Problem 33 - A Complete Resource Book in Chemistry for JEE Main

Question

When ethanal is heated with Fehling's solution it gives a precipitate of (a) \(\mathrm{CuO}\) (b) \(\mathrm{Cu}\) (c) \(\mathrm{Cu}+\mathrm{Cu}_{2} \mathrm{O}+\mathrm{CuO}\) (d) \(\mathrm{Cu}_{2} \mathrm{O}\)

Step-by-Step Solution

Answer

The precipitate formed is (d) \(\mathrm{Cu}_2 \mathrm{O}\).

Step 1: Identify the Reactants

First, identify the reactants in the reaction mentioned. Ethanal, also known as acetaldehyde, is \( ext{CH}_3 ext{CHO}\), and Fehling's solution contains copper(II) ions in an alkaline medium.

Step 2: Determine the Reaction Type

Recognize that the reaction involves an oxidation-reduction process. In Fehling's solution, the copper(II) ions act as an oxidizing agent.

Step 3: Understand the Reaction

When ethanal ( ext{CH}_3 ext{CHO}) is heated with Fehling's solution, the aldehyde group is oxidized. This causes the reduction of the copper(II) ions to copper(I) oxide.

Step 4: Identify the Product

The copper(I) oxide product forms as the final precipitate, which is \( ext{Cu}_2 ext{O}\), recognizable by its red color.

Step 5: Choose the Correct Option

Based on the above steps, the precipitate formed is \( ext{Cu}_2 ext{O}\). The correct option is (d) \( ext{Cu}_2 ext{O}\).

Key Concepts

Oxidation-Reduction Reactions

Oxidation-reduction reactions, also known as redox reactions, are a fundamental type of chemical process where the transfer of electrons occurs between substances. These reactions consist of two main parts: oxidation, where a substance loses electrons, and reduction, where a substance gains electrons. In a redox reaction, the substance that donates electrons is called the reducing agent, while the substance that accepts electrons is known as the oxidizing agent.

When dealing with redox reactions, it’s vital to understand that these processes happen simultaneously - when one compound is oxidized, another is reduced. In the context of Fehling's solution and ethanal, the ethanal acts as the reducing agent. It donates electrons during the reaction, which leads to its oxidation from an aldehyde to the corresponding carboxylate ion.

On the other hand, copper(II) ions in Fehling's solution serve as the oxidizing agent. They accept the electrons and in the process get reduced to copper(I) oxide, a red precipitate (Cu\(_2\)O). These interactions highlight the dynamic equilibrium within redox reactions and underscore how different compounds can change depending on their electron donor-acceptor roles.

Ethanal

Ethanal, commonly known as acetaldehyde, is a simple aldehyde with the chemical formula CH\(_3\)CHO. This compound belongs to the class of organic compounds, specifically carbonyl compounds, characterized by a carbon double-bonded to an oxygen atom (C=O).

As an aldehyde, ethanal has a unique chemical behavior that makes it quite reactive. Its reactivity primarily stems from the presence of the carbonyl group. This feature allows ethanal to participate in various chemical reactions, including oxidation-reduction processes, such as those with Fehling's solution.

In its interactions, like when heated with Fehling’s solution, ethanal readily undergoes oxidation. This means it loses electrons and transforms into a carboxylate ion, which is less reactive. These changes highlight the importance of ethanal as a functional compound in organic chemistry, and its transformative capability during oxidation reactions.

Aldehyde Reactions

Aldehydes are a class of organic compounds that play a central role in numerous chemical reactions. The unique structural feature of aldehydes is their carbonyl group, which contains a carbon atom double-bonded to an oxygen atom. This configuration contributes to their chemical reactivity and ability to undergo a variety of transformations.

One significant type of reaction that aldehydes undergo is oxidation. Aldehydes are easily oxidized to carboxylic acids. In our case, ethanal oxidizes in Fehling's solution. This specific transformation involves ethanal turning into acetic acid as it reacts with copper(II) ions. In this process, the aldehyde group (CH\(_3\)CHO) loses electrons, indicating it is getting oxidized.

Accompanying the oxidation of ethanal, the copper(II) ions in the solution undergo a reduction. This reduction results in the formation of copper(I) oxide (Cu\(_2\)O), which you can identify by its distinct red precipitate color. In summary, aldehyde reactions, particularly oxidation, are crucial for transforming substances and play a significant role in organic chemistry.