Problem 109 - A Complete Resource Book in Chemistry for JEE Main

Question

In addition to aldehyde group glucose contains (a) one secondary OH and four primary OH groups (b) one primary OH and four secondary OH groups (c) two primary OH and three secondary OH groups (d) three primary OH and two secondary OH groups

Step-by-Step Solution

Answer

Glucose contains one primary OH and four secondary OH groups, so the answer is (b).

Step 1: Understanding the Structure of Glucose

Glucose is an aldohexose, which means it contains 6 carbon atoms and one aldehyde group in its structure. In its linear form, it has an aldehyde group at one end (C1) and a primary alcohol group at the other end (C6). The remaining carbon atoms (C2 to C5) are connected by single bonds.

Step 2: Analyze the Alcohol Groups

Within the glucose molecule, examine the hydroxyl (OH) groups attached to each carbon atom: - C1 has the aldehyde group (not hydroxyl). - C2, C3, C4, and C5 each have a secondary alcohol (OH) group, as they are bonded to two other carbon atoms. - C6 has a primary alcohol group, as it is only bonded to one other carbon atom apart from the hydroxyl group.

Step 3: Count the Primary and Secondary OH Groups

From the analysis: - One primary OH group is found at C6. - Four secondary OH groups are present at C2, C3, C4, and C5. Thus, glucose contains one primary OH and four secondary OH groups.

Step 4: Match with the Given Options

Compare the findings with the provided options: - Option (b) states: one primary OH and four secondary OH groups. This matches our analysis, thus option (b) is the correct answer.

Key Concepts

Aldehyde Group

At the heart of the glucose molecule lies its defining feature, the aldehyde group. Situated at the terminal end of its linear form, this group is characterized by the presence of a carbon atom double-bonded to an oxygen atom, and also bonded to a hydrogen atom. In the case of glucose, this aldehyde group is located on the first carbon, often referred to as C1.

The aldehyde group gives glucose its classification as an aldohexose, which indicates that it is an aldehyde-containing sugar with six carbon atoms. This group is reactive, and through processes like the formation of hemiacetals, it allows glucose to cyclize and form ring structures, which are prevalent in its naturally occurring forms. Understanding the role of the aldehyde group is essential, as it significantly influences glucose's chemical behavior, including its ability to reduce other compounds in biochemical reactions.

Primary Alcohol Group

The primary alcohol group in glucose provides an important structural element and is located at the opposite end of the molecule from the aldehyde group. Found on the sixth carbon, or C6, this group consists of a hydroxyl group (OH) attached to a carbon atom also bonded to only one other carbon atom. This distinction is critical, as it determines the reactivity and interactions of glucose.

Primary alcohol groups, like the one found at C6 of glucose, are capable of undergoing oxidation reactions to form carboxylic acids. This characteristic is often exploited in metabolic pathways within the body, where glucose is converted to energy. Additionally, the primary alcohol group plays a pivotal role in creating ester linkages and significantly contributes to glucose's solubility and reactivity with other molecules.

Secondary Alcohol Group

In the glucose molecule, the secondary alcohol groups are located on carbons C2, C3, C4, and C5. These groups are identified by a hydroxyl (OH) group attached to a carbon that is itself bonded to two other carbon atoms. This structural feature lends different properties to these secondary alcohols compared to primary alcohols.

Unlike primary alcohol groups, secondary alcohols are less reactive in oxidation reactions and often require more potent oxidizing agents to convert them to ketones. This characteristic affects glucose metabolism and its involvement in various biochemical pathways. Moreover, the presence of these multiple secondary alcohol groups is key in the formation of complex biomolecules and contributes to the hydrogen bonding that enables glucose to be highly soluble in water, which is crucial for its transport in biological systems. Understanding the distribution and role of these secondary alcohol groups offers insight into the functional complexity of glucose.