PCC (Pyridinium Chlorochromate): Mild Oxidation
Pyridinium Chlorochromate (PCC), also known as the Corey-Suggs Reagent, is a mild oxidizing agent used primarily to oxidize primary alcohols to aldehydes and secondary alcohols to ketones. Its most significant feature is that it prevents the over-oxidation of aldehydes to carboxylic acids.
1. Preparation and Structure
PCC is prepared by dissolving Chromium Trioxide ($CrO_3$) in aqueous Hydrochloric Acid ($HCl$) and adding Pyridine.
Structure: It is a salt consisting of a Pyridinium cation and a Chlorochromate anion.
2. General Reactions
PCC is typically used in anhydrous solvents like Dichloromethane ($CH_2Cl_2$ or DCM).
A. Primary Alcohols $\to$ Aldehydes
This is the main advantage of PCC. Strong oxidants like $KMnO_4$ or Jones Reagent ($CrO_3/H_2SO_4$) oxidize primary alcohols all the way to carboxylic acids. PCC stops at the aldehyde.
B. Secondary Alcohols $\to$ Ketones
3. Detailed Mechanism
The mechanism involves the formation of a chromate ester followed by an elimination reaction.
Step 1: Formation of Chromate Ester
The alcohol oxygen attacks the chromium atom, displacing the chloride ion (or Cl helps as a leaving group during proton shuffling). This forms a Chromate Ester.
Step 2: Elimination (Oxidation Step)
A base (often Pyridine or the Chloride ion) abstracts a proton from the carbon bonded to oxygen. The electrons form the $C=O$ double bond, and the chromium species leaves (Chromium is reduced from Cr(VI) to Cr(IV)).
4. Why does it stop at Aldehyde?
Absence of Water
For an aldehyde to be oxidized further to a carboxylic acid, it must first be hydrated to form a Gem-diol ($R-CH(OH)_2$). Since PCC is used in anhydrous organic solvents ($CH_2Cl_2$), water is not available to form the gem-diol. Therefore, the oxidation halts at the aldehyde stage.
5. Chemoselectivity
PCC is a mild reagent. It oxidizes alcohols but does not affect carbon-carbon double bonds ($C=C$).
Example: Oxidation of Allyl Alcohol
$$ CH_2=CH-CH_2-OH \xrightarrow{PCC} CH_2=CH-CHO \text{ (Acrolein)} $$Strong oxidants might cleave the double bond.
6. Summary Table
| Reagent | Condition | Result with $1^\circ$ Alcohol |
|---|---|---|
| Jones Reagent ($CrO_3/H^+$) | Aqueous Acidic | Carboxylic Acid |
| PCC | Anhydrous ($CH_2Cl_2$) | Aldehyde |
| PDC (Pyridinium Dichromate) | Anhydrous ($CH_2Cl_2$) | Aldehyde |
PCC Quiz
Test your concepts on Oxidation. 10 MCQs with explanations.
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