Alcohols, secondary, oxidation dichromate

By the oxidation of secondary alcohols with potassium dichromate and dilute sulphuric acid, for example ... 

Primary and secondary alcohols undergo oxidation when reacted with a mixture of acidified potassium dichromate (VI) and dilute sulphuric acid. Primary alcohols are oxidised in the sequence... 

The remaining three steps are accomplished without purification of the intermediate products. The secondary hydroxy group is protected by acetylation and the benzyl ether is removed by hydrogenolysis to provide a primary alcohol. The alcohol is oxidized to a carboxylic acid by ruthenium(III) chloride or pyridinium dichromate. This method has been applied to the synthesis of various enzyme inhibitors containing the 1-hydroxyethylene isostere. 

The oxidation of secondary alcohols with sodium dichromate in dilute sulphuric acid gives acceptable yields of ketones since these do not normally undergo extensive further oxidation under the reaction conditions (cf. Section 5.7.1, p. 587, the oxidation of primary alcohols to aldehydes). 

Chromic acid test. This test is able to distinguish primary and secondary alcohols from tertiary alcohols. Using acidified dichromate solution, primary alcohols are oxidized to carboxylic acids secondary alcohols are oxidized to ketones tertiary alcohols are not oxidized. (Note that in those alcohols which are oxidized, the carbon that has the hydroxyl group loses a hydrogen.) In the oxidation, the brown-red color of the chromic acid changes to a blue-green solution. Phenols are oxidized to nondescript brown tarry masses. (Aldehydes are also oxidized under these conditions to carboxylic acids, but ketones remain intact see Experiment 31 for further discussion.)... 

Quinolinium dichromate (17) shows the solubility profile common to most chromium based oxidants—sparingly soluble in chlorinated solvents, but more soluble in more polar solvents. It has been mainly used in dichloromethane at reflux, or in DMF at 30 C to oxidize primary alcohols. Secondary alcohols are also oxidized reasonably well. 

Pyridinium dichromate, prepared from chromium trioxide in a minimum amount of water and pyridine, forms a bright-orange solid and is soluble in water, dimethylformamide, dimethyl sulfoxide, and dimethyl-acetamide sparingly soluble in dichloromethane, chloroform, and acetone and almost insoluble in hexane, toluene, ether, and ethyl acetate. Allylic secondary alcohols are oxidized more rapidly than their saturated analogues. Oxidations are carried out in dichloromethane solutions at 25 °C, and ketones are obtained in high yields (equation 251) [603. ... 

Aldehydes are oxidized not only by the same reagents that oxidize primary and secondary alcohols—permanganate and dichromate—but also by the very mild oxidizing agent silver ion. Oxidation by silver ion requires an alkaline medium to prevent precipitation of the insoluble silver oxide, a complexing agent is added ammonia... 

In either mechanism, a low value for the KIE is expected for non-linear hydrogen transfer, with non-linear transfer associated with similar enthalpies and diiferent energies of activation for the two isotopomers. The oxidation of benzyl alcohol by quinoxalinium dichromate is acid catalysed, being first order in dichromate, substrate and added / -toluenesulfonic acid, and the KIE for PhCD20H oxidation is 6.78. The latter is the product of a primary effect and a secondary a-deuterium effect if we assume the latter is around 1.2 (see Section 3.3.1), the reaction is a typical acid chromate oxidation with a linear... 

Oxidation with sodium dichromate. Primary and secondary alcohols are oxidized to aldehydes and ketones in 80-90% yield by Na2Cr2 07 2H2O and sulfuric acid in DMSO at 70° (90 min.). DMSO is not involved and acts as a solvent. In its absence, considerable charring occurs with further oxidation of the carbonyl product. Cr03 can also be used as oxidant. ... 

Secondary alcohols are oxidized to ketones by chromic acid or an aqueous solution of potassium dichromate,... 

Arts. The color change from orange to green indicates that dichromate has oxidized the unknown and has itself been converted to the green-colored Cr . However, this test does not distinguish hexene from hexanol since alkenes as well as primary and secondary alcohols are oxidized by dichromate. 

Primary alcohols are oxidized to aldehydes and secondary alcohols to ketones by a number of reagents especially pyridinium dichromate (PDC) or pyridinium chloro-chromate (PCC) in dichloromethane. 

A variety of compounds are available that oxidize alcohols. For many years, a commonly used reagent was chromic acid (H2Cr04), which is formed when sodium dichromate (Na2Cr207) is dissolved in aqueous acid. Notice that secondary alcohols are oxidized to ketones. 

Primary alcohols are oxidized by the easily prepared pyridinium dichromate [6] (PDC) in DMF (Eq. 6.3) [7] or by Jones reagent in acetone (Eq. 6.4) [8]. Secondary alcohols are also easily oxidized to ketones under these conditions. Potassium permanganate in aqueous NaOH will oxidize primary alcohols but is not selective, attacking alkene sites as well. 

Alcohols are often oxidized with chromium(VI) compounds. When the alcohol is oxidized, Cr(VI) is reduced in several steps to Cr(III). The specific Cr(VI) species used depends on the scale of the process, the cost of reagents, and limitations that result firom the presence of other functional groups in the reactant. For example, the inexpensive reagent sodium dichromate (Na2Cr207) oxidizes secondary alcohols to ketones. It also oxidizes primary alcohols to aldehydes and then to carboxylic acids. Sodium dichromate in acetic acid (HOAc) is used in large-scale reactions to convert secondary alcohols to ketones. 

By Oxidation of Alcohols.—Barium manganate is readily available and stable, and has been recommended for the oxidation of primary and secondary alcohols to aldehydes and ketones respectively. Chromic acid adsorbed on to silica geP and the recyclable poly[vinyl(pyridinium chromate)] effect the same changes, while acid-stable primary and secondary alcohols are oxidized rapidly using potassium dichromate, sulphuric acid, methylene chloride, and a phase-transfer catalyst." Benzeneseleninic anhydride is an alternative reagent for the oxidation of alcohols under essentially neutral conditions. ... 

Oxidation of secondary alcohols to ke tones (Section 15 10) Many oxidizing agents are available for converting sec ondary alcohols to ketones PDC or PCC may be used as well as other Cr(VI) based agents such as chromic acid or po tassium dichromate and sulfuric acid... 

Dichromate oxidation of secondary alcohols produces ketones in good yield, with little additional oxidation. For example, CH,CH2CH(OH)CH3 can be oxidized to CH CH2COCH3. The difference between the ease of oxidation of aldehydes and that of ketones is used to distinguish them. Aldehydes can reduce silver ions to form a silver mirror—a coating of silver on test-tube walls—with Tollens reagent, a solution of Ag1" ions in aqueous ammonia (Fig. 19.3) ... 

Quinolinium dichromate (QDC) oxidations of primary and secondary alcohols both proceed via a cyclic chromate ester. Acrylonitrile polymerization was observed in the oxidation of several para- and meffl-substituted benzaldehydes to the corresponding benzoic acids by quinolinium chlorochromate (QCC). QCC oxidations of diphenacyl sulfide and of aromatic anils have been studied. 

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