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

The product formed is (b) \(\mathrm{Cu}_{2} \mathrm{O}\).

Step 1: Understand the reaction components

Fehling's solution is a mixture of copper(II) sulfate (\(\mathrm{CuSO}_4\)), sodium potassium tartrate (Rochelle salt), and sodium hydroxide (\(\mathrm{NaOH}\)). Aliphatic aldehydes are simple organic compounds with the form \(\mathrm{RCHO}\), where \(\mathrm{R}\) is an alkyl group.

Step 2: Identify the role of each component

In the presence of aliphatic aldehydes, Fehling's solution acts as an oxidizing agent. The copper(II) sulfate (\(\mathrm{CuSO}_4\)) provides copper(II) ions (\(\mathrm{Cu^{2+}}\)) which are reduced during the reaction.

Step 3: Determine the chemical reaction outcome

During the reaction, the aliphatic aldehyde is oxidized to a carboxylic acid. The copper(II) ions from the \(\mathrm{CuSO}_4\) in Fehling's solution are reduced to form copper(I) oxide, \(\mathrm{Cu_2O}\), which is a red precipitate.

Step 4: Select the correct answer

Based on the understanding that copper(II) ions are reduced to copper(I) oxide when reacting with an aliphatic aldehyde, the correct product formed is \(\mathrm{Cu_{2}O}\).

Key Concepts

Aliphatic Aldehydes

Aliphatic aldehydes are a class of organic compounds that contain a carbonyl group \((RCHO)\) where \(R\) is a simple carbon-containing substituent such as a methyl \((CH_3)\) or ethyl \((C_2H_5)\) group. These compounds are typically straightforward in structure and lack the complex rings found in aromatic aldehydes.

• The carbonyl group is highly reactive due to the presence of a polar carbon-oxygen double bond.
• This makes aliphatic aldehydes susceptible to oxidation, where they can transform into carboxylic acids.

A key characteristic of aliphatic aldehydes is their ability to act as reducing agents in chemical reactions, such as when they react with oxidizing agents.

Oxidizing Agent

An oxidizing agent is a substance that accepts electrons in a chemical reaction, thereby causing another substance to be oxidized. Every redox reaction has an oxidizing agent and a reducing agent. In the context of Fehling's solution reaction:

• The oxidizing agent is Fehling's solution, which contains \(Cu^{2+}\) ions.
• In this reaction, the aldehyde donates electrons (and loses hydrogen atoms), while the copper(II) ions gain electrons, becoming reduced.
• This transformation leads to the formation of copper(I) oxide \(Cu_2O\).

This process is a fundamental representation of oxidation-reduction reactions, where the oxidizing agent promotes the oxidation of another chemical.

Copper(I) Oxide

Copper(I) oxide, \(Cu_2O\), is the product formed alongside a carboxylic acid when aliphatic aldehydes react with Fehling's solution. Unlike copper(II) ions, copper(I) oxide has a characteristic red color and shows as a precipitate. This compound is formed through the reduction of copper(II) ions present in the Fehling's solution:

• The chemical equation involved is \([2 \, Cu^{2+} + 2e^- \, \rightarrow \, Cu_2O]\).
• The reddish color is a distinguishing feature that makes this reaction visually evident.

Copper(I) oxide is insoluble in water, and its formation confirms the presence of an aliphatic aldehyde in the reaction mixture.

Red Precipitate

Red precipitate is the term used to describe the solid that forms at the bottom of a reaction vessel during a chemical process, such as when Fehling's solution reacts with aliphatic aldehydes. In this specific reaction, the red precipitate is due to the formation of copper(I) oxide \(Cu_2O\). The appearance of the red precipitate signifies that:

• The aliphatic aldehyde has been oxidized to a carboxylic acid.
• The once-blue Fehling's solution transforms its color, indicating a successful reduction of copper(II) ions to copper(I) oxide.

This visible change provides clear evidence of the redox process and serves as a qualitative test for aliphatic aldehydes.