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 {Cu_2O}, so the answer is (b).

Step 1: Understand the reaction conditions

When an aliphatic aldehyde is heated with Fehling's solution, a chemical reaction occurs. Fehling's solution is composed of two solutions: Fehling's A, which contains copper(II) sulfate ({CuSO_4}), and Fehling's B, which is a mixture of sodium hydroxide ({NaOH}) and potassium sodium tartrate (Rochelle salt).

Step 2: Identify the reactants and the nature of the reaction

The main reactant, in this case, is the aliphatic aldehyde. In the reaction, the aldehyde acts as a reducing agent, and Fehling's solution acts as an oxidizing agent. As the aldehyde gets oxidized to a carboxylic acid, the copper(II) ions ({Cu^{2+}}) in the Fehling's solution are reduced to form copper(I) oxide.

Step 3: Describe the chemical change

During the reaction, the blue color of the {Cu^{2+}} ions in the Fehling's solution fades, and a red precipitate of copper(I) oxide, {Cu_2O}, is formed. This change indicates the reduction of copper ions by the aldehyde.

Step 4: Conclusion based on the reaction

The correct chemical product formed from this reaction is copper(I) oxide ({Cu_2O}). Therefore, the answer to the exercise is option (b): {Cu_2O}.

Key Concepts

Understanding Aliphatic Aldehydes

Aliphatic aldehydes are organic compounds characterized by the presence of a carbonyl group (\(\text{C=O}\)) bonded to a carbon chain with at least one hydrogen atom attached to the carbonyl carbon.
These compounds are generally part of linear or branched chains, unlike aromatic aldehydes which contain benzene rings.
Their simple structure makes them highly reactive in oxidation-reduction reactions.
Aliphatic aldehydes serve as excellent reducing agents since they have hydrogen atoms that are typically more available to participate in reactions compared to other types. The structural simplicity and high reactivity of these aldehydes are key factors in their interaction with Fehling’s solution, leading to notable color changes as they undergo oxidation to form carboxylic acids.

The Oxidation-Reduction Reaction

In the context of Fehling's solution, the reaction involving aliphatic aldehydes is a typical redox process.
Simply put, a redox (oxidation-reduction) reaction involves the transfer of electrons between two species.
Here, the aliphatic aldehyde undergoes oxidation, which means it loses electrons.Fehling's solution, acting as an oxidizing agent, has components that accept these electrons.
Specifically, the copper(II) ions (\(\text{Cu}^{2+}\)) in the solution are reduced as they gain electrons lost by the aliphatic aldehyde.
This transfer of electrons is what causes the notable change in color and indicates the dynamic nature of redox reactions.
When describing this reaction in detail, it’s important to note that the aldehyde is transformed into a carboxylic acid, highlighting the dual process of oxidation and reduction.

Copper(I) Oxide Formation in Fehling's Reaction

The formation of copper(I) oxide is a defining aspect of the reaction between aliphatic aldehydes and Fehling's solution.
Initially, Fehling’s solution contains a blue-colored complex due to the presence of copper(II) ions (\(\text{Cu}^{2+}\)).
During the reaction, the reduction of copper(II) ions to copper(I) oxide (\(\text{Cu}_2\text{O}\)) occurs.This transformation is evident as the initial blue solution turns into a reddish precipitate, signaling the formation of copper(I) oxide.
The change in color is not just visually noteworthy; it serves as a qualitative indicator of the redox reaction occurring.
The appearance of the red precipitate confirms the completion of the reduction process, marking the successful electron exchange and chemical transformation of the involved substances.

Pathway to Carboxylic Acid Production

Once aliphatic aldehydes are subjected to oxidation by Fehling’s solution, they undergo a chemical transformation into carboxylic acids.
This process involves the addition of an oxygen atom to the aldehyde, creating the carboxyl functional group (\(\text{-COOH}\)).
Carboxylic acids are characterized by this functional group and are known to have higher boiling points and differing solubility compared to the original aldehydesThis alteration in chemical structure reflects the transition of the aldehyde from a reducing agent to a more oxidized form—the carboxylic acid.
In practical terms, this change is crucial in the study and utilization of organic reactions, showcasing how simple organic molecules can be converted into different classes of compounds through oxidation.
This process exemplifies the efficiency and predictability of chemical reactions involving aldehydes and provides valuable insight for organic chemistry applications.