Problem 4 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The most suitable reagent for the conversion of \(\mathrm{R}-\mathrm{CH}_{2}-\mathrm{OH} \rightarrow \mathrm{R}-\mathrm{CHO}\) is: [Main 2014] (a) \(\mathrm{KMnO}_{4}\) (b) \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) (c) \(\mathrm{CrO}_{3}\) (d) PCC (Pyridinium chlorochromate)

Step-by-Step Solution

Answer

PCC (Pyridinium chlorochromate) is the most suitable reagent.

Step 1: Understand the Reaction

The reaction involves the conversion of a primary alcohol (\(\mathrm{R-CH_2-OH}\)) to an aldehyde (\(\mathrm{R-CHO}\)). This transformation requires selective oxidation that stops at the aldehyde stage without further oxidation to a carboxylic acid.

Step 2: Identify Suitable Reagents

We are given several reagents: \(\mathrm{KMnO_4}\), \(\mathrm{K_2Cr_2O_7}\), \(\mathrm{CrO_3}\), and PCC. Most of these reagents are strong oxidizing agents and tend to oxidize alcohols to carboxylic acids, except for one that can achieve this transformation selectively.

Step 3: Review Each Option

- \(\mathrm{KMnO_4}\) is a strong oxidizing agent typically used to convert alcohols directly to carboxylic acids.- \(\mathrm{K_2Cr_2O_7}\) is another strong oxidizing agent, which usually oxidizes primary alcohols to carboxylic acids as well.- \(\mathrm{CrO_3}\) is generally used in acidic medium (Jones oxidation) and also results in over-oxidation.- PCC, or Pyridinium chlorochromate, is known for its ability to oxidize primary alcohols to aldehydes without further oxidation to carboxylic acids.

Step 4: Choose the Most Suitable Reagent

Based on the understanding of each reagent's properties, PCC is the most suitable reagent for converting \(\mathrm{R-CH_2-OH}\) to \(\mathrm{R-CHO}\) as it provides the desired oxidation level without over-oxidizing to a carboxylic acid.

Key Concepts

Alcohol to Aldehyde Conversion

The conversion of alcohols into aldehydes is a fundamental reaction in organic chemistry. Alcohols, especially primary alcohols, contain an \(-OH\) group attached to a carbon atom. To convert this alcohol into an aldehyde, we need to remove two hydrogen atoms from the carbon center, creating a carbonyl group \(-C=O\) instead. This transformation requires oxidation, but it must be carefully controlled. If the reaction continues too far, the aldehyde could further oxidize into a carboxylic acid, which is often not desirable for this type of conversion.

In this reaction, the oxidation must stop at the aldehyde stage. The appropriate reagent choice is critical to achieving this selective oxidation, as it allows for the formation of the desired aldehyde without progressing to a carboxylic acid. Selecting the right oxidizing agent ensures a successful reaction, yielding the intended product with precision.

Oxidizing Agents

Oxidizing agents are substances that cause the oxidation of other molecules by accepting electrons. In the context of alcohol to aldehyde conversions, certain oxidizing agents are needed to achieve selective oxidation.

Commonly used oxidizing agents include:

• \(\mathrm{KMnO_4}\) - a powerful agent that typically leads to over-oxidation to carboxylic acids.
• \(\mathrm{K_2Cr_2O_7}\) - another strong oxidizer often resulting in carboxylic acid formation from primary alcohols.
• \(\mathrm{CrO_3}\) - used in Jones oxidation, which also tends to fully oxidize alcohols to acids.
• PCC (Pyridinium chlorochromate) - preferred for converting primary alcohols specifically to aldehydes, limiting the oxidation at the aldehyde stage.

Understanding each oxidizing agent's strength and selectivity is key to determining the right choice for alcohol to aldehyde transformation. PCC stands out as the ideal agent in this process due to its ability to halt oxidation at the aldehyde level.

Selectivity in Oxidation

The concept of selectivity in oxidation is crucial when converting alcohols to aldehydes. Selectivity means the ability to target a specific product without forming excessive by-products. In organic chemistry, achieving high selectivity involves choosing reagents and conditions that favor one reaction pathway over others.

During the oxidation of primary alcohols, extreme care must be taken to prevent over-oxidation. Certain reagents, like PCC, provide this selectivity by allowing oxidation to stop precisely at the aldehyde stage. This reagent avoids the pitfall of further oxidation, which would produce carboxylic acids.

Therefore, selectivity in oxidation ensures the conversion stops at just the right chemical transformation. It is vital for obtaining the desired compounds while minimizing undesired reactions and by-products, optimizing yield and purity for the target aldehyde product.