Microwave- and Ultrasound-Accelerated Green Oxidation of Alcohols by Potassium Permanganate absorbed on Copper(II) Sulfate Pentahydrate

Abstract

The solvent-free potassium permanganate (KMnO₄)–promoted oxidation of alcohols into the corresponding carbonyl derivatives has been examined in detail with respect to the importance of supporting materials and supporting irradiation. Secondary alcohols are oxidized very efficiently to the corresponding ketones at ambient temperature by KMnO₄ absorbed on a fourfold molar amount of copper(II) sulfate pentahydrate. The reaction is accelerated considerably by ultrasonic irradiation and drastically by microwave irradiation. Under the latter conditions, also hydroquinone can be converted 100% into p-benzoquinone within 4.5 min. Benzylic alcohols are oxidized most efficiently to the corresponding benzaldehydes under heterogeneous reaction conditions.

Keywords:

• Alcohols
• Copper sulfate pentahydrate
• Microwave
• Oxidation
• Permanganate
• Ultrasound

ACKNOWLEDGMENT

We thank Danish International Aid Programme (DANIDA) for financial support via the Enhancement of Research Capacity (ENRECA) program.

Notes

^a 2-Heptanol (0.348 g, 3 mmol) was shaken with the oxidant (KMnO₄ absorbed on the pertiment inorganic salt) together with small glass balls for 1 h.

^b Yields are based on GC/MS analysis.

^c The crystalline inorganic salt was mixed directly with crystalline KMnO₄ to avoid oxidation of the inorganic salt by aqueous KMnO₄.

^a Yields were determined by GC/MS analyses.

^b Method A: the reactants were shaken at 33–34 °C. Method B: the reaction was assisted by ultrasound irradiation. Method C: the reaction was assisted by microwave irradiation. Method D: the reaction mixture was heated in an oil bath at the appropriate temperature (Experimental).

^c Time = reaction time in minutes.

^d W = energy of microwave irradiation in Watts.

^e Temp = oil bath temperature (Experimental).

^a The reaction time (in minutes) was fixed by TLC monitoring.

^b Absolute yield of benzaldehyde (determined by GLC).

^c All starting material was consumed by the reaction.

^a Magnetic stirring at room temperature.

^b Ultrasonic cell at room temperature.

^c Yields were determined on the basis of GC/MS analysis.

^d rt = reaction time in hours.