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Chromate salts and other chromium (VI) oxides have been widely used as oxidizing agents for a variety of substrates including alcohols. These oxidants are frequently used in large excess but can also be used catalytically in conjunction with secondary oxidants. Problems in the use of chromium (VI) complexes as oxidation reagents include the lack of selectivity in oxidations, safety hazards associated with the use of large quantities of toxic compounds, cumbersome preparation and potential danger (ignition) in handling its complexes, difficulties in terms of product isolation and waste disposal and the need for aqueous acidic or basic conditions for reactions of chromate salts. These problems could be eliminated by reagents on solid supports. Many oxidants, especially the older and stronger chromium oxidizing agents, may have their reactivity and selectivity modified by adsorption onto inert supports. There were many chromium oxidants on inert inorganic supports such as: pyridinium chlorochromate on alumina, pyridinium chromate on silica, chromic acid on alumina and silica, chromyl chloride on silica – alumina and resin, poly(vinyl pyridinium chlorochromate (PVPCC) and poly(vinyl pyridinium dichromate, chromium VI oxide prepared and used for the oxidation of organic compounds. In this paper, we wish to report the utilization of zinc dichromate onto alumina as a convenient, efficient and economical reagent for the oxidation of alcohol to carbonyl compounds in CH3CN, H2O and solvent–Free conditions. (Table 1, Scheme 1) The rate of oxidation and yield of product depended on the nature of the substrate and reaction condition. Oxidation of benzylic alcohols proceeded more rapidly than the oxidation of aliphatic alcohols (Table 1). In the oxidation of benzyl alcohols to benzaldehyde, when there is an electron releasing group, the rate and yield of the reaction increased (Entry 1-3). 4-Nitro benzyl alcohol is oxidized to its corresponding aldehyde within a longer time and 80-85% yield. Primary and secondary aliphatic alcohols (Entry 1012) are oxidized to the corresponding carbonyl compounds under these conditions. Reaction in solvent–Free conditions is of a high yield and faster than reactions in water and acetonitrile. The reactions are relatively clean with no tar formation and no over oxidation to carboxylic acids. The reaction of cinnamyl alcohol gave cinnamaldehyde with a high yield without cleavage of benzylic double bond. (Entry 9) The acidity of zinc dichromate/Alumina (pH of 0.01 molar solution: 4.65) is less pronounced than those reported for pyridinium chlorochromate (pH of a 0.01 molar solution: 1.75) and pyridinium fluorochromate (pH of a 0.01 molar solution: 2.45). This reaction has several interesting features: 1) Yields are satisfactory. 2) The reaction is specific, avoiding over oxidation of aromatic aldehyde into acid. 3) The reagent is cheap and easy to prepare. 4) The reaction is performed under mild conditions, in CH3CN, H2O and solvent-free at room temperature. 5) The experimental set-up and work-up are exceedingly simple.

Heterogeneous Method for the Oxidation of Alcohols under Mild Conditions with Zinc Dichromate Adsorbed on Alumina