Problem 127 - A Complete Resource Book in Chemistry for JEE Main

Question

A compound 'A' has a molecular formula \(\mathrm{C}_{2} \mathrm{Cl}_{3} \mathrm{OH}\). It reduces Fehling's solution and on oxidation, gives a monocarboxylic acid 'B'. 'A' can be obtained by the action of chlorine on ethyl alcohol. 'A' is (a) chloroform (b) chloral (c) methyl chloride (d) monochloroacetic acid

Step-by-Step Solution

Answer

The compound 'A' is chloral (option b).

Step 1: Understanding the Problem

We are given a compound 'A' with the molecular formula \( \mathrm{C}_2 \mathrm{Cl}_3 \mathrm{OH} \) that reduces Fehling's solution and upon oxidation yields a monocarboxylic acid 'B'. We need to identify this compound from the given options.

Step 2: Analyzing the Reaction Conditions

The compound 'A' reduces Fehling's solution, indicating it has an aldehyde or alpha-hydroxy ketone group. Upon oxidation, it forms a monocarboxylic acid, suggesting 'A' contains an aldehyde group. Additionally, 'A' forms from chlorination of ethyl alcohol.

Step 3: Identify the Structures

Options provided are: (a) chloroform (\( \mathrm{CHCl}_3 \)), (b) chloral (\( \mathrm{C}_2 \mathrm{Cl}_3 \mathrm{OH} \)), (c) methyl chloride (\( \mathrm{CH}_3\mathrm{Cl} \)), and (d) monochloroacetic acid. We compare these with the properties of 'A'.

Step 4: Conclusion Based on Structure and Reactions

Chloral (option b) fits the molecular formula \( \mathrm{C}_2 \mathrm{Cl}_3 \mathrm{OH} \). It contains a trichloromethyl group and an aldehyde group, which can reduce Fehling's solution and form a carboxylic acid upon oxidation. Chloral also forms by chlorination of ethanol.

Key Concepts

Aldehyde Reaction

Aldehydes are crucial functional groups in organic chemistry due to their reactivity. They contain the group \(-CHO\) and are known for oxidation-reduction reactions. Aldehydes can easily be oxidized to carboxylic acids by mild oxidizing agents. This property is often used to identify the presence of an aldehyde group.
When reacting with Fehling's solution, an aldehyde will reduce the copper(II) ions to copper(I) oxide, which precipitates as a red solid. This transformation is a hallmark reaction indicating the presence of the aldehyde group in a compound.
In the case of the compound in our exercise, the ability to reduce Fehling's solution suggests it has an aldehyde group, making it capable of subsequent oxidation to form a monocarboxylic acid.

Compound Identification

Identifying a compound involves understanding its molecular structure and properties. The given molecular formula \(\mathrm{C}_2\mathrm{Cl}_3\mathrm{OH}\) contains clues about the structure, indicating it is chloral. Recognition of functional groups such as alcohols, aldehydes, or carbons attached to halogens often aids in this process.
In the given exercise, we analyze which of the potential candidates fits the molecular formula and known chemical behaviors. Unlike the others, chloral (\(\mathrm{C}_2\mathrm{Cl}_3\mathrm{OH}\)) has both a trichloromethyl group and an aldehyde group.

• Chloroform and methyl chloride lack aldehyde groups, not fitting our Fehling's test requirement.
• Monochloroacetic acid is a carboxylic acid, not aligning with the reactivity patterns needed for this solution.

The presence of both chloride and aldehyde groups in chloral matches each criterion, identifying it as compound 'A'.

Chlorination of Alcohol

The chlorination of alcohol is a chemical reaction where chlorine atoms replace hydrogen atoms on the carbon atom that is attached to the alcohol group. In our exercise, ethanol undergoes this transformation to form chloral. This is how alcohols can be transformed to various chlorinated derivatives.
The chlorination process involves several steps, often under catalyzed conditions, leading to the substitution of hydrogen atoms by chlorine, especially when carried out under specific conditions like light or heat.

• In the initial step, chlorine radicals are generated through homolytic cleavage.
• These radicals react with ethanol, abstracting hydrogens and replacing them with chlorine.
• Continued reaction leads to trichloro ethanol (chloral), where three chlorine atoms replace respective hydrogens, and an aldehyde group forms.

This reaction underscores the utility of chlorination in modifying the chemical nature of alcohols, aiding in creating desired functional groups.

Fehling's Solution Reduction

Fehling's solution is a chemical test used to differentiate aldehyde and ketone functional groups. It consists of two solutions: Fehling's A, which contains copper(II) sulfate, and Fehling's B, containing potassium sodium tartrate and a strong alkali.
When an aldehyde reacts with Fehling's solution, it reduces the blue copper(II) ions to red copper(I) oxide. This visual change is a positive test for aldehydes, while ketones (except alpha-hydroxy ketones) typically do not react, remaining unchanged.

• The reaction is a redox transformation - the aldehyde is oxidized to a carboxylic acid.
• The copper ions are simultaneously reduced, leading to the observable color change.

This principle was applied to identify the compound in the exercise, as only the aldehyde-containing structure could cause the reduction of Fehling's solution, leading to a positive identification of chloral.