The most suitable reagent for the conversion of R-CH2-OH - R-CHO is
(a) KMnO4
(b) K2Cr2O7
(c) CrO3

Q: Why can't we use $KMnO_4$ for this conversion?

$KMnO_4$ is a very strong oxidizing agent. It will rapidly oxidize the primary alcohol to an aldehyde and then immediately oxidize that aldehyde further into a carboxylic acid. You won't be able to isolate the aldehyde.

Q: What exactly is PCC?

PCC stands for Pyridinium chlorochromate. It is a yellow-orange salt with the formula $[C_5H_5NH]^+[CrO_3Cl]^-$. It is prepared by reacting pyridine with hydrochloric acid and chromium trioxide.

Q: Why does PCC stop at the aldehyde stage?

PCC is typically used in anhydrous (water-free) solvents like dichloromethane. For an aldehyde to be oxidized to a carboxylic acid, it usually needs to react with water to form a hydrate first. Without water, this further oxidation is prevented.

Q: What happens if we use $CrO_3$?

If $CrO_3$ is used in an aqueous acidic medium (known as Jones reagent), it acts as a strong oxidizer and forms a carboxylic acid. However, if used with pyridine in an anhydrous medium (Collins reagent), it can stop at the aldehyde, but PCC is generally preferred for its stability and efficiency.

Q: Does PCC affect secondary alcohols?

Yes, PCC oxidizes secondary alcohols to ketones. Since ketones are generally resistant to further oxidation under these conditions, PCC is also an excellent reagent for this transformation.

Question

The most suitable reagent for the conversion of $R-CH_2-OH \rightarrow R-CHO$ is

Options:

(A) $KMnO_4$

(B) $K_2Cr_2O_7$

(C) $CrO_3$

(D) PCC [Pyridinium chlorochromate]

Sigma Percentile · Accepted Answer

### Analyzing the Conversion

The problem asks us to identify the most suitable reagent for converting a primary alcohol ($R-CH_2-OH$) into an aldehyde ($R-CHO$). This is a classic organic chemistry transformation that requires careful selection of the oxidizing agent.

Oxidation of a primary alcohol can proceed in two stages: first to an aldehyde, and then further to a carboxylic acid. The challenge lies in stopping the reaction at the first stage.

### The Trap of Strong Oxidizing Agents

Let's evaluate the options provided. Potassium permanganate ($KMnO_4$) and potassium dichromate ($K_2Cr_2O_7$) are powerful oxidizing agents. If we use them, they will not stop at the aldehyde stage. Instead, they will aggressively oxidize the primary alcohol all the way to a carboxylic acid ($R-COOH$).

Chromium trioxide ($CrO_3$) in an aqueous acidic medium (Jones reagent) also behaves as a strong oxidizing agent and yields a carboxylic acid. Therefore, none of these three reagents are suitable if our target is strictly an aldehyde.

### The Elegance of Mild Oxidizing Agents

To halt the oxidation at the aldehyde stage, we must employ a **mild oxidizing agent**. This is where **PCC (Pyridinium chlorochromate)** shines. PCC is a complex of chromium trioxide, pyridine, and hydrochloric acid, typically used in an anhydrous solvent like dichloromethane ($CH_2Cl_2$).

Because the reaction environment lacks water, the aldehyde cannot form an aldehyde hydrate, which is the necessary intermediate for further oxidation to a carboxylic acid. Consequently, the oxidation elegantly stops at the aldehyde.

**Final Conclusion:** PCC is the perfect, tailor-made reagent for the selective oxidation of primary alcohols to aldehydes.

Animated Solution for Chemistry - Organic Chemistry: The most suitable reagent for the conversion of $R - C H_{2} - O H \to R - C H O$ is

Select Answer:

Visualized Solution

Analyzing the Conversion

Action of Strong Oxidizing Agents

Action of Mild Oxidizing Agents

Final Conclusion

The Sigma Insight: Alcohols, Phenols, Ethers

Analyzing the Conversion

The Trap of Strong Oxidizing Agents

The Elegance of Mild Oxidizing Agents

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Animated Solution for Chemistry - Organic Chemistry: .math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } The most suitable reagent for the conversion of R−CH2​−OH→R−CHO is

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