What is the detailed protocol for Jones oxidation of a secondary alcohol to a ketone, including chromium trioxide equivalents, sulfuric acid concentration, acetone as solvent, and how to destroy excess chromium safely?
Research Activity · 4 steps
Research will be limited to:
- Reaction: Jones Oxidation
- Substrate: Secondary Alcohol
- Product: Ketone
- Reagent: Chromium Trioxide
Strategy: The user requires a precise laboratory protocol for the Jones oxidation of a secondary alcohol. I need to find the standard stoichiometry (equivalents of CrO3), the specific concentration of sulfuric acid used to prepare the Jones reagent, the volume of acetone solvent relative to the substrate, and the standard methods for quenching the reaction (typically using isopropanol or a reducing agent like sodium bisulfite). I will look for established organic synthesis procedures (e.g., Organic Syntheses or reputable lab manuals).
Queries:
- Jones oxidation protocol secondary alcohol chromium trioxide sulfuric acid equivalents acetone
- standard concentration of Jones reagent preparation CrO3 H2SO4 H2O
- quenching Jones oxidation excess chromium trioxide isopropyl alcohol sodium bisulfite
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Jones oxidation - Wikipedia
The Jones oxidation is used to oxidize secondary alcohols to ketones. The Jones reagent is prepared by dissolving chromium trioxide in aqueous sulfuric acid. The acidic mixture is added to an acetone solution of the secondary alcohol. Two equivalents of chromic acid oxidize three equivalents of the alcohol: 2 HCrO 4− + 3 RR'C(OH)H + 8 H+ + 4 H 2 O → 2 [Cr(H 2 O)6]3+ + 3 RR'CO. The reaction is rapid, exothermic and typically gives high yields. Cr(VI) compounds are carcinogenic. The inorganic products are green, characteristic of chromium(III) aquo complexes.
17.7: Oxidation of Alcohols - Chemistry LibreTexts
The LibreTexts Chemistry page discusses the Jones oxidation, a method for oxidizing secondary alcohols to ketones using chromic acid (H2CrO4). Chromic acid, also known as Jones reagent, is prepared by adding chromium trioxide (CrO3) to aqueous sulfuric acid.
Here's a summary based on the user's specific requirements:
- Reaction: Jones Oxidation
- Substrate: Secondary Alcohol
- Product: Ketone
- Reagent: Chromium Trioxide (CrO3) forms chromic acid (H2CrO4) with aqueous sulfuric acid.
Key Details Related to the Jones Oxidation (Secondary Alcohol to Ketone):
- Reagent Preparation: Chromic acid (Jones reagent) is made by adding chromium trioxide (CrO3) to aqueous sulfuric acid (H2SO4).
- Reaction type: Oxidation reaction, where alcohol is oxidised to carbonyl compound (ketone). The chromium is reduced from Cr(VI) to Cr(IV)
- General Reaction: Secondary alcohols are oxidized to ketones.
- Example: The oxidation of 2-Pentanol to 2-Pentanone is shown.
- Mechanism: The mechanism involves the formation of a chromate ester intermediate, followed by an E2-like elimination to form the ketone and release a reduced chromium species.
Jones Oxidation - Chemistry Steps
The provided text discusses Jones oxidation, a method for oxidizing primary alcohols to carboxylic acids. While it mentions that secondary alcohols are oxidized to ketones by both strong and weak oxidizing agents, it lacks a detailed protocol. The site provides the following limited information:
- Reaction: Jones Oxidation (CrO3 in H2SO4, chromic acid, or dichromate salt such as Na2Cr2O7 in the presence of sulfuric acid)
- Substrate: Secondary alcohols
- Product: Ketones
- Reagent: CrO3 in H2SO4, chromic acid, or dichromate salt such as Na2Cr2O7 in the presence of sulfuric acid
- The mechanism involves nucleophilic addition of the alcohol to protonated chromic oxide, forming a chromate ester, followed by elimination to form the ketone. Then the aldehyde is converted into carboxylic acid by adding water with the same mechanism.
The text does not define specifics such as: Chromium Trioxide equivalents, sulfuric acid concentration, acetone solvent volume, or how to quench the excess chromium reagent.
Jones Oxidation (CrO3 + H2SO4)
ChemicalDesk.Com: Jones Oxidation (CrO3 + H2SO4)
The Jones Oxidation converts secondary alcohols to ketones using a mixture of chromic anhydride and dilute sulfuric acid (CrO3 + H2SO4 + H2O) in acetone.
Reagent Preparation: The Jones Reagent is a mixture of chromic trioxide or sodium dichromate in diluted sulfuric acid, which forms chromic acid in situ.
Mechanism: The alcohol and chromic acid form a chromate ester that either reacts intramolecularly or intermolecularly in the presence of a base (water) to yield the corresponding carbonyl compound (ketone from secondary alcohol).
Procedures (Examples):
- Nortricyclanone Synthesis: Dissolve 70 g (0.70 mole) of chromium trioxide in 100 ml of water. Add to this 112 g (61 ml, 1.10 moles) of concentrated (18M) sulfuric acid followed by 200 ml of water. Cool to 0–5°C. React 110 g (1.00 mole) of nortricyclanol in 600 ml of acetone at 0–5°C, adding the reagent at a rate to maintain the temperature at 20°C. Stir for 3 hours after the addition. Quench: Add sodium bisulfite until the brown color is gone. Work-up includes extraction with petroleum ether, washing with sodium chloride and sodium bicarbonate solutions, drying over magnesium sulfate, and distillation.
