Problem 7 - A Complete Resource Book in Chemistry for JEE Main

Question

Glucose gives silver mirror test with Tollen's reagent. It shows the presence of (a) ketonic group (b) aldehydic group (c) alcoholic group (d) acidic group

Step-by-Step Solution

Answer

The presence of the aldehydic group in glucose allows it to give a silver mirror test with Tollen's reagent.

Step 1: Understanding the Tollen's Reagent Test

Tollen's reagent is a chemical reagent used to distinguish between aldehydes and ketones. When an aldehyde is present, it reduces the Tollen's reagent, leading to the deposition of metallic silver on the walls of the test container, forming a 'silver mirror.' This reaction does not occur with ketones.

Step 2: Identifying Glucose's Relevant Functional Group

Glucose is an aldohexose, meaning it contains an aldehydic group as one of its functional groups. This aldehydic group is at the end of its carbon chain, making it capable of reacting with Tollen's reagent to produce a silver mirror.

Step 3: Making the Conclusion

Because glucose gives a silver mirror test when reacted with Tollen's reagent, this indicates the presence of an aldehydic group. Hence, the test is specifically identifying the aldehydic group in glucose as the active component.

Key Concepts

Aldehydic Group

The aldehydic group is a key component in certain organic compounds' structure. Aldehydic groups are represented by the functional group \(-CHO\). This means that they have a carbon atom (") connected to a hydrogen atom (") and double bonded to an oxygen atom ("). The carbon in the aldehydic group is typically at the end of a carbon chain. This end-position allows it to participate in specific chemical reactions. It is this unique arrangement that classifies it distinctively from other types of carbonyl groups found in ketones, where the carbonyl carbon is located between two other carbon atoms. The ability of aldehydes to react with certain reagents, such as Tollen's reagent, is due to this terminal carbonyl group, which makes them more easily oxidized. In the case of glucose, the aldehydic group enables it to participate in the Tollen's test, forming the characteristic silver mirror.

Chemical Reagent

Chemical reagents are substances or compounds added to a system to cause a chemical reaction or added to test if a reaction occurs. Tollen's reagent is a classic example used in organic chemistry labs. It is an aqueous solution containing silver nitrate and ammonia. When prepared, Tollen's reagent produces a complex ion, essentially making it a gentle oxidizing agent. This reagent is particularly adept at identifying aldehydyic groups. When an aldehyde is present, Tollen's reagent gets reduced while simultaneously oxidizing the aldehyde to a carboxylic acid. In this process, the silver ions in the reagent are reduced to metallic silver, which deposits to form a visible, reflective silver mirror on the test container. This distinctive reaction is absent in ketones and makes Tollen's reagent a reliable tool for differentiating between aldehydes and ketones in a laboratory setting.

Functional Groups

Functional groups are unique groups of atoms within molecules that determine the chemical properties and reactions of those molecules. They are crucial in identifying and dictating the behavior of organic compounds. Some common functional groups include hydroxyl groups in alcohols (") or carbonyl groups in aldehydes (") and ketones. Each functional group has distinct chemical characteristics. For example, aldehydic groups can be easily oxidized, allowing them to react with certain reagents such as Tollen's reagent. This reactional characteristic can be used to distinguish aldehydes from other functional groups like ketones, which do not oxidize with Tollen's reagent. Understanding functional groups aids chemists in predicting the reactivity and interactions of organic compounds, thereby playing a critical role in organic synthesis and analysis.