Problem 98 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

A liquid (X), having a molecular formula \(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{2}\) is hydrolysed with water in the presence of an acid to give a carboxylic acid \((\mathrm{Y})\) and an alcohol \((\mathrm{Z})\). Oxidation of \((\mathrm{Z})\) with chromic acid gives \((\mathrm{Y}) .\) What are the structures of \((\mathrm{X})\), (Y) and \((\mathrm{Z}) ?\)

Step-by-Step Solution

Answer

'X' is ethyl acetate, 'Y' is acetic acid, and 'Z' is ethanol.

Step 1: Identify the Type of Reaction

We are dealing with a hydrolysis reaction in the presence of an acid. This typically indicates an ester being split into a carboxylic acid and an alcohol.

Step 2: Write the General Ester Hydrolysis Reaction

The general reaction for the hydrolysis of an ester \( RCOOR' \) in the presence of an acid is \( RCOOR' + H_2O ightarrow RCOOH + R'OH \), where \( RCOOH \) is the carboxylic acid and \( R'OH \) is the alcohol.

Step 3: Analyze the Molecular Formula

The given molecular formula \( ext{C}_6 ext{H}_{12} ext{O}_2 \) suggests an ester, with one part forming the carboxylic acid and the other forming the alcohol.

Step 4: Determine Structures of 'Y' and 'Z'

Since the oxidation of 'Z' gives 'Y', 'Z' must be a primary alcohol which can be oxidized to a carboxylic acid. The likely alcohol is ethanol \( ext{C}_2 ext{H}_5 ext{OH} \), which oxidizes to acetic acid \( ext{CH}_3 ext{COOH} \). Therefore, 'Y' is acetic acid.

Step 5: Determine the Structure of 'X'

If 'Z' is ethanol, then the remaining part of the ester must form 'Y', which is acetic acid. Therefore, 'X' is ethyl acetate \( ext{CH}_3 ext{COOC}_2 ext{H}_5 \).

Key Concepts

Carboxylic Acid Formation

Carboxylic acids are organic compounds that contain a carboxyl group \( \text{(-COOH)} \). They are formed through various chemical reactions, including ester hydrolysis. In the given exercise, we are tasked with understanding this transformation in the context of a specific ester, ethyl acetate. During ester hydrolysis, the ester molecule reacts with water \( H_2O \) in the presence of an acid catalyst.
This leads to the breaking of the ester bond, resulting in a carboxylic acid and an alcohol. For ethyl acetate, the hydrolysis process yields acetic acid.

• The general reaction equation is: \( RCOOR' + H_2O \rightarrow RCOOH + R'OH \).
• 'RCOOH' is the carboxylic acid, and 'R'OH' the alcohol.

The reaction often involves an acid catalyst to accelerate the process. Hydrolysis can occur under acidic or basic conditions, but in this exercise, we focus on the acidic version. The identification of the products is crucial to successfully understanding and predicting the structures of reactants and resulting compounds.

Alcohol Oxidation

Alcohol oxidation is a chemical reaction that transforms an alcohol into another compound. Typically, primary alcohols become aldehydes, and further oxidize to carboxylic acids. In this exercise, ethanol is the primary alcohol generated after ester hydrolysis. The oxidation of ethanol by chromic acid is a notable example where it turns into acetic acid.

• The process follows two main stages:
• **Stage 1:** Ethanol (\( C_2H_5OH \)) is first oxidized to acetaldehyde (\( CH_3CHO \)). This involves removing hydrogen from ethanol.
• **Stage 2:** Acetaldehyde is further oxidized to acetic acid (\( CH_3COOH \)).

This is possible because ethanol has an oxidizable hydrogen atom on the carbon bonded to the hydroxyl group. Chromic acid \( H_2CrO_4 \) serves as a strong oxidizing agent in this reaction, effectively promoting the conversion process. Understanding alcohol oxidation is essential in mastering organic reactions and identifying related chemical changes.

Molecular Structure Analysis

Molecular structure analysis involves determining the arrangement of atoms within a molecule. In the context of the exercise, understanding the molecular formula and reactivity helps identify each component's structure. We begin with the ester, ethyl acetate \( C_6H_{12}O_2 \), and analyze how hydrolysis transforms its parts.

• The ester can be broken down into two main components due to hydrolysis: the alcohol (ethanol) and the carboxylic acid (acetic acid).
• To determine the potential structures, look at the sub-groups: acetyl (from acetic acid) and ethyl (from ethanol).

By recognizing these groups, you can identify the connectivity between atoms in each molecule.
Using both empirical evidence (e.g., observed chemical reactions) and analytical techniques (e.g., spectroscopy), we validate the structural predictions. Structural analysis allows us to map out the molecules' exact form, providing insights into their physical and chemical behavior.