- Cycloöctanone Synthesis: Dissolve 67 g chromium trioxide in 125 ml distilled water. Add 58 ml conc sulfuric acid (sp. gr. 1.84), and dilute with water to a total volume of 225 ml. React 64 g (0.5 mole) of cyclooctanol in 1.25 l of acetone, keeping the temperature below 35°C. Add the chromic acid oxidizing reagent until the orange color persists for 20 minutes. Quench excess chromium: Add isopropyl alcohol dropwise until the excess chromic acid is destroyed. Add sodium bicarbonate until pH is neutral. Filter and concentrate. Add saturated sodium chloride solution. Extract with ether and distill the ether.
- Methcathinone Synthesis (Example): 1.06 grams CrO3, .92ml H2SO4, and distilled water added to bring the total volume to 4ml. Add the Jones reagent slowly to 2 grams of pseudoephedrine which are dissolved in 20ml acetone, while stirring. Stir the solution warm over night. Add about 20mL of water, and chill in an ice bath. Drop wise and with stirring add about 1 gram of NaOH in a solution with as little water as possible until pH=12. Then extract with toluene.
Tips:
- Residues of toxic Cr(V) and Cr(VI) compounds can be destroyed by the addition of an excess of 2-propanol once the intended reaction is complete. The formation of Cr(III) is indicated by a color change to green.
- Make sure the reaction mass is slightly acidic for it to progress.
- The Jones reagent is prepared by adding chromic anhydride to dilute sulfuric acid in acetone and is added to the alcohol at 0-25C.
- The orange or yellow colored Cr(VI) is reduced to blue green Cr(III) species during the oxidation.
Jones Reagent
The Jones reagent is a mixture of chromium trioxide (CrO3) in sulfuric acid (H2SO4), typically used with acetone as a solvent for Jones oxidation reactions. The reagent is described as a solution of 2M CrO3 in aqueous H2SO4 with a density of 1.298 g/mL. Jones oxidation is used for converting primary alcohols to carboxylic acids. A procedure excerpt details the preparation of the reagent by adding H2SO4 (10.8 mL) dropwise to a solution of CrO3 (12.4 g, 0.123 mol) in H2O (88.4 mL) at 0°C. The reagent is noted to be highly toxic, and the website links to a patent and external resources for further information on oxidation of alcohols and Jones reagent use at Sigma-Aldrich and Wikipedia. While the website describes the Jones Oxidation and has information about the individual components and their concentrations and procedure, it focuses primarily on oxidation of primary alcohols to carboxylic acids.
The Jones oxidation is a classic method for the conversion of secondary alcohols to ketones. It utilizes "Jones reagent"—a solution of chromium trioxide ($\ce{CrO3}$) in aqueous sulfuric acid ($\ce{H2SO4}$)—typically performed in acetone.
1. Preparation of Jones Reagent (Standard 2.67 M Solution)
The most common laboratory formulation for the reagent is the 2.67 M solution, prepared as follows:
* Components: $\ce{26.72 g}$ of chromium trioxide ($\ce{CrO3}$) and $\ce{23 mL}$ of concentrated sulfuric acid ($\ce{H2SO4}$).
* Procedure: Dissolve the $\ce{CrO3}$ in a minimum amount of distilled water (approx. $\ce{40-50 mL}$). Carefully add the concentrated $\ce{H2SO4}$ while cooling in an ice bath (the dissolution is highly exothermic). Once mixed, dilute the solution with distilled water to a final volume of $\ce{100 mL}$.
* Result: This yields a deep orange-red solution where the active oxidant is chromic acid ($\ce{H2CrO4}$).
2. Reaction Protocol
- Substrate Solution: Dissolve 1 equivalent of the secondary alcohol in acetone. The volume of acetone is typically $\ce{10-20 mL}$ per gram of substrate to ensure the resulting chromium salts remain as a manageable slurry.
- Stoichiometry: Theoretically, 2 equivalents of $\ce{Cr(VI)}$ are required to oxidize 3 equivalents of alcohol. In practice, the reagent is added dropwise to the stirred acetone solution at $\ce{0 °C}$ to $\ce{20 °C}$.
- Endpoint: The reaction is self-indicating. The reagent is added until the orange color of the $\ce{Cr(VI)}$ persists for 15–20 minutes, indicating that the alcohol has been fully consumed. During the reaction, the solution will turn from orange to a dark green/blue-green color, characteristic of $\ce{Cr(III)}$ salts.
3. Quenching and Destruction of Excess Chromium
Because $\ce{Cr(VI)}$ is highly toxic and carcinogenic, any excess must be reduced to the less toxic $\ce{Cr(III)}$ state before workup.
* Isopropanol Quench: Add isopropyl alcohol (2-propanol) dropwise to the reaction mixture. The isopropanol reacts with the remaining Jones reagent to form acetone and green $\ce{Cr(III)}$. Continue adding until the orange/brown tint completely disappears and only a green precipitate or solution remains.
* Sodium Bisulfite Quench: Alternatively, an aqueous solution of sodium bisulfite ($\ce{NaHSO3}$) or sodium metabisulfite ($\ce{Na2S2O5}$) can be added. These inorganic reducing agents rapidly convert $\ce{Cr(VI)}$ to $\ce{Cr(III)}$.
4. Workup and Safety
- Workup: After quenching, the mixture is often filtered through a pad of Celite to remove the bulky chromium salts. The filtrate is then concentrated, and the ketone is extracted using an organic solvent (e.g., diethyl ether or ethyl acetate), followed by washes with saturated $\ce{NaHCO3}$ to neutralize any residual acid.
- Safety Note: Chromium(VI) compounds are potent oxidizers and known human carcinogens. All manipulations should be performed in a fume hood with appropriate PPE (nitrile gloves, lab coat, and safety goggles). Chromium waste must be collected and disposed of according to hazardous heavy metal protocols.
For specific experimental variations, such as the synthesis of cycloöctanone or nortricyclanone, historical procedures can be found in Organic Syntheses.