The conversion of a primary alcohol (R–CH₂–OH) to an aldehyde (R–CHO) is a selective oxidation reaction. The key challenge is to stop the oxidation at the aldehyde stage and prevent further oxidation to the carboxylic acid (R–COOH).

Let's evaluate the reagents:

Step 1: Analyze Common Oxidizing Agents

Strong oxidizing agents like $K_2{\mathrm{Cr}}_2{O}_7$ (potassium dichromate) and ${\mathrm{KMnO}}_4$ (potassium permanganate) are very powerful. They are typically used in aqueous acidic conditions and will oxidize a primary alcohol all the way to a carboxylic acid. They are not suitable for stopping at the aldehyde.

Step 2: Analyze Chromium Trioxide (CrO₃)

Chromium trioxide is also a strong oxidizing agent. Under standard conditions (e.g., in aqueous acid), it behaves similarly to $K_2{\mathrm{Cr}}_2{O}_7$ and leads to over-oxidation to the carboxylic acid.

Step 3: The Role of PCC (Pyridinium Chlorochromate)

PCC is a complex of chromium trioxide with pyridine and HCl. Its unique structure makes it a mild and anhydrous oxidizing agent. It is soluble in organic solvents like dichloromethane. Because the reaction occurs in the absence of water, the aldehyde product is not hydrated. The hydrate form is more susceptible to further oxidation. PCC selectively oxidizes primary alcohols to aldehydes without over-oxidizing them.

The general reaction is:

$R-{\mathrm{CH}}_2-\mathrm{OH}\underset{\mathrm{PCC},{\mathrm{CH}}_2{\mathrm{Cl}}_2}{\to }R-\mathrm{CHO}$

Final Answer: Therefore, the most suitable reagent for this conversion is PCC (Pyridinium Chlorochromate).

Related Topics & Formulae

Oxidation of Alcohols: The oxidation level of an alcohol depends on its class (primary, secondary, tertiary) and the strength of the oxidizing agent used.

• Primary Alcohols: Can be oxidized to aldehydes (with mild agents like PCC) or carboxylic acids (with strong agents like KMnO₄, K₂Cr₂O₇/H⁺).
• Secondary Alcohols: Are oxidized to ketones. Most oxidizing agents (including PCC, KMnO₄, K₂Cr₂O₇) will stop at the ketone stage.
• Tertiary Alcohols: Are resistant to oxidation under normal conditions.

Other Mild Oxidizing Agents: While PCC is very common, other reagents like Dess-Martin periodinane (DMP) or the Swern oxidation also achieve the selective oxidation of primary alcohols to aldehydes.