reactivity of glucose is largely altered, essentially masking the key functional groups that participate in typical reactions like those with hydroxylamine.

Schiff's Test

Schiff's test is a distinctive chemical test developed to identify aldehyde groups in compounds. The test involves decolorizing a solution of fuchsin (a magenta dye) with sulfurous acid—a reagent known as Schiff's reagent.
This clear solution takes on a pink hue when exposed to aldehydes, as they react to restore the original color of the dye. For glucose, a free aldehyde group is present in its open-chain form.
It will react in this test, confirming the presence of the aldehyde functional group through the reappearance of color. This test is instrumental in confirming aldehydes in compounds like glucose, although its reliability can vary depending on other structural factors.

Aldehyde Reactions

Aldehyde reactions play a pivotal role in organic chemistry, particularly with glucose. Being an aldose, glucose contains an aldehyde group, especially notable in its open-chain form.
This aldehyde group allows glucose to engage in specific reactions, such as the formation of oximes with hydroxylamine.
Aldehydes tend to undergo oxidation reactions as well, transforming into carboxylic acids like gluconic acid when treated with mild oxidizing agents.

• These reactions underscore the versatility of glucose in forming derivatives.
• The presence of the aldehyde group makes glucose reactive and chemically interesting.

Acetylation, conversely, blocks these reactions, as seen with glucose pentaacetate.

Crystalline Forms of Glucose

Glucose crystallizes in two primary forms: alpha (α) and beta (β) glucopyranose. These forms arise due to differences in the arrangement of atoms around the carbon atom that determines the ring form.
In α-glucose, the hydroxyl group on the first carbon is below the plane of the ring, while in β-glucose, it is above the plane. These positional differences yield diverse crystalline structures and slightly differing chemical properties.

• The mutarotation, or change between these forms, can be observed under certain conditions.
• Crystalline forms play a crucial role in the functional properties of glucose in biology and industry.

Understanding these crystalline forms is essential for applications where structure impacts glucose's behavior.