Problem 222 - A Complete Resource Book in Chemistry for JEE Main

Question

The best reagent to convert pent-3-en-2-ol into pent3-en \(-2\)-one is (a) acidic permanganate (b) acidic dichromate (c) chromic anhydride in glacial acetic acid (d) pyridinium chloro-chromate

Step-by-Step Solution

Answer

The best reagent is pyridinium chloro-chromate (option (d)).

Step 1: Identify the Functional Group

First, identify that the starting compound, pent-3-en-2-ol, contains an alcohol group (–OH) that needs to be oxidized into a ketone (carbonyl group, C=O) to form pent-3-en-2-one.

Step 2: Understand the Reaction Requirement

We need an oxidizing agent that can convert the secondary alcohol (H group attached to one hydrogen and two carbon atoms) into a ketone, without affecting the double bond present in the molecule.

Step 3: Evaluate Options for Oxidants

Analyze the given reagents. Acidic permanganate and acidic dichromate (options (a) and (b)) are strong oxidizing agents that can oxidize alkenes, meaning they could affect the double bond. Therefore they are not suitable. Chromic anhydride in glacial acetic acid (option c) is also a strong oxidant, which might lead to over-oxidation.

Step 4: Choose a Suitable Oxidant

Pyridinium chloro-chromate (PCC) (option (d)) is known for its capacity to oxidize alcohols to ketones or aldehydes without affecting other functional groups like double bonds. Therefore, it is the most suitable reagent for this transformation.

Key Concepts

Oxidizing Agents

Oxidizing agents play a crucial role in chemical reactions by facilitating the transfer of electrons. They come into the picture when a substance needs to lose electrons, which is a common requirement when converting alcohols to ketones. Oxidizing agents can be strong or mild, and choosing the right one is essential for the desired chemical transformation.

• Strong oxidizing agents, like potassium permanganate or chromic anhydride, may oxidize multiple bonds and even break down complex structures. This is often unwelcome when specific functional groups need to remain intact, such as when a double bond should not be oxidized.
• Mild oxidizing agents, like pyridinium chloro-chromate (PCC), are preferred when the goal is to convert alcohols to carbonyl compounds, like ketones, while preserving other sensitive parts of the molecule. PCC, for instance, can effectively oxidize a secondary alcohol to a ketone without affecting a double bond present in the structure.

Choosing the correct oxidizing agent involves understanding the functional groups present and the reaction's desired outcome.

Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. Recognizing and understanding these groups is fundamental in organic chemistry.

• The primary functional group involved in oxidizing a secondary alcohol to a ketone is the hydroxyl group (-OH), which is characteristic of alcohols. In the starting substance, pent-3-en-2-ol, this group is the target for oxidation.
• The end product of the reaction has a carbonyl group (C=O) characteristic of ketones. This transformation is achieved without affecting double bonds, which can be seen as unsaturated functional groups.

The presence and interaction of these groups dictate how a chemical reaction proceeds and the type of oxidizing agents that can be used.

Alcohol to Ketone Conversion

The conversion of alcohols to ketones is a common task in organic chemistry, particularly when dealing with secondary alcohols. This conversion generally involves the oxidation of the carbon atom bonded to the hydroxyl group.

• A secondary alcohol has the structure R₂CH(OH), where 'R' represents organic groups. During oxidation, the hydroxyl group is converted to a carbonyl group, transforming the alcohol into a ketone.
• In the case of pent-3-en-2-ol to pent-3-en-2-one, the secondary alcohol is oxidized using a mild oxidizing agent, such as PCC, which selectively oxidizes the alcohol without altering the existing double bond.

Understanding the process of alcohol to ketone conversion often involves recognizing the stability and selectivity of the oxidizing agent used, ensuring other functional groups remain untouched.