Problem 74 - A Complete Resource Book in Chemistry for JEE Main

Question

An organic compound ' \(X\) ' with a molecular formula \(\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2}\) when reacts with excess of methyl magnesium chloride following by acidification gives an alcohol ' \(\mathrm{Y}\) ' only, when this alcohol ' \(\mathrm{Y}\) ' is oxidized with \(\mathrm{NaOCl}\) followed by acidification, acidic acid is formed. Here the compound ' \(\mathrm{X}\) ' is (a) Propyl methanoate (b) Isopropyl methanoate (c) Ethyl ethanoate (d) Both (a) and (b)

Step-by-Step Solution

Answer

Both propyl methanoate and isopropyl methanoate (options a and b) can form acetic acid.

Step 1: Analyze the given reaction information

We know compound X has the molecular formula \(\mathrm{C}_{4}\mathrm{H}_{8}\mathrm{O}_{2}\). It reacts with excess methyl magnesium chloride followed by acidification to yield alcohol Y. This indicates that X is likely an ester, as Grignard reagents typically react with esters to give alcohols.

Step 2: Identify possible ester structures

For \(\mathrm{C}_{4}\mathrm{H}_{8}\mathrm{O}_{2}\), possible ester structures include propyl methanoate (\(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{OCOCH}_{3}\)) and isopropyl methanoate. These esters can undergo a reaction with methyl magnesium chloride to produce different alcohols.

Step 3: Reaction of esters with methyl magnesium chloride

When esters react with methyl magnesium chloride, the ester group is converted into a tertiary alcohol. Both propyl methanoate and isopropyl methanoate would react with methyl magnesium chloride to produce tertiary alcohols, specifically producing 2-methylpropan-2-ol from propyl methanoate and isopropyl methanoate.

Step 4: Oxidation of tertiary alcohol to form acetic acid

The tertiary alcohol produced, \(\mathrm{Y}\), when oxidized with \(\mathrm{NaOCl}\) and then acidified, forms acetic acid. This confirms that both propyl methanoate and isopropyl methanoate could indeed react in such a manner to eventually yield acetic acid upon full oxidation.

Key Concepts

Grignard Reagent

Grignard reagents are a pivotal component in organic chemistry, commonly used for forming carbon-carbon bonds. These reagents are typically alkyl, vinyl, or aryl-magnesium halides, represented as RMgX, where R is an organic group and X is a halogen (often chlorine or bromine). In the presence of a Grignard reagent, organic compounds can undergo a variety of reactions. For instance, when an ester reacts with a Grignard reagent, it forms a tertiary alcohol after hydrolysis.

Understanding how Grignard reagents work is essential for several synthetic pathways. They are highly nucleophilic, meaning they readily donate electrons to electrophilic carbon atoms found in carbonyl groups (like those in esters), effectively attacking and altering these molecules. After this nucleophilic attack, an acidic reagent, such as water or a dilute acid, is typically used to complete the transformation into alcohol.

• Used for forming carbon-carbon bonds.
• Activates carbonyl groups through nucleophilic attacks.
• Commonly results in alcohols when reacting with compounds like esters.

By reacting with esters specifically, the Grignard reagent can expand both the complexity and variety of alcohols that can be synthesized, which is highlighted in transformations like those seen in compound 'X' from the original exercise.

Ester Reactions

Esters are commonly known for their pleasant smells and flavors, but chemically, they play a critical role in organic synthesis. Esters typically have the functional group RCOOR', where R and R' are hydrocarbon chains. In reactions such as those involving Grignard reagents, esters undergo significant transformations. When a Grignard reagent reacts with an ester, it involves breaking the ester linkage and forming a more complex alcohol.

The reaction process involves the nucleophilic attack by the Grignard reagent on the carbonyl carbon of the ester. Initially, the ester group is converted into a ketone intermediate, but with an excess of the Grignard reagent, a secondary addition occurs, transforming it into a tertiary alcohol. This step is crucial as it determines the structure and type of alcohol produced.

• Known for sweet-smelling compounds.
• Highly reactive with nucleophiles like Grignard reagents.
• Lead to alcohols when reacted with Grignard reagents.

Thus, esters serve as critical intermediates in synthesizing complex molecules, allowing for the expansion of carbon frameworks in controlled laboratory settings, as exemplified in the exercise reaction with compound 'X'.

Oxidation of Alcohols

The oxidation of alcohols is a fundamental reaction in organic chemistry, yielding aldehydes, ketones, or carboxylic acids depending on the type of alcohol. Tertiary alcohols, like the one derived from the original exercise's reaction scheme, generally do not oxidize to form aldehydes or ketones. Instead, under specific conditions, such as using sodium hypochlorite (NaOCl), tertiary alcohols undergo an oxidative cleavage to yield carboxylic acids.

Oxidation is essentially the process of increasing the number of carbon-oxygen bonds in a molecule. Primary alcohols typically oxidize first to aldehydes and then to carboxylic acids, while secondary alcohols form ketones. Tertiary alcohols resist the typical oxidation path and require harsher conditions, often leading to a breakdown of their carbon skeleton.

• Primary alcohols oxidize to aldehydes and carboxylic acids.
• Secondary alcohols oxidize to ketones.
• Tertiary alcohols resist oxidation but can break down under strong conditions.

In more complex applications, such as the circumstance discussed in the exercise, NaOCl acts as a mild oxidant, aiding in the transformation of Y, a tertiary alcohol, into acetic acid, emphasizing its utility in laboratory and industrial settings.