Problem 27 - A Complete Resource Book in Chemistry for JEE Main

Question

When aliphatic aldehyde heat with Fehling solution product formed is (a) \(\mathrm{CuO}\) (b) \(\mathrm{Cu}_{2} \mathrm{O}\) (c) \(\mathrm{CuSO}_{4}\) (d) \(\mathrm{Cu}\)

Step-by-Step Solution

Answer

(b) \\( ext{Cu}_2 ext{O}\\) is formed.

Step 1: Understand the Reaction

Fehling's solution is used as a test for aldehydes and contains a complexed Cu(II) ion in alkaline solution. When it's heated with an aliphatic aldehyde, it oxidizes the aldehyde to a carboxylic acid and the copper(II) ions are reduced.

Step 2: Identify the Oxidation Process

In the reaction, aldehyde (R-CHO) is oxidized to the corresponding carboxylic acid (R-COOH), while the copper(II) ions, which are originally blue due to the formation of Cu(II) complex, are reduced.

Step 3: Identify the Reduction Product

The Cu(II) ions are reduced to copper(I) oxide. The reduction product \( ext{Cu}_2 ext{O}\) is a red precipitate, which is commonly observed in the reaction.

Step 4: Match with Given Options

The reduction product of copper in this reaction is \( ext{Cu}_2 ext{O}\), which corresponds to option (b). This matches our understanding that \( ext{Cu}_2 ext{O}\) is formed as a precipitate during the reaction with an aliphatic aldehyde.

Key Concepts

Aliphatic Aldehydes

Aliphatic aldehydes are a type of organic compound characterized by their carbonyl group  ( 6CHO), which is linked to an aliphatic carbon chain 6 a straight or branched chain of carbon atoms. These aldehydes are known for their relatively simple structure in comparison to aromatic aldehydes, which contain a benzene ring. Aliphatic aldehydes are quite reactive due to the presence of the carbonyl group.
They readily participate in various chemical reactions, such as oxidation, due to the electrophilic nature of the carbon atom. They are used in many industrial and chemical processes, such as in the production of perfumes and flavors.

When it comes to detection, aliphatic aldehydes can be identified through the use of Fehling's solution, a blue solution used in qualitative analysis. If an aliphatic aldehyde is present, the solution will change color, showcasing its ability to oxidize under the right conditions.

Oxidation-Reduction Reactions

Oxidation-reduction reactions, also known as redox reactions, are processes where the oxidation state of atoms is changed. They involve two crucial chemical changes: oxidation, which is the loss of electrons, and reduction, which is the gain of electrons.

In the specific case of Fehling's solution with an aliphatic aldehyde, the aldehyde is the substance that undergoes oxidation. Here, the carbonyl group 6CHO of the aldehyde transforms into a carboxylic acid 6COOH, demonstrating a classic case of oxidation. Simultaneously, the copper ions in Fehling's solution undergo reduction. The copper starts in a +2 oxidation state and gains electrons to become copper(I) oxide, which has a +1 oxidation state. These complementary changes highlight the importance of recognizing oxidation-reduction as coupled processes that occur naturally and are balanced in chemical reactions.

• Oxidation: Loss of electrons (e.g., aldehyde to acid)
• Reduction: Gain of electrons (e.g., Cu(II) to Cu(I))

The significant visual indicator of this reaction is the formation of a red precipitate of copper(I) oxide, signaling the successful reduction of copper ions.

Copper(I) Oxide

Copper(I) oxide, also known as cuprous oxide, is a red compound that forms as a precipitate during the reaction of Fehling's solution with aliphatic aldehydes. This precipitate is a hallmark of such reactions and serves as a visual cue for the occurrence of a redox reaction.

Cuprous oxide has the chemical formula \(\mathrm{Cu}_2\mathrm{O}\) and is distinct in color from other copper compounds like copper(II) oxide, which is usually black. The formation of \(\mathrm{Cu}_2\mathrm{O}\) is a direct result of the reduction of copper ions (Cu(II) to Cu(I)) during the reaction process. When the aldehyde is oxidized, copper(II) ions in Fehling's solution grab some electrons and turn into copper(I) ions, which then bond with oxygen to form the red compound.

Besides its role in chemical reactions, \(\mathrm{Cu}_2\mathrm{O}\) holds importance in various industrial applications. It is used as a pigment, a fungicide, and in some electrical and electronic applications due to its semiconducting properties. Its consistency in causing a notable color change makes it a valuable detector in chemical laboratories.