Question

Which of the following cannot reduce ammonical silver nitrate solution? (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NHOH}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) (c) \(\mathrm{HCOOH}\) (d) \(\mathrm{CH}_{3} \mathrm{CHO}\)

Short answer

Aniline (\(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\)) cannot reduce ammoniacal silver nitrate solution.

Step-by-step solution

Understand the Question

The task is to identify which compound cannot reduce an ammoniacal silver nitrate solution. The ammoniacal silver nitrate solution is also known as Tollen's reagent, which is reduced by compounds with aldehyde groups or similar structures.

Analyze the Options for Reducing Properties

Let's analyze each compound to see if it has the potential to reduce the Tollen's reagent:(a) \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NHOH} \) (Phenylhydroxylamine) can act as a reducing agent due to the presence of the hydroxylamine group. (b) \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2} \) (Aniline) does not have an aldehyde or similar reducing functional group. (c) \( \mathrm{HCOOH} \) (Formic acid) decomposes to produce carbon dioxide and hydrogen, which can reduce the silver nitrate to metallic silver. (d) \( \mathrm{CH}_{3} \mathrm{CHO} \) (Acetaldehyde) is an aldehyde and is known to reduce Tollen's reagent.

Evaluate and Identify the Non-Reducing Compound

Based on the analysis, compound (b) \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2} \) (Aniline) cannot reduce the ammoniacal silver nitrate solution because it lacks the necessary functional group to undergo oxidation and reduce the silver ions.

Key concepts

Reduction Reactions

Reduction reactions are a type of chemical reaction where a molecule gains electrons. In these reactions, there is typically a decrease in oxidation state of the molecule involved. When we talk about reduction in the context of Tollen's reagent, it involves the reduction of silver ions, resulting in the formation of a silver mirror on the inside of a test tube.
Tollen's reagent consists of silver nitrate, ammonia, and water. When a reducing agent, like an aldehyde, is present, it reduces the silver ions to metallic silver. This is an example of a redox reaction where the silver ions accept electrons and get reduced.

• Reduction requires a reducing agent that donates electrons.
• Silver ions ( ext{Ag}^{+}) are reduced to silver metal (Ag) in Tollen's reaction.
• Aldehydes often act as reducing agents in these reactions.

Understanding reduction reactions involves looking at the electron transfer and recognizing that not every compound has the ability to donate electrons or possess the correct functional group for this reaction.

Functional Groups in Chemistry

Functional groups are specific atoms or groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. Knowing about functional groups helps chemists predict how a particular compound will behave under different conditions. Each functional group has distinct properties and types of reactions it can undergo.
In the context of Tollen's reagent and our exercise:

• Aldehydes, characterized by the functional group R-CHO, are typically able to reduce Tollen’s reagent due to their ability to donate electrons.
• Hydroxylamine groups, like in phenylhydroxylamine ( ext{C}_{6} ext{H}_{5} ext{NHOH}), can act as reducing agents because they can donate electrons.
• Aniline ( ext{C}_{6} ext{H}_{5} ext{NH}_{2}) lacks a reducing functional group and cannot reduce silver ions.
• Carboxylic acids, such as formic acid ( ext{HCOOH}), can decompose to produce hydrogen, which acts as a reducing agent.

Recognizing these groups and their behavior is fundamental in understanding the chemical properties of substances and their reactions.

Aldehydes and Ketones

Aldehydes and ketones are classes of organic compounds that are both polar due to the presence of a carbonyl group (C=O). However, they differ in structure and reactivity. Aldehydes have at least one hydrogen atom attached to the carbonyl carbon, while ketones have two alkyl groups attached to it.
Aldehydes are more reactive than ketones in redox reactions. This is because the presence of the hydrogen atom makes the carbonyl carbon more susceptible to nucleophilic attack. The classic reaction with Tollen's reagent demonstrates this reactivity, where aldehydes can be easily oxidized to carboxylic acids, reducing silver ions to silver metal in the process.

• Aldehydes ( ext{R-CHO}) are excellent reducing agents for Tollen's reagent, such as acetaldehyde ( ext{CH}_{3} ext{CHO}).
• Ketones generally do not react with Tollen's reagent, as they lack the necessary hydrogen atom to undergo oxidation.

This intrinsic difference makes aldehydes particularly useful in tests for reactive carbonyl compounds, such as the silver mirror test, while ketones like acetone remain largely unreactive without more forceful conditions.