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

Option (b): one primary OH and four secondary OH groups.

Step 1: Understand the Structure of Glucose

Glucose is an aldohexose, which means it has six carbon atoms, one aldehyde group at the first carbon, and five hydroxyl (-OH) groups. One hydroxyl group is attached to each of the remaining five carbon atoms of the chain.

Step 2: Identify the Functional Groups in Glucose

In the chain structure of glucose, the first carbon has an aldehyde group, and the sixth carbon, which is at the end of the chain, has a hydroxyl group attached making it a primary alcohol. The rest of the carbons (C2 to C5) in the chain have secondary alcohols.

Step 3: Count Primary and Secondary OH Groups

In glucose, the -OH group at carbon 6 is attached to a primary carbon (since it is at the end of the chain). Therefore, it is a primary OH group. The -OH groups on carbons 2, 3, 4, and 5 are attached to secondary carbons since they are connected to two other carbons. Hence, there are four secondary hydroxyl groups and one primary hydroxyl group.

Step 4: Choose the Correct Option

The correct combination of primary and secondary OH groups that matches glucose is one primary OH and four secondary OH groups. Therefore, the answer to the question is option (b).

Key Concepts

Aldehyde Group

The aldehyde group is a key functional group in glucose, defining its classification as an aldohexose. An aldehyde group has the structure -CHO, where the carbonyl group (C=O) is bonded to a hydrogen atom. In glucose, this group is present at the first carbon atom of the molecule. This position is crucial because it determines the chemical reactivity of glucose.

• The carbon in the aldehyde group is bonded to an oxygen through a double bond, forming a C=O (carbonyl group).
• It's also bonded to a hydrogen atom, creating the -CHO structure.

Understanding the aldehyde group is vital as it participates in reactions such as oxidation, where glucose is converted into other sugar derivatives. Recognizing this group in structural formulas helps identify and predict the behavior and interactions of glucose in chemical pathways.

Primary Alcohol

A primary alcohol in chemistry has its hydroxyl (-OH) group attached to a carbon atom that is only connected to one other carbon atom. Primary alcohols are often found at the ends of carbon chains.

• In glucose, the primary alcohol is the hydroxyl group on the sixth carbon (C6).
• Since C6 is at the end of the chain, it's connected to only one other carbon (C5), making the hydroxyl group a primary alcohol.

Primary alcohols, such as the one in glucose, are important in biochemical reactions, particularly in energy metabolism. They undergo oxidation to form aldehydes and acids, which are crucial in various cellular processes.

Secondary Alcohol

The term secondary alcohol refers to a hydroxyl group (-OH) attached to a secondary carbon atom. A secondary carbon is one that is bonded to two other carbon atoms. In the glucose molecule, four out of the five hydroxyl groups are considered secondary alcohols.

• These secondary alcohols are located on carbons 2, 3, 4, and 5 of the glucose structure.
• Each of these carbons connects to both an adjacent carbon and a hydroxyl group, classifying them as secondary carbons.

Secondary alcohols play a significant role in the structural integrity of glucose. They are pivotal in hydrogen bonding and intermolecular interactions, influencing how glucose behaves in aqueous solutions and participates in metabolic pathways.