Problem 46 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

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

Step-by-Step Solution

Answer

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

Step 1: Understanding the Reaction

Fehling's solution is used as a test for aldehydes. When an aldehyde reacts with Fehling's solution, it gets oxidized to an acid, and the copper(II) ions in the solution are reduced.

Step 2: Identify the Products of Fehling's Reaction with an Aldehyde

Aldehydes reduce the Cu(II) ions in Fehling's solution to Cu(I) oxide, which is a red precipitate of \( \mathrm{Cu}_2\mathrm{O} \). The aldehyde itself is oxidized to a carboxylic acid.

Step 3: Conclusion

Based on the reaction of aldihydes with Fehling's solution, the precipitate formed upon heating acetaldehyde with Fehling's solution is \( \mathrm{Cu}_2\mathrm{O} \).

Key Concepts

Aldehyde Oxidation

Aldehyde oxidation is a key chemical reaction where an aldehyde compound is transformed into a carboxylic acid. This reaction is an essential step in various industrial and laboratory processes, often used to identify or quantify the presence of aldehydes.
Most aldehydes can undergo oxidation easily due to their molecular structure. An aldehyde group has a carbon atom double-bonded to oxygen (a carbonyl group) and single-bonded to hydrogen. Upon oxidation, the hydrogen atom is removed, and an oxygen atom that forms a new hydroxyl group (O-H) is added, converting the aldehyde to a carboxylic acid.

• This process typically requires an oxidizing agent, like Fehling's solution, which contains copper(II) ions as the oxidizing species.
• When aldehydes are oxidized, their chemical properties change, as they lose two electrons to the oxidizing agent.
• From a functional standpoint, this change results in the formation of a red precipitate that indicates a successful oxidation.

Understanding aldehyde oxidation helps in studying reactions and behaviors of other related organic compounds, especially in biological and food chemistry.

Copper(I) Oxide Formation

Copper(I) oxide, known chemically as Cu₂O, is a red, insoluble solid that forms during specific chemical reactions involving reducing agents, such as aldehydes. In the context of Fehling's solution, this compound is significant.
Fehling's solution contains copper(II) sulfate, which provides Cu²⁺ ions. During the reaction with aldehydes, these are reduced to copper(I) oxide, forming a reddish precipitate.

• This transformation is a classic example of a redox reaction, where copper undergoes reduction (gains electrons), and the aldehyde undergoes oxidation (loses electrons).
• Copper(I) oxide plays a critical role as a visual indicator. Its formation signifies the presence of an aldehyde and its subsequent oxidation.
• The deep red color of Cu₂O aids in easy visual identification, making this reaction a popular choice for qualitative analysis in laboratories.

The stability and insolubility of copper(I) oxide contribute to its use in various practical applications, beyond simple laboratory tests.

Acetaldehyde Reaction with Fehling's Solution

When acetaldehyde is treated with Fehling's solution, a specific chemical interaction takes place, highlighting the characteristics and reactivity of acetaldehyde as an aldehyde.
Fehling's solution serves both as a test and a reagent, containing blue copper(II) ions in an alkaline solution. When acetaldehyde is present, it reacts with these ions.

• The acetaldehyde undergoes oxidation to form acetic acid, a carboxylic acid, in this reaction.
• Meanwhile, the copper(II) ions in Fehling's solution are reduced to copper(I) oxide, resulting in the creation of the distinctive red precipitate of Cu₂O.
• This red precipitate confirms the presence of acetaldehyde and serves as a clear visual cue that oxidation has occurred.

This reaction is frequently utilized in laboratories to distinguish aldehydes from other functional groups, given the pronounced and identifiable outcome. Together with understanding aldehyde oxidation, it illustrates fundamental principles of organic chemistry reactions.