Oxidizing Action of Potassium Dichromate ($K_2Cr_2O_7$)
Acidified Potassium Dichromate ($K_2Cr_2O_7$) is a powerful oxidizing agent used extensively in organic chemistry. In acidic medium (usually dilute $H_2SO_4$), it releases nascent oxygen which oxidizes alcohols and aldehydes.
1. The Active Species
In acidic solution, dichromate ions react to form Chromic Acid ($H_2CrO_4$), which acts as the oxidizer. The reduction half-reaction shows the conversion of Chromium(VI) to Chromium(III).
Characteristic Color Change
The reaction is accompanied by a distinct color change from Orange ($Cr_2O_7^{2-}$) to Green ($Cr^{3+}$). This is often used as a qualitative test for primary and secondary alcohols (and aldehydes).
2. Oxidation of Alcohols
The extent of oxidation depends on the nature of the alcohol.
A. Primary Alcohols ($1^\circ$)
Primary alcohols are first oxidized to aldehydes. However, in aqueous acidic media, the aldehyde is rapidly oxidized further to a Carboxylic Acid. Stopping at the aldehyde stage is difficult with $K_2Cr_2O_7$ (unlike PCC).
Example: Ethanol $\to$ Acetaldehyde $\to$ Acetic Acid.
B. Secondary Alcohols ($2^\circ$)
Secondary alcohols are oxidized to Ketones. Ketones are resistant to further oxidation under these conditions, so the reaction stops here.
Example: Isopropyl Alcohol $\to$ Acetone.
C. Tertiary Alcohols ($3^\circ$)
Tertiary alcohols have no $\alpha$-hydrogen on the carbon bearing the -OH group. Therefore, they do not react with $K_2Cr_2O_7$ under normal conditions. (Under drastic conditions, C-C bond cleavage may occur).
3. Oxidation of Aldehydes
Aldehydes are very easily oxidized to Carboxylic Acids by acidified $K_2Cr_2O_7$. This distinguishes aldehydes from ketones (which do not react).
4. Mechanism (Chromate Ester)
The mechanism is similar to Jones Oxidation.
- Formation of a Chromate Ester ($R-O-CrO_3H$).
- Base-assisted elimination of the $\alpha$-proton.
- Formation of $C=O$ double bond and reduction of Chromium.
5. Comparison: $K_2Cr_2O_7$ vs $KMnO_4$
| Feature | Acidified $K_2Cr_2O_7$ | Alkaline/Acidic $KMnO_4$ |
|---|---|---|
| Strength | Strong, but slightly milder than KMnO4 | Very Strong |
| Color Change | Orange $\to$ Green | Purple $\to$ Colorless (or Brown $MnO_2$) |
| Alkylbenzene Oxidation | Poor/Slow (Toluene resists) | Excellent (Toluene $\to$ Benzoic Acid) |
Dichromate Oxidation Quiz
Test your concepts on Oxidation reactions. 10 MCQs with explanations.
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