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 (PCC).

Step 1: Understand the Reaction

The reaction described is the conversion of an alcohol (pent-3-en-2-ol) into a ketone (pent-3-en-2-one). This is an oxidation reaction where an alcohol is oxidized to a ketone.

Step 2: Evaluate Each Option

Evaluate each option based on its ability to oxidize secondary alcohols to ketones. (a) Acidic permanganate is strong but typically over-oxidizes alcohols to carboxylic acids. (b) Acidic dichromate can oxidize alcohols to ketones but might be too strong. (c) Chromic anhydride in glacial acetic acid is a known reagent for selective oxidation. (d) Pyridinium chloro-chromate (PCC) is known for oxidizing alcohols to aldehydes and ketones without over-oxidizing.

Step 3: Select Best Reagent

The best choice must selectively oxidize the secondary alcohol to a ketone without further oxidation. PCC (option d) is renowned for its ability to convert alcohols to aldehydes or ketones without over-oxidizing to carboxylic acids, making it the ideal choice for this reaction.

Key Concepts

Oxidation of Alcohols

In organic chemistry, the oxidation of alcohols is a crucial process. It involves converting an alcohol into a more oxidized form, such as a ketone or aldehyde. This transformation is significant because it can change the properties and reactivity of a molecule. The oxidation process usually involves the removal of hydrogen atoms from the alcohol group.
For secondary alcohols like pent-3-en-2-ol, oxidation typically yields ketones. Understanding the nature of this transformation is key to predicting the outcome of reactions in organic synthesis.
Let’s summarize the general steps:

• Start with an alcohol functional group.
• Apply an oxidizing agent.
• End with a ketone or aldehyde with increased oxidation state.

The choice of oxidizing agent is crucial for achieving the desired product selectively and efficiently.

Reagents in Organic Chemistry

Reagents are essential tools in organic chemistry, acting as the workhorses that drive chemical reactions forward. They are substances or compounds added to cause a chemical reaction, and the choice of reagent has a significant impact on the reaction outcome.
Different reagents have specific properties that make them suitable for certain reactions. For example:

• Strong oxidizers like acidic permanganate can over-oxidize alcohols to carboxylic acids.
• Acidic dichromate is potent but can be too harsh, leading to over-oxidation.
• Chromic anhydride in glacial acetic acid allows selective oxidation without over-oxidizing.
• Pyridinium chloro-chromate (PCC) is preferred for its ability to convert alcohols to aldehydes or ketones smoothly, without further oxidation to acids.

Choosing the right reagent ensures the transformation is achieved with precision, minimizing side reactions and maximizing yield.

Alcohol to Ketone Conversion

The process of converting alcohols to ketones is a common transformation in organic chemistry, essential for synthesizing compounds with desired properties. Secondary alcohols like pent-3-en-2-ol are ideal candidates for oxidation to ketones.
To effectively carry out this conversion, a suitable reagent like Pyridinium chloro-chromate (PCC) is used. PCC is known for:

• Its ability to oxidize secondary alcohols to ketones without requiring additional catalysts.
• Operating under mild conditions, thus preventing over-oxidation to carboxylic acids.

Here’s how it works: the PCC reagent acknowledges the presence of the hydroxyl group in the alcohol and facilitates the oxidation, removing hydrogen atoms to generate a ketone.
This conversion is significant because ketones serve as important building blocks in various chemical syntheses, offering stability and reactivity useful for further chemical manipulation.