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PCC and CrO₃ Reagents

Selective vs. Strong Oxidation of Alcohols.

By chemca Team • Updated Jan 2026

Oxidation of alcohols is a fundamental transformation. The choice of reagent determines whether a primary alcohol stops at the Aldehyde stage or continues to the Carboxylic Acid.

1. PCC (Pyridinium Chlorochromate)

The Mild Oxidizer

Composition: A complex of Chromium Trioxide ($CrO_3$), Pyridine, and HCl. ($C_5H_5NH^+ CrO_3Cl^-$). Used in dry Dichloromethane ($CH_2Cl_2$).

A. Primary Alcohols to Aldehydes: PCC is the best reagent to stop oxidation at the aldehyde stage.

$$ R-CH_2OH \xrightarrow{PCC} R-CHO $$

B. Secondary Alcohols to Ketones:

$$ R-CH(OH)-R' \xrightarrow{PCC} R-CO-R' $$

Selectivity: PCC does NOT affect carbon-carbon double bonds ($C=C$).
Example: $CH_3-CH=CH-CH_2OH \xrightarrow{PCC} CH_3-CH=CH-CHO$

2. Chromic Oxide ($CrO_3$) - Jones Reagent

The Strong Oxidizer

Composition: $CrO_3$ dissolved in aqueous diluted Sulphuric Acid ($H_2SO_4$) and Acetone. This generates Chromic Acid ($H_2CrO_4$) in situ.

A. Primary Alcohols to Carboxylic Acids: In aqueous acidic medium, the aldehyde initially formed forms a hydrate, which is easily oxidized further to acid.

$$ R-CH_2OH \xrightarrow{CrO_3 / H^+} \underset{\text{Carboxylic Acid}}{R-COOH} $$

B. Secondary Alcohols to Ketones:

$$ R-CH(OH)-R' \xrightarrow{CrO_3 / H^+} R-CO-R' $$

3. Collins Reagent

The Alternative Mild Oxidizer

Composition: A complex of Chromium Trioxide with two molecules of Pyridine ($CrO_3 \cdot 2C_5H_5N$) in Dichloromethane.

Function: Similar to PCC. It oxidizes primary alcohols to Aldehydes without over-oxidation.

$$ R-CH_2OH \xrightarrow{CrO_3 \cdot 2Py} R-CHO $$

Note: It is hygroscopic and difficult to prepare compared to PCC.

4. Quick Comparison

Substrate	Reagent: PCC ($CH_2Cl_2$)	Reagent: Jones ($CrO_3/H^+$)
Primary Alcohol	Aldehyde ($RCHO$)	Carboxylic Acid ($RCOOH$)
Secondary Alcohol	Ketone ($RCOR'$)	Ketone ($RCOR'$)
Tertiary Alcohol	No Reaction	No Reaction
Double Bond ($C=C$)	Unreactive	Generally Unreactive*

*Jones reagent is acidic and may cause acid-catalyzed hydration/rearrangement of double bonds in sensitive molecules.

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