Problem 3 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Consider the following reactions: [Main Sep. 02, 2020 (I)] (i) Glucose \(+\mathrm{ROH} \stackrel{\text { dry } \mathrm{HCl}}{\longrightarrow}\) Acetal \(\frac{x \text { eq. of }}{\left(\mathrm{CH}_{3} \mathrm{CO}\right)_{2} \mathrm{O}}\) acetyl derivative (ii) Glucose \(\stackrel{\mathrm{Ni} / \mathrm{H}_{2}}{\longrightarrow} \mathrm{A} \frac{y \text { eq. of }}{\left(\mathrm{CH}_{3} \mathrm{CO}\right)_{2} \mathrm{O}}\) acetyl derivative (iii) Glucose \(\frac{z \text { eq. of }}{\left(\mathrm{CH}_{3} \mathrm{CO}\right)_{2} \mathrm{O}}\) acetyl derivative ' \(x^{\prime}, y^{\prime}\) and ' \(z^{\prime}\) in these reactions are respectively. (a) \(5,4 \& 5\) (b) \(4,6 \& 5\) (c) \(4,5 \& 5\) (d) \(5,6 \& 5\)

Step-by-Step Solution

Answer

(b) 4, 6, & 5

Step 1: Analyze Reaction (i)

In Reaction (i), glucose reacts with alcohol in the presence of dry HCl to form an acetal followed by an acetylation reaction which results in the acetyl derivative. Since the glucose molecule contains 5 hydroxyl groups, one acetal formation replaces one hydroxyl group, leaving 4 remaining hydroxyl groups for acetylation. Therefore, for Reaction (i), the number of equivalents of acetic anhydride (\((CH_3CO)_2O\)) needed is 4.

Step 2: Analyze Reaction (ii)

In Reaction (ii), glucose is hydrogenated in the presence of nickel catalyst, which converts the aldehyde group of glucose into an additional hydroxyl group, forming sorbitol. Sorbitol has a total of 6 hydroxyl groups available for acetylation. So, for Reaction (ii), the number of equivalents of acetic anhydride required is 6.

Step 3: Analyze Reaction (iii)

In Reaction (iii), glucose reacts directly with acetic anhydride, acetylating each of its 5 hydroxyl groups. Hence, for Reaction (iii), the number of equivalents of acetic anhydride is 5.

Step 4: Choose the Correct Option

Based on the analyses: \(x = 4\) for Reaction (i), \(y = 6\) for Reaction (ii), and \(z = 5\) for Reaction (iii). The correct answer is therefore (b) \(4, 6, \& 5\).

Key Concepts

Glucose Reactions

Glucose is a fundamental sugar molecule, often referred to as a monosaccharide, that is central to various biochemical processes. Chemically, it consists of a six-carbon chain with an aldehyde group at one end and hydroxyl groups attached to the remaining carbon atoms. These hydroxyl groups make glucose highly reactive and suitable for numerous chemical transformations.

In organic chemistry, glucose reactions often focus on modifying these functional groups to explore new molecular structures or to derive various useful derivatives. Understanding these reactions is crucial, especially for students preparing for examinations like the Joint Entrance Examination (JEE), as they demonstrate principles such as substitution, addition, and elimination in organic chemistry.

• Glucose can undergo reactions such as oxidation, reduction, and acetal formation.
• It serves as a precursor for synthesizing important industrial and pharmaceutical compounds.

By closely analyzing its interactions, students can better grasp how changes in molecular structure affect chemical behavior, which is a key aspect of organic chemistry.

Acetal Formation

Acetals are formed when aldehydes or ketones react with alcohols under acidic conditions. This transformation is significant in organic synthesis as it temporarily protects carbonyl groups during multi-step chemical reactions.

In the context of glucose, acetal formation involves converting the aldehyde functional group into an acetal by reacting it with an alcohol. Using a catalyst like dry HCl, this reaction forms a glycosidic bond, leaving hydroxyl groups available for further chemical processes such as acetylation.

• The main advantage of acetal formation is protecting reactive aldehyde groups against unwanted reactions.
• This step is essential before acetylating the remaining hydroxyl groups on the glucose molecule.

Acetal formation is a strategic move in organic synthesis to control the reactivity of complex molecules, ensuring that specific functional groups are temporarily inactivated or "protected" from reactants.

Acetylation Reaction

Acetylation is a chemical reaction that introduces an acetyl group ( CH_3CO) into a compound. In organic chemistry, it's widely used to alter the properties of molecules, such as solubility and reactivity.

When applied to glucose or its derivatives, acetylation often follows acetal formation to modify the available hydroxyl groups. The reagent commonly used in this process is acetic anhydride. During result analysis, the number of equivalents of acetic anhydride required signifies how many hydroxyl groups are acetylated.

• The acetylation increases the hydrophobicity of glucose, impacting its solubility in organic solvents.
• This process is key in producing glucose-based molecules that have potential applications in pharmaceuticals and agro-chemicals.

Understanding acetylation's role in organic transformations is crucial as it allows the manipulation of molecular properties for targeted chemical synthesis.

Hydrogenation of Glucose

Hydrogenation refers to the addition of hydrogen to a molecule, often involving unsaturated compounds to make them saturated. For glucose, hydrogenation specifically targets the aldehyde group to convert it into an alcohol group.

This reaction requires a catalyst, typically nickel (Ni), and produces sorbitol from glucose. The conversion of the aldehyde to a secondary alcohol group adds an additional hydroxyl group, resulting in a total of six hydroxyl groups available for further reactions such as acetylation.

• Sorbitol formed from hydrogenation is distinct from glucose due to its increased number of hydroxyl groups.
• This transformation is not only significant in organic chemistry but also has applications in the food industry as a sweetener and humectant.

Comprehending the hydrogenation of glucose is essential for organic chemistry students, highlighting essential catalytic processes and the transformation of molecular structures.