Jones Reagent: Oxidation of Alcohols
Jones Reagent is a strong oxidizing agent consisting of Chromium Trioxide ($CrO_3$) dissolved in aqueous Sulfuric Acid ($H_2SO_4$) and Acetone. It is primarily used to oxidize primary alcohols to carboxylic acids and secondary alcohols to ketones.
1. General Reactions
A. Primary Alcohols ($1^\circ$)
Primary alcohols are oxidized completely to Carboxylic Acids.
B. Secondary Alcohols ($2^\circ$)
Secondary alcohols are oxidized to Ketones.
2. Preparation and Active Species
The reagent is prepared by dissolving $CrO_3$ in dilute sulfuric acid. The active species formed in situ is Chromic Acid ($H_2CrO_4$).
Acetone is added as a co-solvent to dissolve the organic substrate and moderate the reaction.
3. Detailed Mechanism
The oxidation proceeds via the formation of a Chromate Ester.
Step 1: Chromate Ester Formation
The alcohol attacks the chromic acid, eliminating water to form a Chromate Ester.
Step 2: Elimination ($E2$-like)
A base (usually water) abstracts an $\alpha$-proton. The electrons form the $C=O$ double bond, and the chromium species leaves (Cr reduces from +6 to +4).
This yields an Aldehyde ($R-CHO$).
Step 3: Hydration and Further Oxidation (For Primary Alcohols)
In the presence of water and acid, the aldehyde is rapidly hydrated to form a Gem-diol (Hydrate).
This gem-diol reacts again with chromic acid (Steps 1 & 2 repeat) to undergo a second oxidation, yielding the Carboxylic Acid ($R-COOH$).
4. Comparison with PCC/PDC
The "Water" Factor
Jones Reagent (Aqueous/Acidic): Contains water. Aldehyde hydrates to gem-diol $\to$ Oxidizes further to Acid.
PCC / PDC (Anhydrous/DCM): No water present. Aldehyde cannot hydrate $\to$ Oxidation stops at Aldehyde.
5. Limitations
- Acid Sensitivity: The reaction conditions are strongly acidic. Compounds with acid-sensitive groups (like acetals, double bonds susceptible to hydration) may decompose.
- Tertiary Alcohols: Do not react (no $\alpha$-hydrogen), but may undergo acid-catalyzed dehydration to alkenes.
6. The "Jones Test"
The reduction of Chromium from Cr(VI) (Orange) to Cr(III) (Green) serves as a visual test for the presence of primary or secondary alcohols.
Color Change: Orange $\rightarrow$ Green.
Jones Reagent Quiz
Test your concepts on Alcohol Oxidation. 10 MCQs with explanations.
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