Problem 12 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

But 2-ene on reaction with alkaline \(\mathrm{KMnO}_{4}\) at elevated temperature followed by acidification will give : [Main April 12, 2019 (I)] (a) CC(O)C(C)O (b) one molecule of \(\mathrm{CH}_{3} \mathrm{CHO}\) and one molecule of \(\mathrm{CH}_{3} \mathrm{COOH}\) (c) 2 molecules of \(\mathrm{CH}_{3} \mathrm{COOH}\) (d) 2 molecules of \(\mathrm{CH}_{3} \mathrm{CHO}\)

Step-by-Step Solution

Answer

The product is two molecules of CH3COOH (option c).

Step 1: Understand the reaction

But-2-ene undergoes oxidative cleavage when reacted with alkaline potassium permanganate (KMnO4) at elevated temperatures. This reaction splits the double bond and converts the alkene into carbonyl compounds or carboxylic acids after acidification.

Step 2: Predict the products of oxidative cleavage

The starting material, But-2-ene, has the structure: CH3-CH=CH-CH3. Upon oxidative cleavage, the double bond is split, forming two molecules. Each of the carbons involved in the double bond ends up bonded to an oxygen atom, leading to the formation of two intermediate ketones which further oxidize to acids.

Step 3: Convert intermediates after cleavage

Each fragment from the cleavage of CH3-CH=CH-CH3 results in the formation of acetic acid (CH3COOH) after the reaction with alkaline KMnO4 followed by acidification. Both carbon fragments are fully oxidized to carboxylic acids.

Step 4: Choose the correct option

The oxidative cleavage converts But-2-ene into two molecules of acetic acid (CH3COOH). Therefore, the correct choice among the options given is (c) 2 molecules of CH3COOH.

Key Concepts

Oxidative Cleavage

Oxidative cleavage is a crucial reaction in organic chemistry that involves breaking a carbon-carbon double bond. This reaction splits the alkene into two separate carbon fragments with each carbon acquiring a double bond with oxygen, forming carbonyl compounds. The reagent commonly used for facilitating oxidative cleavage is potassium permanganate ( KMnO_4 ). This process is powerful because it can transform simpler alkenes into more complex and functional oxygen-containing molecules.
Oxidative cleavage encompasses a series of steps:

• The double bond in the alkene is cleaved.
• Oxidation occurs, leading to carbonyl or carboxylic acid formation.
• Depending on the conditions, either aldehydes or carboxylic acids result.

This reaction is essential because it provides a method to break down larger, intricate molecules into smaller, functional components, helpful in synthetic and industrial chemistry. In the example provided, But-2-ene undergoes this reaction with KMnO_4 , resulting in two molecules of acetic acid.

Alkene Reactions

Alkenes, characterized by their carbon-carbon double bond, engage in a variety of chemical reactions. Among these reactions, oxidative cleavage stands out for breaking these double bonds effectively. But-2-ene, an alkene with a structure CH_3-CH=CH-CH_3 , is subject to diverse transformations when conditions vary.
Some common alkene reactions include:

• Addition reactions, where atoms or groups add across the double bond.
• Polymerization, forming long chains or structures from the alkene units.
• Oxidation reactions, such as oxidative cleavage, converting alkenes into oxygen-containing molecules.

In the context of the exercise, the reaction of But-2-ene with KMnO_4 and the specific conditions leads to the oxidative cleavage of the double bond. This results in breaking But-2-ene into two separate molecules, each subsequently converting to acetic acid through oxidation.

KMnO4 Reaction Mechanism

The KMnO_4 reaction mechanism is notable for its effectiveness in oxidizing alkenes into carbonyl compounds. Potassium permanganate, as a strong oxidizing agent, plays a significant role in organic synthesis due to its ability to promote oxidative transformations.
Key steps in the KMnO_4 mechanism include:

• The double bond of the alkene forms a cyclic manganese ester.
• Hydrolysis of this ester leads to the formation of diol intermediates.
• Further oxidation of the diols results in carbonyl compounds, often carboxylic acids, when the reaction is under strong oxidizing conditions such as elevated temperatures and alkaline medium.

In the given exercise, KMnO_4 facilitates oxidative cleavage of But-2-ene, resulting in the transformation of the alkene into two molecules of acetic acid. This highlights the utility of KMnO_4 in increasing the oxidation state of organic compounds, making it a valuable tool in both laboratory and industrial chemistry.