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: Understand the Properties of Compound 'A'

Compound 'A' has the molecular formula \( \mathrm{C}_{2} \mathrm{Cl}_{3} \mathrm{OH} \). It reduces Fehling's solution, indicating it has an aldehyde group that can be oxidized to a carboxylic acid. 'A' gives a monocarboxylic acid 'B' upon oxidation. These properties suggest 'A' is a compound with an aldehyde group.

Step 2: Consider the Formation of 'A'

The compound 'A' is formed from ethyl alcohol and chlorine. Introducing chlorine to ethyl alcohol generally results in substitution reactions, likely forming a compound with the formula \( \mathrm{C}_{2} \mathrm{Cl}_{3} \mathrm{OH} \), which matches the formula for 'A'.

Step 3: Analyze the Options

Option (a) chloroform \( \mathrm{CHCl_3} \), does not reduce Fehling's solution. Option (c) methyl chloride \( \mathrm{CH_3Cl} \), and (d) monochloroacetic acid \( \mathrm{ClCH_2COOH} \) also do not match the properties of 'A'. The structure of 'A' must be \( \mathrm{CCl_3CHO} \) chloral, which can both reduce Fehling’s solution and be oxidized to \( \mathrm{CCl_3COOH} \) chloral hydrate.

Step 4: Confirm 'A' as Chloral

Chloral \( \mathrm{CCl_3CHO} \) is formed by chlorinating ethyl alcohol and it can reduce Fehling’s solution due to the aldehyde group. Upon oxidation, it forms trichloroacetic acid, a monocarboxylic acid, verifying that compound 'A' is chloral.

Key Concepts

Fehling's Solution

Fehling's solution is a chemical reagent used to differentiate between aldehydes and ketones. It's primarily employed to identify the presence of aldehyde groups. When an aldehyde is present, the Fehling's solution undergoes a noticeable color change, usually from a deep blue to a brick-red precipitate. This reaction occurs because the aldehyde is oxidized to a carboxylic acid, while the copper(II) ions in Fehling’s solution are reduced to copper(I) oxide.

• This makes Fehling's solution a common tool in organic chemistry for detecting aldehyde-functional groups in compounds.
• However, it's important to note that Fehling's solution does not react with ketones due to their resistant structure to oxidation by mild oxidizing agents.

Chlorination

Chlorination involves the introduction of chlorine into a molecule, often resulting in the substitution of hydrogen atoms with chlorine atoms. This process is used in various chemical syntheses to create tailored compounds with desired properties. In organic chemistry, chlorination can significantly alter the characteristics and reactivity of a molecule.

• For instance, chlorine adds electronegativity to the molecule, making it more reactive or polar.
• When ethyl alcohol (Cmathrm{C}_2H_5OHC) is chlorinated, as described in the original exercise, it can result in different products depending on the reaction conditions.
• In the case of chloral (Cmathrm{CCl_3CHOC), the chlorination is quite extensive, as three hydrogens are substituted by chlorine atoms.

Aldehydes

Aldehydes are a class of organic compounds characterized by the presence of a carbonyl group (Cmathrm{C=O}C) with a hydrogen atom bonded to the carbon atom in the carbonyl group. This structural component makes aldehydes quite reactive, particularly in oxidation reactions.
Aldehyde Characteristics:

• The carbonyl group in aldehydes is polar, resulting in higher reactivity compared to aliphatic ketones.
• Aldehydes are easily oxidized to carboxylic acids, making them susceptible to oxidation reactions, such as their reaction with Fehling's solution.
• Chloral (Cmathrm{CCl_3CHOC) is an example of an aldehyde where the aldehyde functional group enables its reactivity towards oxidation, making it a unique compound in organic synthesis.

Oxidation Reactions

Oxidation reactions involve the addition of oxygen or the removal of hydrogen from a molecule. In organic chemistry, these reactions are critical for converting alcohols to aldehydes and further to carboxylic acids, or converting aldehydes directly to carboxylic acids.
Key Points on Oxidation:

• When an aldehyde, such as chloral (Cmathrm{CCl_3CHOC), is oxidized, it undergoes transformation into a carboxylic acid, in this case, trichloroacetic acid (Cmathrm{CCl_3COOHC).
• Oxidizing reagents can vary in strength, with some being mild like Fehling's solution, which is suitable for oxidizing aldehydes to acids without over-oxidizing to other products.
• Understanding oxidation reactions is crucial for synthesizing various compounds and manipulating their structures for desired chemical properties.