Sodium Dichromate1

[7778-12-0]  · Cr2Na2O7  · Sodium Dichromate  · (MW 298.02)

(reagent for oxidizing alkylaromatics to carboxylic acids, oxidizing benzylic methylene groups to carbonyl groups, oxidizing alkenes to form a,b-unsaturated compounds, and for converting alcohols to carbonyl compounds)

Physical Data: mp (loses 2H2O at 100 °C) 356.7 °C (anhydr); d = 2.52 g cm-3.13

Solubility: sol H2O (238 g/100 mL at 0 °C), acetic acid, DMSO.

Form Supplied in: red crystals.

Handling, Storage, and Precautions: highly toxic and a cancer suspect agent. All chromium(VI) reagents must be handled with care in a fume hood. The mutagenicity of CrVI compounds is well documented.2

Introduction.

Sodium dichromate, which contains three oxygen equivalents per mol, has a wide range of oxidizing powers which are dependent on the solvent systems and experimental conditions.

Oxidation of Alkylaromatics to Carboxylic Acids.

Aqueous sodium dichromate selectively oxidizes arylmethanes and heteroarylmethanes to the corresponding carboxylic acids in excellent yields.3 For example, 1-methylnaphthalene is oxidized to 1-naphthoic acid in 95% yield (eq 1). Aromatic dicarboxylic acids are also available from dialkyl-substituted aromatic nuclei. In general, the lower the pH the more likely is attack on the aromatic nucleus.

Oxidation of Benzylic Methylene Groups to Carbonyl Groups.

Sodium dichromate in acetic acid oxidizes 3-ethyl-2,6-dimethylpyrazine to 3-acetyl-2,6-dimethylpyrazine (57%).4 The heterocyclic ring is essentially destroyed under these experimental conditions. 2-Acetylfluorene is oxidized to the fluorene-2-carboxylic acid by sodium dichromate in acetic anhydride/acetic acid (eq 2)5 and acenaphthene is converted to acenaphthenequinone with sodium dichromate in anhydrous acetic acid in the presence of cerium(IV) acetate catalyst (eq 3).6 The octahydrophenanthrene derivative (1), an intermediate in the synthesis of morphine analogs, is oxidized to the benzylic ketone by sodium dichromate in sulfuric acid (eq 4).7

Allylic Oxidation of Alkenes to a,b-Unsaturated Compounds.

Sodium dichromate, Dipyridine Chromium(VI) Oxide (Collins reagent), Chromium(VI) Oxide-3,5-Dimethylpyrazole (DMP/CrO3), Di-t-butyl Chromate, and sodium chromate are very useful for oxidizing polycyclic alkenes, especially terpenes and steroids, to a,b-unsaturated compounds. Limitations may include side reactions such as cleavage of the carbon-carbon double bond and lack of regiochemistry in the oxidation. Sodium dichromate in acetic acid oxidizes methyl acetyloleanolate to the 11-keto derivative (eq 5).8

Oxidation of Alcohols to Carbonyl Compounds.

Primary alcohols are oxidized to aldehydes in excellent yields by Chromium(VI) Oxide-Silica Gel, chromium(VI) oxide/graphite, chromium(VI) oxide/Kieselguhr, and other chromium oxidants. Primary alcohols are oxidized to aldehydes, acids, and esters by sodium dichromate.9-13 Removal of the low molecular weight aldehyde as fast as it is formed increases the yield.9,10 a,b-Unsaturated aldehydes may be prepared (35-50%) by stirring the a,b-unsaturated alcohols in aqueous sulfuric acid with sodium dichromate, followed by extraction of the product.14-16 Benzyl alcohols are oxidized to aromatic aldehydes in good to excellent yields.17 4-Hydroxymethyl-3-methyl-5-phenylisoxazoles are oxidized to 3-methyl-5-phenylisoxazole-4-carbaldehydes by sodium dichromate in sulfuric acid/DMSO (eq 6).18 Sodium dichromate in DMSO containing small amounts of sulfuric acid oxidizes primary and secondary saturated, allylic, and benzylic alcohols to carbonyl compounds (80-90%).19

Oxidation of Secondary Alcohols to Ketones.

Secondary alcohols are generally oxidized by sodium dichromate to ketones in excellent yields.19-22 A two-phase procedure using sodium dichromate, acetic acid, and benzene oxidized a cis/trans mixture of 1,5-dihydroxydecalins to a cis/trans mixture of decalin-1,5-diones (71-76%).23 This system oxidizes 2,5,5-trimethylcyclohex-2-en-1-ol to 2,5,5-trimethylcyclohex-2-en-1-one in 91% yield.24 A two-phase system of sodium dichromate, sulfuric acid, water, and ether oxidizes (-)-menthol to (-)-menthone in 97% yield.25,26 The latter procedure has been used for the selective oxidation of steroidal 16b,20a-diols to 16-keto-20a-ols.27 Dichloromethane has also been used as the organic phase in the sodium dichromate/sulfuric acid oxidation of a cis/trans mixture of 2-cyclopentene-1,4-diol to the 2-cyclopentene-1,4-dione.28 The organic phase protects the product from undesirable side reactions such as oxidation and epimerization. Of the three organic solvents, ethyl ether appears to be the one of choice owing to the ease of product isolation. Other examples of the two-phase sodium dichromate oxidation of alcohols are shown in eqs 7 and 8.29 Addition of a phase-transfer catalyst improves the two-phase transfer procedures.30,31

Other Applications.

