Oxidation by chromic anhydride

Of the compounds we have dealt with so far, alcohols also dissolve in sulfuric acid. Alcohols can be distinguished from alkenes, however, by the fact that alcohols give a negative test with bromine in carbon tetrachloride and a negative Baeyer test—so long as we are not misled by impurities. Primary and secondary alcohols are oxidized by chromic anhydride, CrOa, in aqueous sulfuric acid within two seconds, the clear orange solution turns blue-green and becomes opaque. 

Alkynes and dienes respond to characterization tests in the same way as alkenes they decolorize bromine in carbon tetrachloride without evolution of hydrogen bromide, and they decolorize cold, neutral, dilute permanganate they are not oxidized by chromic anhydride. They are, however, more unsaturated than alkenes. This property can be detected by determination of their molecular formulas (CnH2n-2) and by a quantitative hydrogenation (two moles of hydrogen are taken up per mole of hydrocarbon). 

N-Nitroso N phenylglycine, 46, 96 reaction with acetic anhydride to yield 3 phenylsydnone, 46, 96 Nitrosyl chloride, addition to bicyclo-[2 2 ljhepta 2,5 diene, 46, 75 2,4-Nonanedione, 47, 92 Nonane, 1,1,3 trichloro-, 46,104 Nortricyclanol, 46, 74 oxidation by chromic acid, 46, 78 Nortricyclanone, 46, 77 Nortncj clyl acetate 46, 74 frombicyclo[2 2 ljhepta 2,5 dieneand acetic acid, 46, 74 saponification of, 46, 75... 

Hofmann degradation of 0,7V-dimethyllythranidine (77) methiodide followed by catalytic hydrogenation gave a product whose methiodide underwent the same sequence of reactions yielding de-iV-product 80 (mp 133.5-135°). Oxidation with chromic anhydride in pyridine afforded a diketone 81 (mp 116-118°). 

The Wittig reaction of dialdehyde 175, prepared by chromic anhydride-pyridine oxidation of diol 94 (53), with 176 in dilute methylene chloride solution produced cyclophane 177 in 86% yield. Epoxidation of 177 with m-chloroperbenzoic acid followed by hydrogenolysis over Pd/C, acetylation, and PtOz-Raney Ni-catalyzed hydrogenation afforded the cis-substituted piperidine derivative (178). 

By oxidation with chromic anhydride in acetic acid, 3-amino-2,4,5-triphenylpyrrole leads to compound 84,78,90 while l-acetyl-2,3,5-triphenylpyrrole undergoes bond opening between the pyrrole a- and j8-carbons.83 However, if the nitrogen in 2,3,5-triphenylpyrrole bears alkyl or aryl substituents, different behavior is observed in acetic acid —the pyrrole nucleus is still opened, but between the a-carbon and the heteroatom.91... 

Cleavage of ketones by oxidation is infrequently used for preparation of monocarboxylic acids. Trimethylacetic acid is made in 75% yield from pinacolone, (CHj)jCCOCHj, by oxidation with chromic anhydride in aqueous acetic acid. Cleavage on only one side of the carbonyl group is possible in this case. 

Mercuric Chromate, HgCr04, is formed by heating an equivalent mixture of mercuric oxide and chromic anhydride with a little water. It yields dark red rhombic prisms, which are decomposed by water with formation of the basic salt, SHgO.CrOa. Many basic salts have been described, but it has been shown from an investigation of the system, HgO—CrOg—HgO, that the above is the only one which exists as a separate chemical entity. It may also be produced by precipitation of... 

Oxidations with chromium trioxide (chromic oxide or chromic anhydride) and with chromic acid are carried out in different solvents, usually by adding solutions of chromic oxide or chromic acid to the solutions of the alcohols. When chromium trioxide dissolved in 80% acetic acid is added to a stirred solution of cis-2-phenylcyclohexanol in acetic acid at 50 °C and the mixture is allowed to stand at room temperature for 1 day, an 80% yield of 2-phenylcyclohexanone is obtained [576], Other solvents used are dimethylformamide [542], hexamethylphosphoric triamide (HMPA) [543], acetone [578, 5 i], ether [55 ], dichloromethane [555, 617], and benzene [571] (equation 249). 

Oxidation of allylic alcohols. Steroidal allylic alcohols are oxidized to the corresponding a,/3-unsaturated ketones in high yield by chromic anhydride in HMPT.2 The reaction is rapid in the case of equatorial alcohols but requires some weeks in the case of axial alcohols. Nonallylic hydroxyl groups are not affected. If the alcohol is only slightly soluble in HMPT, purified acetone is used as cosolvent. The reaction, if slow, can be carried out at 50°. Benzyl alcohol is oxidized, but in low yield (30%). 

Upon oxidation with chromic anhydride, the three muricholic acids, like hyocholic acid, yielded 3-keto-6,7-secocholanic acid-6,7-dioic acid (II) [Hsia et al. (30)]. Independent of the studies of Haslewood (24) and Ziegler (7), the St. Louis group identified this product by deriving it from lactone IV [Yamasaki and Chang (33)] which was hydrolyzed and subjected to chromic oxidation. The 3-keto-seco acid (II) was difficult to crystallize, while its 2,4-dinitrophenyl hydrazone derivative was found to be crystalline and easy to characterize and was suitably used for identification. 

The 6a-hydroxyl group in ty-muricholic and hyocholic acid was verified by the identification of hyodeoxycholic acid derived from these trihydroxy acids. After acetylation in a mixture of acetic anhydride and pyridine at room temperature, hyocholic acid yielded a diacetate whereas cy-muricholic acid yielded both a diacetate and a triacetate. The diacetates were oxidized with chromic anhydride and an ethylenedithioketal was prepared, which after desulfuration with Raney nickel yielded hyodeoxycholic acid [Hsia et al. (45)]. 

Scheme 8.2. A representation of a pathway for the oxidation of cyclohexanol to cyclohexanone by chromic anhydride.

Primary alcohols are oxidized in well-defined conditions by chromic anhydride CrOs, giving the corresponding carboxylic acids, while chromium reduces from state+VI to + III. In the same conditions, secondary alcohols give the corresponding ketones and tertiary alcohols give orange-colored chromic esters. This color contrasts with the green color of chromic salts obtained with both previous alcohols. 

Lee JG, Kwak KH (1992) Oxidation of aldehydes to aeyl azides by chromic anhydride-azidotrimethylsilane. Tetrahedron Lett 33(22) 3165-3166... 

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