Sodium dichromate in aqueous sulfuric acid oxidizes phenols, anilines, and aminophenols to p-quinones.32,33 3-Iodo-4-aminophenol is oxidized to 2-iodobenzoquinone in 95% yield. Sodium dichromate in acetic acid/sulfuric acid oxidizes phenols with an alkyl group para to the oxygen function to quinones and coupling products (eq 9).34 The minor product arises from oxidative elimination of the tertiary alkyl group from the para position. Sodium dichromate in acetic acid, sulfuric acid, and water (Kiliani reagent)35,36 oxidizes the triol digitogenin to digitogenic acid (a 13-keto-2,3-dioic acid).35

Borinate esters derived from the hydroboration of homoallylic alcohols were oxidized with sodium dichromate in aqueous sulfuric acid to racemic and enantiomeric g-butyrolactones (2-oxotetrahydrofurans).37 Hydroboration of 1-alkyl- and 1-aryl-1-indenes followed by sodium dichromate oxidation leads to the corresponding 1-alkyl- and 1-aryl-2-indanones (70-80%).38


1. (a) Wiberg, K. B. Oxidation in Organic Chemistry, Part A; Wiberg, K. B. Ed.; Academic: New York, 1965; Chapter II. (b) Freeman, F. Organic Syntheses By Oxidation With Metal Compounds; Mijs, W. J.; de Jonge, C. R. H. I., Eds.; Plenum: New York, 1986; Chapter 2. (c) Lee, D. G. The Oxidation of Organic Compounds by Permanganate Ion and Hexavalent Chromium; Open Court: La Salle, IL, 1980. (d) Stewart, R. Oxidation Mechanisms: Applications to Organic Chemistry; W. A. Benjamin: New York, 1964. (e) Cainelli, G.; Cardillo, G. Chromium Oxidations in Organic Chemistry; Springer: Berlin, 1984.
2. Cupo, D. Y.; Wetterhahn, K. E. Cancer Res. 1985, 45, 1146 and references cited therein.
3. Friedman, L.; Fishel, D. L.; Shechter, H. JOC 1965, 30, 1453.
4. Wolt, J. JOC 1975, 40, 1178.
5. Rieveschl, G., Jr.; Ray, F. E. OSC 1955, 3, 420.
6. Allen, C. F. H.; VanAllan, J. A. OSC 1955, 3, 1.
7. Shiotani, S. JOC 1975, 40, 2033.
8. Corey, E. J.; Ursprung, J. J. JACS 1956, 78, 183.
9. Hurd, C. D.; Meinert, R. N. OSC 1943, 2, 541.
10. Sauer, J. C. OSC 1963, 4, 813.
11. Wiberg, K. B.; Evans, R. J. JACS 1958, 80, 3019.
12. Robertson, G. R. OSC 1941, 1, 138.
13. Lee, D. G.; Spitzer, U. A. JOC 1970, 35, 3589.
14. Delaby, R.; Guillot-Allegre, S. BSF 1933, 301.
15. Martin, C. J.; Schepartz, A. I.; Daubert, B. F. JACS 1948, 70, 2601.
16. Jacobson, M. JACS 1950, 72, 1489.
17. Gindraux, L. HCA 1929, 12, 921.
18. Heck, R.; Ofenloch, R.; Wolf, R. S 1990, 62.
19. Rao, Y. S.; Filler, R. JOC 1974, 39, 3304.
20. Nickels, J. E.; Heintzelman, W. JACS 1950, 15, 1142.
21. Adkins, H.; Hager, G. F. JACS 1949, 71, 2965.
22. Conant, J. B.; Quayle, O. R. OSC 1941, 1, 211.
23. Johnson, W. S.; Gutsche, C. D.; Banerjee, D. K. JACS 1951, 73, 5464.
24. Ellis, J. E.; Dutcher, J. S.; Heathcock, C. H. JOC 1976, 41, 2670.
25. Brown, H. C.; Garg, C. P. JOC 1961, 83, 2952.
26. Acharya, S. P.; Brown, H. C. JOC 1970, 35, 3874.
27. Noguchi, S.; Imanishi, M.; Morita, K. CPB 1964, 12, 1184.
28. Rasmusson, G. H.; House, H. O.; Zaweski, E. F.; De Puy, C. H. OS 1962, 42, 36.
29. Brown, H. C.; Garg, C. P.; Liu, K.-T. JOC 1971, 36, 387.
30. Landini, D.; Montanari, F.; Rolla, F. S 1979, 134.
31. Pletcher, D.; Tait, S. J. D. TL 1978, 1601.
32. Smith, L. I.; Byers, D. J. JACS 1941, 63, 612.
33. Kvalnes, D. E. JACS 1934, 56, 667.
34. Albert, H. E.; JACS 1954, 76, 4983.
35. Kiliani, H.; Merk, B. CB 1901, 34, 3562.
36. Pelletier, S. W.; Locke, D. M. JACS 1965, 87, 761.
37. Mandal, A. K.; Mahajan, S. W. S 1991, 311.
38. Kirkiacharian, B. S.; Koutsourakis, P. G. S 1990, 815.

Fillmore Freeman

University of California, Irvine, CA, USA



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